[PATCH v4] c++: Check for indirect change of active union member in constexpr [PR101631,PR102286]

[PATCH v4] c++: Check for indirect change of active union member in constexpr [PR101631,PR102286]

[Nathaniel Shead]

Now that bootstrap has finished, I have gotten regressions in the
following libstdc++ tests:

Running libstdc++:libstdc++-dg/conformance.exp ...
FAIL: 20_util/bitset/access/constexpr.cc -std=gnu++23 (test for excess errors)
FAIL: 20_util/bitset/access/constexpr.cc -std=gnu++26 (test for excess errors)
FAIL: 20_util/variant/constexpr.cc -std=gnu++20 (test for excess errors)
FAIL: 20_util/variant/constexpr.cc -std=gnu++26 (test for excess errors)
FAIL: 21_strings/basic_string/cons/char/constexpr.cc -std=gnu++20 (test for excess errors)
FAIL: 21_strings/basic_string/cons/char/constexpr.cc -std=gnu++26 (test for excess errors)
FAIL: 21_strings/basic_string/cons/wchar_t/constexpr.cc -std=gnu++20 (test for excess errors)
FAIL: 21_strings/basic_string/cons/wchar_t/constexpr.cc -std=gnu++26 (test for excess errors)
FAIL: 21_strings/basic_string/modifiers/swap/constexpr-wchar_t.cc -std=gnu++20 (test for excess errors)
FAIL: 21_strings/basic_string/modifiers/swap/constexpr-wchar_t.cc -std=gnu++26 (test for excess errors)
FAIL: 21_strings/basic_string/modifiers/swap/constexpr.cc -std=gnu++20 (test for excess errors)
FAIL: 21_strings/basic_string/modifiers/swap/constexpr.cc -std=gnu++26 (test for excess errors)
FAIL: std/ranges/adaptors/join_with/1.cc -std=gnu++23 (test for excess errors)
UNRESOLVED: std/ranges/adaptors/join_with/1.cc -std=gnu++23 compilation failed to produce executable
FAIL: std/ranges/adaptors/join_with/1.cc -std=gnu++26 (test for excess errors)
UNRESOLVED: std/ranges/adaptors/join_with/1.cc -std=gnu++26 compilation failed to produce executable

On investigation though it looks like the issue might be with libstdc++
rather than the patch itself; running the failing tests using clang with
libstdc++ also produces similar errors, and my reading of the code
suggests that this is correct.

What's the way forward here? Should I look at creating a patch to fix
the libstdc++ issues before resubmitting this patch for the C++
frontend? Or should I submit a version of this patch without the
`std::construct_at` changes and wait till libstdc++ gets fixed for that?

On Sat, Sep 23, 2023 at 01:01:20AM +1000, Nathaniel Shead wrote:
> On Fri, Sep 22, 2023 at 02:21:15PM +0100, Jason Merrill wrote:
> > On 9/21/23 09:41, Nathaniel Shead wrote:
> > > I've updated the error messages, and also fixed another bug I found
> > > while retesting (value-initialised unions weren't considered to have any
> > > active member yet).
> > > 
> > > Bootstrapped and regtested on x86_64-pc-linux-gnu.
> > > 
> > > -- >8 --
> > > 
> > > This patch adds checks for attempting to change the active member of a
> > > union by methods other than a member access expression.
> > > 
> > > To be able to properly distinguish `*(&u.a) = ` from `u.a = `, this
> > > patch redoes the solution for c++/59950 to avoid extranneous *&; it
> > > seems that the only case that needed the workaround was when copying
> > > empty classes.
> > > 
> > > This patch also ensures that constructors for a union field mark that
> > > field as the active member before entering the call itself; this ensures
> > > that modifications of the field within the constructor's body don't
> > > cause false positives (as these will not appear to be member access
> > > expressions). This means that we no longer need to start the lifetime of
> > > empty union members after the constructor body completes.
> > > 
> > > As a drive-by fix, this patch also ensures that value-initialised unions
> > > are considered to have activated their initial member for the purpose of
> > > checking stores, which catches some additional mistakes pre-C++20.
> > > 
> > > 	PR c++/101631
> > > 
> > > gcc/cp/ChangeLog:
> > > 
> > > 	* call.cc (build_over_call): Fold more indirect refs for trivial
> > > 	assignment op.
> > > 	* class.cc (type_has_non_deleted_trivial_default_ctor): Create.
> > > 	* constexpr.cc (cxx_eval_call_expression): Start lifetime of
> > > 	union member before entering constructor.
> > > 	(cxx_eval_store_expression): Activate member for
> > > 	value-initialised union. Check for accessing inactive union
> > > 	member indirectly.
> > > 	* cp-tree.h (type_has_non_deleted_trivial_default_ctor):
> > > 	Forward declare.
> > > 
> > > gcc/testsuite/ChangeLog:
> > > 
> > > 	* g++.dg/cpp1y/constexpr-89336-3.C: Fix union initialisation.
> > > 	* g++.dg/cpp1y/constexpr-union6.C: New test.
> > > 	* g++.dg/cpp2a/constexpr-union2.C: New test.
> > > 	* g++.dg/cpp2a/constexpr-union3.C: New test.
> > > 	* g++.dg/cpp2a/constexpr-union4.C: New test.
> > > 	* g++.dg/cpp2a/constexpr-union5.C: New test.
> > > 
> > > Signed-off-by: Nathaniel Shead <nathanieloshead@gmail.com>
> > > ---
> > >   gcc/cp/call.cc                                |  11 +-
> > >   gcc/cp/class.cc                               |   8 ++
> > >   gcc/cp/constexpr.cc                           | 129 +++++++++++++-----
> > >   gcc/cp/cp-tree.h                              |   1 +
> > >   .../g++.dg/cpp1y/constexpr-89336-3.C          |   2 +-
> > >   gcc/testsuite/g++.dg/cpp1y/constexpr-union6.C |  13 ++
> > >   gcc/testsuite/g++.dg/cpp2a/constexpr-union2.C |  30 ++++
> > >   gcc/testsuite/g++.dg/cpp2a/constexpr-union3.C |  45 ++++++
> > >   gcc/testsuite/g++.dg/cpp2a/constexpr-union4.C |  29 ++++
> > >   gcc/testsuite/g++.dg/cpp2a/constexpr-union5.C |  71 ++++++++++
> > >   10 files changed, 296 insertions(+), 43 deletions(-)
> > >   create mode 100644 gcc/testsuite/g++.dg/cpp1y/constexpr-union6.C
> > >   create mode 100644 gcc/testsuite/g++.dg/cpp2a/constexpr-union2.C
> > >   create mode 100644 gcc/testsuite/g++.dg/cpp2a/constexpr-union3.C
> > >   create mode 100644 gcc/testsuite/g++.dg/cpp2a/constexpr-union4.C
> > >   create mode 100644 gcc/testsuite/g++.dg/cpp2a/constexpr-union5.C
> > > 
> > > diff --git a/gcc/cp/call.cc b/gcc/cp/call.cc
> > > index e8dafbd8ba6..c1fb8807d3f 100644
> > > --- a/gcc/cp/call.cc
> > > +++ b/gcc/cp/call.cc
> > > @@ -10330,10 +10330,7 @@ build_over_call (struct z_candidate *cand, int flags, tsubst_flags_t complain)
> > >   	   && DECL_OVERLOADED_OPERATOR_IS (fn, NOP_EXPR)
> > >   	   && trivial_fn_p (fn))
> > >       {
> > > -      /* Don't use cp_build_fold_indirect_ref, op= returns an lvalue even if
> > > -	 the object argument isn't one.  */
> > > -      tree to = cp_build_indirect_ref (input_location, argarray[0],
> > > -				       RO_ARROW, complain);
> > > +      tree to = cp_build_fold_indirect_ref (argarray[0]);
> > >         tree type = TREE_TYPE (to);
> > >         tree as_base = CLASSTYPE_AS_BASE (type);
> > >         tree arg = argarray[1];
> > > @@ -10341,7 +10338,11 @@ build_over_call (struct z_candidate *cand, int flags, tsubst_flags_t complain)
> > >         if (is_really_empty_class (type, /*ignore_vptr*/true))
> > >   	{
> > > -	  /* Avoid copying empty classes.  */
> > > +	  /* Avoid copying empty classes, but ensure op= returns an lvalue even
> > > +	     if the object argument isn't one. This isn't needed in other cases
> > > +	     since MODIFY_EXPR is always considered an lvalue.  */
> > > +	  to = cp_build_addr_expr (to, tf_none);
> > > +	  to = cp_build_indirect_ref (input_location, to, RO_ARROW, complain);
> > >   	  val = build2 (COMPOUND_EXPR, type, arg, to);
> > >   	  suppress_warning (val, OPT_Wunused);
> > >   	}
> > > diff --git a/gcc/cp/class.cc b/gcc/cp/class.cc
> > > index b71333af1f8..e31aeb8e68b 100644
> > > --- a/gcc/cp/class.cc
> > > +++ b/gcc/cp/class.cc
> > > @@ -5688,6 +5688,14 @@ type_has_virtual_destructor (tree type)
> > >     return (dtor && DECL_VIRTUAL_P (dtor));
> > >   }
> > > +/* True iff class TYPE has a non-deleted trivial default
> > > +   constructor.  */
> > > +
> > > +bool type_has_non_deleted_trivial_default_ctor (tree type)
> > > +{
> > > +  return TYPE_HAS_TRIVIAL_DFLT (type) && locate_ctor (type);
> > > +}
> > > +
> > >   /* Returns true iff T, a class, has a move-assignment or
> > >      move-constructor.  Does not lazily declare either.
> > >      If USER_P is false, any move function will do.  If it is true, the
> > > diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc
> > > index a673a6022f1..5e50fc2c792 100644
> > > --- a/gcc/cp/constexpr.cc
> > > +++ b/gcc/cp/constexpr.cc
> > > @@ -3146,40 +3146,34 @@ cxx_eval_call_expression (const constexpr_ctx *ctx, tree t,
> > >   	    cxx_set_object_constness (ctx, new_obj, /*readonly_p=*/false,
> > >   				      non_constant_p, overflow_p);
> > > +	  /* If this is a constructor, we are beginning the lifetime of the
> > > +	     object we are initializing.  */
> > > +	  if (new_obj
> > > +	      && DECL_CONSTRUCTOR_P (fun)
> > > +	      && TREE_CODE (new_obj) == COMPONENT_REF
> > > +	      && TREE_CODE (TREE_TYPE (TREE_OPERAND (new_obj, 0))) == UNION_TYPE)
> > > +	    {
> > > +	      tree activate = build2 (INIT_EXPR, TREE_TYPE (new_obj),
> > > +				      new_obj,
> > > +				      build_constructor (TREE_TYPE (new_obj),
> > > +							 NULL));
> > > +	      cxx_eval_constant_expression (ctx, activate,
> > > +					    lval, non_constant_p, overflow_p);
> > > +	      ggc_free (activate);
> > > +	    }
> > > +
> > >   	  tree jump_target = NULL_TREE;
> > >   	  cxx_eval_constant_expression (&call_ctx, body,
> > >   					vc_discard, non_constant_p, overflow_p,
> > >   					&jump_target);
> > >   	  if (DECL_CONSTRUCTOR_P (fun))
> > > -	    {
> > > -	      /* This can be null for a subobject constructor call, in
> > > -		 which case what we care about is the initialization
> > > -		 side-effects rather than the value.  We could get at the
> > > -		 value by evaluating *this, but we don't bother; there's
> > > -		 no need to put such a call in the hash table.  */
> > > -	      result = lval ? ctx->object : ctx->ctor;
> > > -
> > > -	      /* If we've just evaluated a subobject constructor call for an
> > > -		 empty union member, it might not have produced a side effect
> > > -		 that actually activated the union member.  So produce such a
> > > -		 side effect now to ensure the union appears initialized.  */
> > > -	      if (!result && new_obj
> > > -		  && TREE_CODE (new_obj) == COMPONENT_REF
> > > -		  && TREE_CODE (TREE_TYPE
> > > -				(TREE_OPERAND (new_obj, 0))) == UNION_TYPE
> > > -		  && is_really_empty_class (TREE_TYPE (new_obj),
> > > -					    /*ignore_vptr*/false))
> > > -		{
> > > -		  tree activate = build2 (MODIFY_EXPR, TREE_TYPE (new_obj),
> > > -					  new_obj,
> > > -					  build_constructor (TREE_TYPE (new_obj),
> > > -							     NULL));
> > > -		  cxx_eval_constant_expression (ctx, activate, lval,
> > > -						non_constant_p, overflow_p);
> > > -		  ggc_free (activate);
> > > -		}
> > > -	    }
> > > +	    /* This can be null for a subobject constructor call, in
> > > +	       which case what we care about is the initialization
> > > +	       side-effects rather than the value.  We could get at the
> > > +	       value by evaluating *this, but we don't bother; there's
> > > +	       no need to put such a call in the hash table.  */
> > > +	    result = lval ? ctx->object : ctx->ctor;
> > >   	  else if (VOID_TYPE_P (TREE_TYPE (res)))
> > >   	    result = void_node;
> > >   	  else
> > > @@ -6127,6 +6121,14 @@ cxx_eval_store_expression (const constexpr_ctx *ctx, tree t,
> > >   						  mutable_p)
> > >   		     && const_object_being_modified == NULL_TREE)
> > >   	      const_object_being_modified = probe;
> > > +
> > > +	    /* Track named member accesses for unions to validate modifications
> > > +	       that change active member.  */
> > > +	    if (!evaluated && TREE_CODE (probe) == COMPONENT_REF)
> > > +	      vec_safe_push (refs, probe);
> > > +	    else
> > > +	      vec_safe_push (refs, NULL_TREE);
> > > +
> > >   	    vec_safe_push (refs, elt);
> > >   	    vec_safe_push (refs, TREE_TYPE (probe));
> > >   	    probe = ob;
> > > @@ -6135,6 +6137,7 @@ cxx_eval_store_expression (const constexpr_ctx *ctx, tree t,
> > >   	case REALPART_EXPR:
> > >   	  gcc_assert (probe == target);
> > > +	  vec_safe_push (refs, NULL_TREE);
> > >   	  vec_safe_push (refs, probe);
> > >   	  vec_safe_push (refs, TREE_TYPE (probe));
> > >   	  probe = TREE_OPERAND (probe, 0);
> > > @@ -6142,6 +6145,7 @@ cxx_eval_store_expression (const constexpr_ctx *ctx, tree t,
> > >   	case IMAGPART_EXPR:
> > >   	  gcc_assert (probe == target);
> > > +	  vec_safe_push (refs, NULL_TREE);
> > >   	  vec_safe_push (refs, probe);
> > >   	  vec_safe_push (refs, TREE_TYPE (probe));
> > >   	  probe = TREE_OPERAND (probe, 0);
> > > @@ -6230,6 +6234,7 @@ cxx_eval_store_expression (const constexpr_ctx *ctx, tree t,
> > >         enum tree_code code = TREE_CODE (type);
> > >         tree reftype = refs->pop();
> > >         tree index = refs->pop();
> > > +      bool is_access_expr = refs->pop() != NULL_TREE;
> > >         if (code == COMPLEX_TYPE)
> > >   	{
> > > @@ -6270,21 +6275,72 @@ cxx_eval_store_expression (const constexpr_ctx *ctx, tree t,
> > >         type = reftype;
> > > -      if (code == UNION_TYPE && CONSTRUCTOR_NELTS (*valp)
> > > -	  && CONSTRUCTOR_ELT (*valp, 0)->index != index)
> > > +      if (code == UNION_TYPE
> > > +	  && TREE_CODE (t) == MODIFY_EXPR
> > > +	  && (CONSTRUCTOR_NELTS (*valp) == 0
> > > +	      || CONSTRUCTOR_ELT (*valp, 0)->index != index))
> > >   	{
> > > -	  if (cxx_dialect < cxx20)
> > > +	  /* Probably a change of active member for a union. Ensure that
> > > +	     this is valid.  */
> > > +	  no_zero_init = true;
> > > +
> > > +	  tree init = *valp;
> > > +	  if (!CONSTRUCTOR_NO_CLEARING (*valp) && CONSTRUCTOR_NELTS (*valp) == 0)
> > > +	    /* This is a value-initialized union, process the initialization
> > > +	       to ensure that the active member is properly set.  */
> > > +	    init = build_value_init (TREE_TYPE (*valp), tf_warning_or_error);
> > > +
> > > +	  bool has_active_member = CONSTRUCTOR_NELTS (init) != 0;
> > > +	  tree inner = strip_array_types (reftype);
> > > +
> > > +	  if (has_active_member && CONSTRUCTOR_ELT (init, 0)->index == index)
> > > +	    /* After handling value-initialization, this wasn't actually a
> > > +	       change of active member like we initially thought.  */
> > > +	    no_zero_init = false;
> > > +	  else if (has_active_member && cxx_dialect < cxx20)
> > >   	    {
> > >   	      if (!ctx->quiet)
> > >   		error_at (cp_expr_loc_or_input_loc (t),
> > >   			  "change of the active member of a union "
> > > -			  "from %qD to %qD",
> > > -			  CONSTRUCTOR_ELT (*valp, 0)->index,
> > > +			  "from %qD to %qD is not a constant expression "
> > > +			  "before C++20",
> > > +			  CONSTRUCTOR_ELT (init, 0)->index,
> > >   			  index);
> > >   	      *non_constant_p = true;
> > >   	    }
> > > -	  else if (TREE_CODE (t) == MODIFY_EXPR
> > > -		   && CONSTRUCTOR_NO_CLEARING (*valp))
> > > +	  else if (!is_access_expr
> > > +		   || (CLASS_TYPE_P (inner)
> > > +		       && !type_has_non_deleted_trivial_default_ctor (inner)))
> > > +	    {
> > > +	      /* Diagnose changing active union member after initialization
> > > +		 without a valid member access expression, as described in
> > > +		 [class.union.general] p5.  */
> > > +	      if (!ctx->quiet)
> > > +		{
> > > +		  if (has_active_member)
> > > +		    error_at (cp_expr_loc_or_input_loc (t),
> > > +			      "accessing %qD member instead of initialized "
> > > +			      "%qD member in constant expression",
> > > +			      index, CONSTRUCTOR_ELT (init, 0)->index);
> > > +		  else
> > > +		    error_at (cp_expr_loc_or_input_loc (t),
> > > +			      "accessing uninitialized member %qD",
> > > +			      index);
> > > +		  if (is_access_expr)
> > > +		    inform (DECL_SOURCE_LOCATION (index),
> > > +			    "%qD does not implicitly begin its lifetime "
> > > +			    "because %qT does not have a non-deleted "
> > > +			    "trivial default constructor",
> > > +			    index, inner);
> > 
> > It now occurs to me that this should suggest using placement new instead.
> > And that we should test using placement new to begin the lifetime of such
> > members.
> > 
> > Jason
> > 
> 
> Good point. On testing placement new I found I wasn't checking for
> indirect change of active member during initialisation either, but this
> was pretty trivial to add. How does this look?
> 
> Bootstrap + regtest ongoing, but dg.exp completed with no errors. I'm
> away from my computer next week so I thought I'd send this through now
> anyway.
> 
> -- >8 --
> 
> This patch adds checks for attempting to change the active member of a
> union by methods other than a member access expression.
> 
> To be able to properly distinguish `*(&u.a) = ` from `u.a = `, this
> patch redoes the solution for c++/59950 to avoid extranneous *&; it
> seems that the only case that needed the workaround was when copying
> empty classes.
> 
> This patch also ensures that constructors for a union field mark that
> field as the active member before entering the call itself; this ensures
> that modifications of the field within the constructor's body don't
> cause false positives (as these will not appear to be member access
> expressions). This means that we no longer need to start the lifetime of
> empty union members after the constructor body completes.
> 
> As a drive-by fix, this patch also ensures that value-initialised unions
> are considered to have activated their initial member for the purpose of
> checking stores, which catches some additional mistakes pre-C++20.
> 
> 	PR c++/101631
> 	PR c++/102286
> 
> gcc/cp/ChangeLog:
> 
> 	* call.cc (build_over_call): Fold more indirect refs for trivial
> 	assignment op.
> 	* class.cc (type_has_non_deleted_trivial_default_ctor): Create.
> 	* constexpr.cc (cxx_eval_call_expression): Start lifetime of
> 	union member before entering constructor.
> 	(cxx_eval_store_expression): Activate member for
> 	value-initialised union. Check for accessing inactive union
> 	member indirectly.
> 	* cp-tree.h (type_has_non_deleted_trivial_default_ctor):
> 	Forward declare.
> 
> gcc/testsuite/ChangeLog:
> 
> 	* g++.dg/cpp1y/constexpr-89336-3.C: Fix union initialisation.
> 	* g++.dg/cpp1y/constexpr-union6.C: New test.
> 	* g++.dg/cpp2a/constexpr-union2.C: New test.
> 	* g++.dg/cpp2a/constexpr-union3.C: New test.
> 	* g++.dg/cpp2a/constexpr-union4.C: New test.
> 	* g++.dg/cpp2a/constexpr-union5.C: New test.
> 	* g++.dg/cpp2a/constexpr-union6.C: New test.
> 
> Signed-off-by: Nathaniel Shead <nathanieloshead@gmail.com>
> ---
>  gcc/cp/call.cc                                |  11 +-
>  gcc/cp/class.cc                               |   8 ++
>  gcc/cp/constexpr.cc                           | 135 +++++++++++++-----
>  gcc/cp/cp-tree.h                              |   1 +
>  .../g++.dg/cpp1y/constexpr-89336-3.C          |   2 +-
>  gcc/testsuite/g++.dg/cpp1y/constexpr-union6.C |  13 ++
>  gcc/testsuite/g++.dg/cpp2a/constexpr-union2.C |  30 ++++
>  gcc/testsuite/g++.dg/cpp2a/constexpr-union3.C |  45 ++++++
>  gcc/testsuite/g++.dg/cpp2a/constexpr-union4.C |  29 ++++
>  gcc/testsuite/g++.dg/cpp2a/constexpr-union5.C |  71 +++++++++
>  gcc/testsuite/g++.dg/cpp2a/constexpr-union6.C |  43 ++++++
>  11 files changed, 345 insertions(+), 43 deletions(-)
>  create mode 100644 gcc/testsuite/g++.dg/cpp1y/constexpr-union6.C
>  create mode 100644 gcc/testsuite/g++.dg/cpp2a/constexpr-union2.C
>  create mode 100644 gcc/testsuite/g++.dg/cpp2a/constexpr-union3.C
>  create mode 100644 gcc/testsuite/g++.dg/cpp2a/constexpr-union4.C
>  create mode 100644 gcc/testsuite/g++.dg/cpp2a/constexpr-union5.C
>  create mode 100644 gcc/testsuite/g++.dg/cpp2a/constexpr-union6.C
> 
> diff --git a/gcc/cp/call.cc b/gcc/cp/call.cc
> index e8dafbd8ba6..c1fb8807d3f 100644
> --- a/gcc/cp/call.cc
> +++ b/gcc/cp/call.cc
> @@ -10330,10 +10330,7 @@ build_over_call (struct z_candidate *cand, int flags, tsubst_flags_t complain)
>  	   && DECL_OVERLOADED_OPERATOR_IS (fn, NOP_EXPR)
>  	   && trivial_fn_p (fn))
>      {
> -      /* Don't use cp_build_fold_indirect_ref, op= returns an lvalue even if
> -	 the object argument isn't one.  */
> -      tree to = cp_build_indirect_ref (input_location, argarray[0],
> -				       RO_ARROW, complain);
> +      tree to = cp_build_fold_indirect_ref (argarray[0]);
>        tree type = TREE_TYPE (to);
>        tree as_base = CLASSTYPE_AS_BASE (type);
>        tree arg = argarray[1];
> @@ -10341,7 +10338,11 @@ build_over_call (struct z_candidate *cand, int flags, tsubst_flags_t complain)
>  
>        if (is_really_empty_class (type, /*ignore_vptr*/true))
>  	{
> -	  /* Avoid copying empty classes.  */
> +	  /* Avoid copying empty classes, but ensure op= returns an lvalue even
> +	     if the object argument isn't one. This isn't needed in other cases
> +	     since MODIFY_EXPR is always considered an lvalue.  */
> +	  to = cp_build_addr_expr (to, tf_none);
> +	  to = cp_build_indirect_ref (input_location, to, RO_ARROW, complain);
>  	  val = build2 (COMPOUND_EXPR, type, arg, to);
>  	  suppress_warning (val, OPT_Wunused);
>  	}
> diff --git a/gcc/cp/class.cc b/gcc/cp/class.cc
> index b71333af1f8..e31aeb8e68b 100644
> --- a/gcc/cp/class.cc
> +++ b/gcc/cp/class.cc
> @@ -5688,6 +5688,14 @@ type_has_virtual_destructor (tree type)
>    return (dtor && DECL_VIRTUAL_P (dtor));
>  }
>  
> +/* True iff class TYPE has a non-deleted trivial default
> +   constructor.  */
> +
> +bool type_has_non_deleted_trivial_default_ctor (tree type)
> +{
> +  return TYPE_HAS_TRIVIAL_DFLT (type) && locate_ctor (type);
> +}
> +
>  /* Returns true iff T, a class, has a move-assignment or
>     move-constructor.  Does not lazily declare either.
>     If USER_P is false, any move function will do.  If it is true, the
> diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc
> index a673a6022f1..8e6f60d03d2 100644
> --- a/gcc/cp/constexpr.cc
> +++ b/gcc/cp/constexpr.cc
> @@ -3146,40 +3146,34 @@ cxx_eval_call_expression (const constexpr_ctx *ctx, tree t,
>  	    cxx_set_object_constness (ctx, new_obj, /*readonly_p=*/false,
>  				      non_constant_p, overflow_p);
>  
> +	  /* If this is a constructor, we are beginning the lifetime of the
> +	     object we are initializing.  */
> +	  if (new_obj
> +	      && DECL_CONSTRUCTOR_P (fun)
> +	      && TREE_CODE (new_obj) == COMPONENT_REF
> +	      && TREE_CODE (TREE_TYPE (TREE_OPERAND (new_obj, 0))) == UNION_TYPE)
> +	    {
> +	      tree activate = build2 (INIT_EXPR, TREE_TYPE (new_obj),
> +				      new_obj,
> +				      build_constructor (TREE_TYPE (new_obj),
> +							 NULL));
> +	      cxx_eval_constant_expression (ctx, activate,
> +					    lval, non_constant_p, overflow_p);
> +	      ggc_free (activate);
> +	    }
> +
>  	  tree jump_target = NULL_TREE;
>  	  cxx_eval_constant_expression (&call_ctx, body,
>  					vc_discard, non_constant_p, overflow_p,
>  					&jump_target);
>  
>  	  if (DECL_CONSTRUCTOR_P (fun))
> -	    {
> -	      /* This can be null for a subobject constructor call, in
> -		 which case what we care about is the initialization
> -		 side-effects rather than the value.  We could get at the
> -		 value by evaluating *this, but we don't bother; there's
> -		 no need to put such a call in the hash table.  */
> -	      result = lval ? ctx->object : ctx->ctor;
> -
> -	      /* If we've just evaluated a subobject constructor call for an
> -		 empty union member, it might not have produced a side effect
> -		 that actually activated the union member.  So produce such a
> -		 side effect now to ensure the union appears initialized.  */
> -	      if (!result && new_obj
> -		  && TREE_CODE (new_obj) == COMPONENT_REF
> -		  && TREE_CODE (TREE_TYPE
> -				(TREE_OPERAND (new_obj, 0))) == UNION_TYPE
> -		  && is_really_empty_class (TREE_TYPE (new_obj),
> -					    /*ignore_vptr*/false))
> -		{
> -		  tree activate = build2 (MODIFY_EXPR, TREE_TYPE (new_obj),
> -					  new_obj,
> -					  build_constructor (TREE_TYPE (new_obj),
> -							     NULL));
> -		  cxx_eval_constant_expression (ctx, activate, lval,
> -						non_constant_p, overflow_p);
> -		  ggc_free (activate);
> -		}
> -	    }
> +	    /* This can be null for a subobject constructor call, in
> +	       which case what we care about is the initialization
> +	       side-effects rather than the value.  We could get at the
> +	       value by evaluating *this, but we don't bother; there's
> +	       no need to put such a call in the hash table.  */
> +	    result = lval ? ctx->object : ctx->ctor;
>  	  else if (VOID_TYPE_P (TREE_TYPE (res)))
>  	    result = void_node;
>  	  else
> @@ -6127,6 +6121,14 @@ cxx_eval_store_expression (const constexpr_ctx *ctx, tree t,
>  						  mutable_p)
>  		     && const_object_being_modified == NULL_TREE)
>  	      const_object_being_modified = probe;
> +
> +	    /* Track named member accesses for unions to validate modifications
> +	       that change active member.  */
> +	    if (!evaluated && TREE_CODE (probe) == COMPONENT_REF)
> +	      vec_safe_push (refs, probe);
> +	    else
> +	      vec_safe_push (refs, NULL_TREE);
> +
>  	    vec_safe_push (refs, elt);
>  	    vec_safe_push (refs, TREE_TYPE (probe));
>  	    probe = ob;
> @@ -6135,6 +6137,7 @@ cxx_eval_store_expression (const constexpr_ctx *ctx, tree t,
>  
>  	case REALPART_EXPR:
>  	  gcc_assert (probe == target);
> +	  vec_safe_push (refs, NULL_TREE);
>  	  vec_safe_push (refs, probe);
>  	  vec_safe_push (refs, TREE_TYPE (probe));
>  	  probe = TREE_OPERAND (probe, 0);
> @@ -6142,6 +6145,7 @@ cxx_eval_store_expression (const constexpr_ctx *ctx, tree t,
>  
>  	case IMAGPART_EXPR:
>  	  gcc_assert (probe == target);
> +	  vec_safe_push (refs, NULL_TREE);
>  	  vec_safe_push (refs, probe);
>  	  vec_safe_push (refs, TREE_TYPE (probe));
>  	  probe = TREE_OPERAND (probe, 0);
> @@ -6230,6 +6234,7 @@ cxx_eval_store_expression (const constexpr_ctx *ctx, tree t,
>        enum tree_code code = TREE_CODE (type);
>        tree reftype = refs->pop();
>        tree index = refs->pop();
> +      bool is_access_expr = refs->pop() != NULL_TREE;
>  
>        if (code == COMPLEX_TYPE)
>  	{
> @@ -6270,21 +6275,78 @@ cxx_eval_store_expression (const constexpr_ctx *ctx, tree t,
>  
>        type = reftype;
>  
> -      if (code == UNION_TYPE && CONSTRUCTOR_NELTS (*valp)
> -	  && CONSTRUCTOR_ELT (*valp, 0)->index != index)
> +      /* Check for implicit change of active member for a union.  */
> +      if (code == UNION_TYPE
> +	  && (TREE_CODE (t) == MODIFY_EXPR
> +	      /* Also check if initializations have implicit change of active
> +		 member earlier up the access chain.  */
> +	      || !refs->is_empty())
> +	  && (CONSTRUCTOR_NELTS (*valp) == 0
> +	      || CONSTRUCTOR_ELT (*valp, 0)->index != index))
>  	{
> -	  if (cxx_dialect < cxx20)
> +	  no_zero_init = true;
> +
> +	  tree init = *valp;
> +	  if (TREE_CODE (t) == MODIFY_EXPR
> +	      && !CONSTRUCTOR_NO_CLEARING (*valp)
> +	      && CONSTRUCTOR_NELTS (*valp) == 0)
> +	    /* Modifying a value-initialized union, process the initialization
> +	       to ensure that the active member is properly set.  */
> +	    init = build_value_init (TREE_TYPE (*valp), tf_warning_or_error);
> +
> +	  bool has_active_member = CONSTRUCTOR_NELTS (init) != 0;
> +	  tree inner = strip_array_types (reftype);
> +
> +	  if (has_active_member && CONSTRUCTOR_ELT (init, 0)->index == index)
> +	    /* After handling value-initialization, this wasn't actually a
> +	       change of active member like we initially thought.  */
> +	    no_zero_init = false;
> +	  else if (has_active_member && cxx_dialect < cxx20)
>  	    {
>  	      if (!ctx->quiet)
>  		error_at (cp_expr_loc_or_input_loc (t),
>  			  "change of the active member of a union "
> -			  "from %qD to %qD",
> -			  CONSTRUCTOR_ELT (*valp, 0)->index,
> +			  "from %qD to %qD is not a constant expression "
> +			  "before C++20",
> +			  CONSTRUCTOR_ELT (init, 0)->index,
>  			  index);
>  	      *non_constant_p = true;
>  	    }
> -	  else if (TREE_CODE (t) == MODIFY_EXPR
> -		   && CONSTRUCTOR_NO_CLEARING (*valp))
> +	  else if (!is_access_expr
> +		   || (TREE_CODE (t) == MODIFY_EXPR
> +		       && CLASS_TYPE_P (inner)
> +		       && !type_has_non_deleted_trivial_default_ctor (inner)))
> +	    {
> +	      /* Diagnose changing active union member after initialization
> +		 without a valid member access expression, as described in
> +		 [class.union.general] p5.  */
> +	      if (!ctx->quiet)
> +		{
> +		  if (has_active_member)
> +		    error_at (cp_expr_loc_or_input_loc (t),
> +			      "accessing %qD member instead of initialized "
> +			      "%qD member in constant expression",
> +			      index, CONSTRUCTOR_ELT (init, 0)->index);
> +		  else
> +		    error_at (cp_expr_loc_or_input_loc (t),
> +			      "accessing uninitialized member %qD",
> +			      index);
> +		  if (is_access_expr)
> +		    inform (DECL_SOURCE_LOCATION (index),
> +			    "%qD does not implicitly begin its lifetime "
> +			    "because %qT does not have a non-deleted "
> +			    "trivial default constructor, use "
> +			    "%<std::construct_at%> instead",
> +			    index, inner);
> +		  else
> +		    inform (DECL_SOURCE_LOCATION (index),
> +			    "initializing %qD requires a member access "
> +			    "expression as the left operand of the assignment",
> +			    index);
> +		}
> +	      *non_constant_p = true;
> +	    }
> +	  else if (has_active_member && CONSTRUCTOR_NO_CLEARING (init))
>  	    {
>  	      /* Diagnose changing the active union member while the union
>  		 is in the process of being initialized.  */
> @@ -6292,11 +6354,10 @@ cxx_eval_store_expression (const constexpr_ctx *ctx, tree t,
>  		error_at (cp_expr_loc_or_input_loc (t),
>  			  "change of the active member of a union "
>  			  "from %qD to %qD during initialization",
> -			  CONSTRUCTOR_ELT (*valp, 0)->index,
> +			  CONSTRUCTOR_ELT (init, 0)->index,
>  			  index);
>  	      *non_constant_p = true;
>  	    }
> -	  no_zero_init = true;
>  	}
>  
>        ctors.safe_push (valp);
> diff --git a/gcc/cp/cp-tree.h b/gcc/cp/cp-tree.h
> index fd6bf9fd4a0..40aa9f0be2b 100644
> --- a/gcc/cp/cp-tree.h
> +++ b/gcc/cp/cp-tree.h
> @@ -6815,6 +6815,7 @@ extern bool trivial_default_constructor_is_constexpr (tree);
>  extern bool type_has_constexpr_default_constructor (tree);
>  extern bool type_has_constexpr_destructor	(tree);
>  extern bool type_has_virtual_destructor		(tree);
> +extern bool type_has_non_deleted_trivial_default_ctor (tree);
>  extern bool classtype_has_move_assign_or_move_ctor_p (tree, bool user_declared);
>  extern bool classtype_has_non_deleted_move_ctor (tree);
>  extern tree classtype_has_depr_implicit_copy	(tree);
> diff --git a/gcc/testsuite/g++.dg/cpp1y/constexpr-89336-3.C b/gcc/testsuite/g++.dg/cpp1y/constexpr-89336-3.C
> index 9d370ddefcf..6a60966f6f8 100644
> --- a/gcc/testsuite/g++.dg/cpp1y/constexpr-89336-3.C
> +++ b/gcc/testsuite/g++.dg/cpp1y/constexpr-89336-3.C
> @@ -18,7 +18,7 @@ constexpr int
>  bar ()
>  {
>    union U { int a[5]; long b; };
> -  union V { union U u; short v; };
> +  union V { short v; union U u; };
>    V w {};
>    w.v = 5;
>    w.u.a[3] = w.u.a[1] = w.v;		// { dg-error "change of the active member of a union from" "" { target c++17_down } }
> diff --git a/gcc/testsuite/g++.dg/cpp1y/constexpr-union6.C b/gcc/testsuite/g++.dg/cpp1y/constexpr-union6.C
> new file mode 100644
> index 00000000000..ff7ebf19f89
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp1y/constexpr-union6.C
> @@ -0,0 +1,13 @@
> +// { dg-do compile { target c++14 } }
> +
> +union U {
> +  int a;
> +  float b;
> +};
> +
> +constexpr bool foo() {
> +  U u {};
> +  u.b = 1.0f;  // { dg-error "change of the active member" "" { target c++17_down } }
> +  return u.b == 1.0f;
> +}
> +constexpr bool x = foo();
> diff --git a/gcc/testsuite/g++.dg/cpp2a/constexpr-union2.C b/gcc/testsuite/g++.dg/cpp2a/constexpr-union2.C
> new file mode 100644
> index 00000000000..1712395d7e7
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp2a/constexpr-union2.C
> @@ -0,0 +1,30 @@
> +// PR c++/101631
> +// { dg-do compile { target c++20 } }
> +
> +struct sso {
> +  union {
> +    int buf[10];
> +    int* alloc;
> +  };
> +};
> +
> +constexpr bool direct() {
> +  sso val;
> +  val.alloc = nullptr;
> +  val.buf[5] = 42;
> +  return true;
> +}
> +constexpr bool ok = direct();
> +
> +
> +constexpr void perform_assignment(int& left, int right) noexcept {
> +  left = right;  // { dg-error "accessing .+ member instead of initialized" }
> +}
> +
> +constexpr bool indirect() {
> +  sso val;
> +  val.alloc = nullptr;
> +  perform_assignment(val.buf[5], 42);  // { dg-message "in .constexpr. expansion" }
> +  return true;
> +}
> +constexpr bool err = indirect();  // { dg-message "in .constexpr. expansion" }
> diff --git a/gcc/testsuite/g++.dg/cpp2a/constexpr-union3.C b/gcc/testsuite/g++.dg/cpp2a/constexpr-union3.C
> new file mode 100644
> index 00000000000..6d30bb2498f
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp2a/constexpr-union3.C
> @@ -0,0 +1,45 @@
> +// { dg-do compile { target c++20 } }
> +
> +struct S
> +{
> +  union {
> +    char buf[8];
> +    char* ptr;
> +  };
> +  unsigned len;
> +
> +  constexpr S(const char* s, unsigned n)
> +  {
> +    char* p;
> +    if (n > 7)
> +      p = ptr = new char[n+1];
> +    else
> +      p = buf;
> +    for (len = 0; len < n; ++len)
> +      p[len] = s[len];  // { dg-error "accessing uninitialized member" }
> +    p[len] = '\0';
> +  }
> +
> +  constexpr ~S()
> +  {
> +    if (len > 7)
> +      delete[] ptr;
> +  }
> +};
> +
> +constexpr bool test1()
> +{
> +  S s("test", 4);  // { dg-message "in .constexpr. expansion" }
> +  return true;
> +}
> +
> +constexpr bool a = test1();  // { dg-message "in .constexpr. expansion" }
> +
> +
> +constexpr bool test2()
> +{
> +  S s("hello world", 11);
> +  return true;
> +}
> +
> +constexpr bool b = test2();
> diff --git a/gcc/testsuite/g++.dg/cpp2a/constexpr-union4.C b/gcc/testsuite/g++.dg/cpp2a/constexpr-union4.C
> new file mode 100644
> index 00000000000..429ab203ac2
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp2a/constexpr-union4.C
> @@ -0,0 +1,29 @@
> +// { dg-do compile { target c++20 } }
> +
> +// from [class.union.general] p5
> +
> +union A { int x; int y[4]; };
> +struct B { A a; };
> +union C { B b; int k; };
> +constexpr int f() {
> +  C c;                  // does not start lifetime of any union member
> +  c.b.a.y[3] = 4;       // OK, S(c.b.a.y[3]) contains c.b and c.b.a.y;
> +                        // creates objects to hold union members c.b and c.b.a.y
> +  return c.b.a.y[3];    // OK, c.b.a.y refers to newly created object (see [basic.life])
> +}
> +constexpr int a = f();
> +
> +struct X { const int a; int b; };
> +union Y { X x; int k; };// { dg-message "does not implicitly begin its lifetime" }
> +constexpr int g() {
> +  Y y = { { 1, 2 } };   // OK, y.x is active union member ([class.mem])
> +  int n = y.x.a;
> +  y.k = 4;              // OK, ends lifetime of y.x, y.k is active member of union
> +
> +  y.x.b = n;            // { dg-error "accessing .* member instead of initialized .* member" }
> +                        // undefined behavior: y.x.b modified outside its lifetime,
> +                        // S(y.x.b) is empty because X's default constructor is deleted,
> +                        // so union member y.x's lifetime does not implicitly start
> +  return 0;
> +}
> +constexpr int b = g();  // { dg-message "in .constexpr. expansion" }
> diff --git a/gcc/testsuite/g++.dg/cpp2a/constexpr-union5.C b/gcc/testsuite/g++.dg/cpp2a/constexpr-union5.C
> new file mode 100644
> index 00000000000..c94d7cc0fc9
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp2a/constexpr-union5.C
> @@ -0,0 +1,71 @@
> +// { dg-do compile { target c++20 } }
> +
> +union U { int a; int b; int c[2]; };
> +
> +constexpr int test1() {
> +  U u;
> +  u.a = 10;
> +  *&u.b = 20;  // { dg-error "accessing" }
> +  return u.b;
> +}
> +constexpr int x1 = test1();  // { dg-message "in .constexpr. expansion" }
> +
> +constexpr int test2() {
> +  U u;
> +  u.a = 10;
> +  (0, u.b) = 20;  // { dg-error "accessing" }
> +  return u.b;
> +}
> +constexpr int x2 = test2();  // { dg-message "in .constexpr. expansion" }
> +
> +constexpr int test3() {
> +  U u;
> +  u.a = 0;
> +  int* p = &u.b;
> +  p[u.a] = 10;  // { dg-error "accessing" }
> +  return u.b;
> +}
> +constexpr int x3 = test3();  // { dg-message "in .constexpr. expansion" }
> +
> +constexpr int test4() {
> +  U u;
> +  u.a = 0;
> +  int* p = &u.b;
> +  u.a[p] = 10;  // { dg-error "accessing" }
> +  return u.b;
> +}
> +constexpr int x4 = test4();  // { dg-message "in .constexpr. expansion" }
> +
> +struct S { U u[10]; };
> +constexpr int test5() {
> +  S s;
> +  s.u[4].a = 10;
> +  6[s.u].b = 15;
> +  return 4[s.u].a + s.u[6].b;
> +}
> +static_assert(test5() == 25);
> +
> +constexpr int test6() {
> +  U u;
> +  u.a = 5;
> +  u.c[0] = 3;
> +  1[u.c] = 8;
> +  return 1[u.c] + u.c[0];
> +}
> +static_assert(test6() == 11);
> +
> +constexpr int test7() {
> +  U u {};  // value initialisation initialises first member
> +  int* p = &u.a;
> +  *p = 10;
> +  return *p;
> +}
> +constexpr int x7 = test7();
> +
> +constexpr int test8() {
> +  U u;  // default initialisation leaves no member initialised
> +  int* p = &u.a;
> +  *p = 10;  // { dg-error "accessing" }
> +  return *p;
> +}
> +constexpr int x8 = test8();  // { dg-message "in .constexpr. expansion" }
> diff --git a/gcc/testsuite/g++.dg/cpp2a/constexpr-union6.C b/gcc/testsuite/g++.dg/cpp2a/constexpr-union6.C
> new file mode 100644
> index 00000000000..7c37716892c
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp2a/constexpr-union6.C
> @@ -0,0 +1,43 @@
> +// { dg-do compile { target c++20 } }
> +// PR c++/102286
> +
> +#include "construct_at.h"
> +
> +struct S { const int a; int b; };
> +union U { int k; S s; };
> +
> +constexpr int test1() {
> +  U u {};
> +  std::construct_at(&u.s, S{ 1, 2 });
> +  return u.s.b;
> +}
> +static_assert(test1() == 2);
> +
> +constexpr int test2() {
> +  U u {};
> +  int* p = &u.s.b;
> +  std::construct_at(p, 5);  // { dg-message "in .constexpr. expansion" }
> +  return u.s.b;
> +}
> +constexpr int x2 = test2();  // { dg-message "in .constexpr. expansion" }
> +
> +struct S2 { int a; int b; };
> +union U2 { int k; S2 s; };
> +constexpr int test3() {
> +  U2 u;
> +  int* p = &u.s.b;
> +  std::construct_at(p, 8);  // { dg-message "in .constexpr. expansion" }
> +  return u.s.b;
> +};
> +constexpr int x3 = test3();  // { dg-message "in .constexpr. expansion" }
> +
> +constexpr int test4() {
> +  union {
> +    int data[1];
> +  } u;
> +  std::construct_at(u.data, 0);  // { dg-message "in .constexpr. expansion" }
> +  return 0;
> +}
> +constexpr int x4 = test4();  // { dg-message "in .constexpr. expansion" }
> +
> +// { dg-error "accessing uninitialized member" "" { target *-*-* } 0 }
> -- 
> 2.41.0
> 

Re: [PATCH v4] c++: Check for indirect change of active union member in constexpr [PR101631,PR102286]

[Jonathan Wakely]

[-- Attachment #1: Type: text/plain, Size: 44068 bytes --]

On Sat, 23 Sept 2023, 01:39 Nathaniel Shead via Libstdc++, <
libstdc++@gcc.gnu.org> wrote:

> Now that bootstrap has finished, I have gotten regressions in the
> following libstdc++ tests:
>
> Running libstdc++:libstdc++-dg/conformance.exp ...
> FAIL: 20_util/bitset/access/constexpr.cc -std=gnu++23 (test for excess
> errors)
> FAIL: 20_util/bitset/access/constexpr.cc -std=gnu++26 (test for excess
> errors)
> FAIL: 20_util/variant/constexpr.cc -std=gnu++20 (test for excess errors)
> FAIL: 20_util/variant/constexpr.cc -std=gnu++26 (test for excess errors)
> FAIL: 21_strings/basic_string/cons/char/constexpr.cc -std=gnu++20 (test
> for excess errors)
> FAIL: 21_strings/basic_string/cons/char/constexpr.cc -std=gnu++26 (test
> for excess errors)
> FAIL: 21_strings/basic_string/cons/wchar_t/constexpr.cc -std=gnu++20 (test
> for excess errors)
> FAIL: 21_strings/basic_string/cons/wchar_t/constexpr.cc -std=gnu++26 (test
> for excess errors)
> FAIL: 21_strings/basic_string/modifiers/swap/constexpr-wchar_t.cc
> -std=gnu++20 (test for excess errors)
> FAIL: 21_strings/basic_string/modifiers/swap/constexpr-wchar_t.cc
> -std=gnu++26 (test for excess errors)
> FAIL: 21_strings/basic_string/modifiers/swap/constexpr.cc -std=gnu++20
> (test for excess errors)
> FAIL: 21_strings/basic_string/modifiers/swap/constexpr.cc -std=gnu++26
> (test for excess errors)
> FAIL: std/ranges/adaptors/join_with/1.cc -std=gnu++23 (test for excess
> errors)
> UNRESOLVED: std/ranges/adaptors/join_with/1.cc -std=gnu++23 compilation
> failed to produce executable
> FAIL: std/ranges/adaptors/join_with/1.cc -std=gnu++26 (test for excess
> errors)
> UNRESOLVED: std/ranges/adaptors/join_with/1.cc -std=gnu++26 compilation
> failed to produce executable
>
> On investigation though it looks like the issue might be with libstdc++
> rather than the patch itself; running the failing tests using clang with
> libstdc++ also produces similar errors, and my reading of the code
> suggests that this is correct.
>
> What's the way forward here? Should I look at creating a patch to fix
> the libstdc++ issues before resubmitting this patch for the C++
> frontend? Or should I submit a version of this patch without the
> `std::construct_at` changes and wait till libstdc++ gets fixed for that?
>

I think we should fix libstdc++. There are probably only a few places that
need a fix, which cause all those failures.

I can help with those fixes. I'll look into it after the weekend.



> On Sat, Sep 23, 2023 at 01:01:20AM +1000, Nathaniel Shead wrote:
> > On Fri, Sep 22, 2023 at 02:21:15PM +0100, Jason Merrill wrote:
> > > On 9/21/23 09:41, Nathaniel Shead wrote:
> > > > I've updated the error messages, and also fixed another bug I found
> > > > while retesting (value-initialised unions weren't considered to have
> any
> > > > active member yet).
> > > >
> > > > Bootstrapped and regtested on x86_64-pc-linux-gnu.
> > > >
> > > > -- >8 --
> > > >
> > > > This patch adds checks for attempting to change the active member of
> a
> > > > union by methods other than a member access expression.
> > > >
> > > > To be able to properly distinguish `*(&u.a) = ` from `u.a = `, this
> > > > patch redoes the solution for c++/59950 to avoid extranneous *&; it
> > > > seems that the only case that needed the workaround was when copying
> > > > empty classes.
> > > >
> > > > This patch also ensures that constructors for a union field mark that
> > > > field as the active member before entering the call itself; this
> ensures
> > > > that modifications of the field within the constructor's body don't
> > > > cause false positives (as these will not appear to be member access
> > > > expressions). This means that we no longer need to start the
> lifetime of
> > > > empty union members after the constructor body completes.
> > > >
> > > > As a drive-by fix, this patch also ensures that value-initialised
> unions
> > > > are considered to have activated their initial member for the
> purpose of
> > > > checking stores, which catches some additional mistakes pre-C++20.
> > > >
> > > >   PR c++/101631
> > > >
> > > > gcc/cp/ChangeLog:
> > > >
> > > >   * call.cc (build_over_call): Fold more indirect refs for trivial
> > > >   assignment op.
> > > >   * class.cc (type_has_non_deleted_trivial_default_ctor): Create.
> > > >   * constexpr.cc (cxx_eval_call_expression): Start lifetime of
> > > >   union member before entering constructor.
> > > >   (cxx_eval_store_expression): Activate member for
> > > >   value-initialised union. Check for accessing inactive union
> > > >   member indirectly.
> > > >   * cp-tree.h (type_has_non_deleted_trivial_default_ctor):
> > > >   Forward declare.
> > > >
> > > > gcc/testsuite/ChangeLog:
> > > >
> > > >   * g++.dg/cpp1y/constexpr-89336-3.C: Fix union initialisation.
> > > >   * g++.dg/cpp1y/constexpr-union6.C: New test.
> > > >   * g++.dg/cpp2a/constexpr-union2.C: New test.
> > > >   * g++.dg/cpp2a/constexpr-union3.C: New test.
> > > >   * g++.dg/cpp2a/constexpr-union4.C: New test.
> > > >   * g++.dg/cpp2a/constexpr-union5.C: New test.
> > > >
> > > > Signed-off-by: Nathaniel Shead <nathanieloshead@gmail.com>
> > > > ---
> > > >   gcc/cp/call.cc                                |  11 +-
> > > >   gcc/cp/class.cc                               |   8 ++
> > > >   gcc/cp/constexpr.cc                           | 129
> +++++++++++++-----
> > > >   gcc/cp/cp-tree.h                              |   1 +
> > > >   .../g++.dg/cpp1y/constexpr-89336-3.C          |   2 +-
> > > >   gcc/testsuite/g++.dg/cpp1y/constexpr-union6.C |  13 ++
> > > >   gcc/testsuite/g++.dg/cpp2a/constexpr-union2.C |  30 ++++
> > > >   gcc/testsuite/g++.dg/cpp2a/constexpr-union3.C |  45 ++++++
> > > >   gcc/testsuite/g++.dg/cpp2a/constexpr-union4.C |  29 ++++
> > > >   gcc/testsuite/g++.dg/cpp2a/constexpr-union5.C |  71 ++++++++++
> > > >   10 files changed, 296 insertions(+), 43 deletions(-)
> > > >   create mode 100644 gcc/testsuite/g++.dg/cpp1y/constexpr-union6.C
> > > >   create mode 100644 gcc/testsuite/g++.dg/cpp2a/constexpr-union2.C
> > > >   create mode 100644 gcc/testsuite/g++.dg/cpp2a/constexpr-union3.C
> > > >   create mode 100644 gcc/testsuite/g++.dg/cpp2a/constexpr-union4.C
> > > >   create mode 100644 gcc/testsuite/g++.dg/cpp2a/constexpr-union5.C
> > > >
> > > > diff --git a/gcc/cp/call.cc b/gcc/cp/call.cc
> > > > index e8dafbd8ba6..c1fb8807d3f 100644
> > > > --- a/gcc/cp/call.cc
> > > > +++ b/gcc/cp/call.cc
> > > > @@ -10330,10 +10330,7 @@ build_over_call (struct z_candidate *cand,
> int flags, tsubst_flags_t complain)
> > > >              && DECL_OVERLOADED_OPERATOR_IS (fn, NOP_EXPR)
> > > >              && trivial_fn_p (fn))
> > > >       {
> > > > -      /* Don't use cp_build_fold_indirect_ref, op= returns an
> lvalue even if
> > > > -  the object argument isn't one.  */
> > > > -      tree to = cp_build_indirect_ref (input_location, argarray[0],
> > > > -                                RO_ARROW, complain);
> > > > +      tree to = cp_build_fold_indirect_ref (argarray[0]);
> > > >         tree type = TREE_TYPE (to);
> > > >         tree as_base = CLASSTYPE_AS_BASE (type);
> > > >         tree arg = argarray[1];
> > > > @@ -10341,7 +10338,11 @@ build_over_call (struct z_candidate *cand,
> int flags, tsubst_flags_t complain)
> > > >         if (is_really_empty_class (type, /*ignore_vptr*/true))
> > > >           {
> > > > -   /* Avoid copying empty classes.  */
> > > > +   /* Avoid copying empty classes, but ensure op= returns an lvalue
> even
> > > > +      if the object argument isn't one. This isn't needed in other
> cases
> > > > +      since MODIFY_EXPR is always considered an lvalue.  */
> > > > +   to = cp_build_addr_expr (to, tf_none);
> > > > +   to = cp_build_indirect_ref (input_location, to, RO_ARROW,
> complain);
> > > >             val = build2 (COMPOUND_EXPR, type, arg, to);
> > > >             suppress_warning (val, OPT_Wunused);
> > > >           }
> > > > diff --git a/gcc/cp/class.cc b/gcc/cp/class.cc
> > > > index b71333af1f8..e31aeb8e68b 100644
> > > > --- a/gcc/cp/class.cc
> > > > +++ b/gcc/cp/class.cc
> > > > @@ -5688,6 +5688,14 @@ type_has_virtual_destructor (tree type)
> > > >     return (dtor && DECL_VIRTUAL_P (dtor));
> > > >   }
> > > > +/* True iff class TYPE has a non-deleted trivial default
> > > > +   constructor.  */
> > > > +
> > > > +bool type_has_non_deleted_trivial_default_ctor (tree type)
> > > > +{
> > > > +  return TYPE_HAS_TRIVIAL_DFLT (type) && locate_ctor (type);
> > > > +}
> > > > +
> > > >   /* Returns true iff T, a class, has a move-assignment or
> > > >      move-constructor.  Does not lazily declare either.
> > > >      If USER_P is false, any move function will do.  If it is true,
> the
> > > > diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc
> > > > index a673a6022f1..5e50fc2c792 100644
> > > > --- a/gcc/cp/constexpr.cc
> > > > +++ b/gcc/cp/constexpr.cc
> > > > @@ -3146,40 +3146,34 @@ cxx_eval_call_expression (const
> constexpr_ctx *ctx, tree t,
> > > >               cxx_set_object_constness (ctx, new_obj,
> /*readonly_p=*/false,
> > > >                                         non_constant_p, overflow_p);
> > > > +   /* If this is a constructor, we are beginning the lifetime of the
> > > > +      object we are initializing.  */
> > > > +   if (new_obj
> > > > +       && DECL_CONSTRUCTOR_P (fun)
> > > > +       && TREE_CODE (new_obj) == COMPONENT_REF
> > > > +       && TREE_CODE (TREE_TYPE (TREE_OPERAND (new_obj, 0))) ==
> UNION_TYPE)
> > > > +     {
> > > > +       tree activate = build2 (INIT_EXPR, TREE_TYPE (new_obj),
> > > > +                               new_obj,
> > > > +                               build_constructor (TREE_TYPE
> (new_obj),
> > > > +                                                  NULL));
> > > > +       cxx_eval_constant_expression (ctx, activate,
> > > > +                                     lval, non_constant_p,
> overflow_p);
> > > > +       ggc_free (activate);
> > > > +     }
> > > > +
> > > >             tree jump_target = NULL_TREE;
> > > >             cxx_eval_constant_expression (&call_ctx, body,
> > > >                                           vc_discard,
> non_constant_p, overflow_p,
> > > >                                           &jump_target);
> > > >             if (DECL_CONSTRUCTOR_P (fun))
> > > > -     {
> > > > -       /* This can be null for a subobject constructor call, in
> > > > -          which case what we care about is the initialization
> > > > -          side-effects rather than the value.  We could get at the
> > > > -          value by evaluating *this, but we don't bother; there's
> > > > -          no need to put such a call in the hash table.  */
> > > > -       result = lval ? ctx->object : ctx->ctor;
> > > > -
> > > > -       /* If we've just evaluated a subobject constructor call for
> an
> > > > -          empty union member, it might not have produced a side
> effect
> > > > -          that actually activated the union member.  So produce
> such a
> > > > -          side effect now to ensure the union appears initialized.
> */
> > > > -       if (!result && new_obj
> > > > -           && TREE_CODE (new_obj) == COMPONENT_REF
> > > > -           && TREE_CODE (TREE_TYPE
> > > > -                         (TREE_OPERAND (new_obj, 0))) == UNION_TYPE
> > > > -           && is_really_empty_class (TREE_TYPE (new_obj),
> > > > -                                     /*ignore_vptr*/false))
> > > > -         {
> > > > -           tree activate = build2 (MODIFY_EXPR, TREE_TYPE (new_obj),
> > > > -                                   new_obj,
> > > > -                                   build_constructor (TREE_TYPE
> (new_obj),
> > > > -                                                      NULL));
> > > > -           cxx_eval_constant_expression (ctx, activate, lval,
> > > > -                                         non_constant_p,
> overflow_p);
> > > > -           ggc_free (activate);
> > > > -         }
> > > > -     }
> > > > +     /* This can be null for a subobject constructor call, in
> > > > +        which case what we care about is the initialization
> > > > +        side-effects rather than the value.  We could get at the
> > > > +        value by evaluating *this, but we don't bother; there's
> > > > +        no need to put such a call in the hash table.  */
> > > > +     result = lval ? ctx->object : ctx->ctor;
> > > >             else if (VOID_TYPE_P (TREE_TYPE (res)))
> > > >               result = void_node;
> > > >             else
> > > > @@ -6127,6 +6121,14 @@ cxx_eval_store_expression (const
> constexpr_ctx *ctx, tree t,
> > > >                                                     mutable_p)
> > > >                        && const_object_being_modified == NULL_TREE)
> > > >                 const_object_being_modified = probe;
> > > > +
> > > > +     /* Track named member accesses for unions to validate
> modifications
> > > > +        that change active member.  */
> > > > +     if (!evaluated && TREE_CODE (probe) == COMPONENT_REF)
> > > > +       vec_safe_push (refs, probe);
> > > > +     else
> > > > +       vec_safe_push (refs, NULL_TREE);
> > > > +
> > > >               vec_safe_push (refs, elt);
> > > >               vec_safe_push (refs, TREE_TYPE (probe));
> > > >               probe = ob;
> > > > @@ -6135,6 +6137,7 @@ cxx_eval_store_expression (const constexpr_ctx
> *ctx, tree t,
> > > >           case REALPART_EXPR:
> > > >             gcc_assert (probe == target);
> > > > +   vec_safe_push (refs, NULL_TREE);
> > > >             vec_safe_push (refs, probe);
> > > >             vec_safe_push (refs, TREE_TYPE (probe));
> > > >             probe = TREE_OPERAND (probe, 0);
> > > > @@ -6142,6 +6145,7 @@ cxx_eval_store_expression (const constexpr_ctx
> *ctx, tree t,
> > > >           case IMAGPART_EXPR:
> > > >             gcc_assert (probe == target);
> > > > +   vec_safe_push (refs, NULL_TREE);
> > > >             vec_safe_push (refs, probe);
> > > >             vec_safe_push (refs, TREE_TYPE (probe));
> > > >             probe = TREE_OPERAND (probe, 0);
> > > > @@ -6230,6 +6234,7 @@ cxx_eval_store_expression (const constexpr_ctx
> *ctx, tree t,
> > > >         enum tree_code code = TREE_CODE (type);
> > > >         tree reftype = refs->pop();
> > > >         tree index = refs->pop();
> > > > +      bool is_access_expr = refs->pop() != NULL_TREE;
> > > >         if (code == COMPLEX_TYPE)
> > > >           {
> > > > @@ -6270,21 +6275,72 @@ cxx_eval_store_expression (const
> constexpr_ctx *ctx, tree t,
> > > >         type = reftype;
> > > > -      if (code == UNION_TYPE && CONSTRUCTOR_NELTS (*valp)
> > > > -   && CONSTRUCTOR_ELT (*valp, 0)->index != index)
> > > > +      if (code == UNION_TYPE
> > > > +   && TREE_CODE (t) == MODIFY_EXPR
> > > > +   && (CONSTRUCTOR_NELTS (*valp) == 0
> > > > +       || CONSTRUCTOR_ELT (*valp, 0)->index != index))
> > > >           {
> > > > -   if (cxx_dialect < cxx20)
> > > > +   /* Probably a change of active member for a union. Ensure that
> > > > +      this is valid.  */
> > > > +   no_zero_init = true;
> > > > +
> > > > +   tree init = *valp;
> > > > +   if (!CONSTRUCTOR_NO_CLEARING (*valp) && CONSTRUCTOR_NELTS
> (*valp) == 0)
> > > > +     /* This is a value-initialized union, process the
> initialization
> > > > +        to ensure that the active member is properly set.  */
> > > > +     init = build_value_init (TREE_TYPE (*valp),
> tf_warning_or_error);
> > > > +
> > > > +   bool has_active_member = CONSTRUCTOR_NELTS (init) != 0;
> > > > +   tree inner = strip_array_types (reftype);
> > > > +
> > > > +   if (has_active_member && CONSTRUCTOR_ELT (init, 0)->index ==
> index)
> > > > +     /* After handling value-initialization, this wasn't actually a
> > > > +        change of active member like we initially thought.  */
> > > > +     no_zero_init = false;
> > > > +   else if (has_active_member && cxx_dialect < cxx20)
> > > >               {
> > > >                 if (!ctx->quiet)
> > > >                   error_at (cp_expr_loc_or_input_loc (t),
> > > >                             "change of the active member of a union "
> > > > -                   "from %qD to %qD",
> > > > -                   CONSTRUCTOR_ELT (*valp, 0)->index,
> > > > +                   "from %qD to %qD is not a constant expression "
> > > > +                   "before C++20",
> > > > +                   CONSTRUCTOR_ELT (init, 0)->index,
> > > >                             index);
> > > >                 *non_constant_p = true;
> > > >               }
> > > > -   else if (TREE_CODE (t) == MODIFY_EXPR
> > > > -            && CONSTRUCTOR_NO_CLEARING (*valp))
> > > > +   else if (!is_access_expr
> > > > +            || (CLASS_TYPE_P (inner)
> > > > +                && !type_has_non_deleted_trivial_default_ctor
> (inner)))
> > > > +     {
> > > > +       /* Diagnose changing active union member after initialization
> > > > +          without a valid member access expression, as described in
> > > > +          [class.union.general] p5.  */
> > > > +       if (!ctx->quiet)
> > > > +         {
> > > > +           if (has_active_member)
> > > > +             error_at (cp_expr_loc_or_input_loc (t),
> > > > +                       "accessing %qD member instead of initialized
> "
> > > > +                       "%qD member in constant expression",
> > > > +                       index, CONSTRUCTOR_ELT (init, 0)->index);
> > > > +           else
> > > > +             error_at (cp_expr_loc_or_input_loc (t),
> > > > +                       "accessing uninitialized member %qD",
> > > > +                       index);
> > > > +           if (is_access_expr)
> > > > +             inform (DECL_SOURCE_LOCATION (index),
> > > > +                     "%qD does not implicitly begin its lifetime "
> > > > +                     "because %qT does not have a non-deleted "
> > > > +                     "trivial default constructor",
> > > > +                     index, inner);
> > >
> > > It now occurs to me that this should suggest using placement new
> instead.
> > > And that we should test using placement new to begin the lifetime of
> such
> > > members.
> > >
> > > Jason
> > >
> >
> > Good point. On testing placement new I found I wasn't checking for
> > indirect change of active member during initialisation either, but this
> > was pretty trivial to add. How does this look?
> >
> > Bootstrap + regtest ongoing, but dg.exp completed with no errors. I'm
> > away from my computer next week so I thought I'd send this through now
> > anyway.
> >
> > -- >8 --
> >
> > This patch adds checks for attempting to change the active member of a
> > union by methods other than a member access expression.
> >
> > To be able to properly distinguish `*(&u.a) = ` from `u.a = `, this
> > patch redoes the solution for c++/59950 to avoid extranneous *&; it
> > seems that the only case that needed the workaround was when copying
> > empty classes.
> >
> > This patch also ensures that constructors for a union field mark that
> > field as the active member before entering the call itself; this ensures
> > that modifications of the field within the constructor's body don't
> > cause false positives (as these will not appear to be member access
> > expressions). This means that we no longer need to start the lifetime of
> > empty union members after the constructor body completes.
> >
> > As a drive-by fix, this patch also ensures that value-initialised unions
> > are considered to have activated their initial member for the purpose of
> > checking stores, which catches some additional mistakes pre-C++20.
> >
> >       PR c++/101631
> >       PR c++/102286
> >
> > gcc/cp/ChangeLog:
> >
> >       * call.cc (build_over_call): Fold more indirect refs for trivial
> >       assignment op.
> >       * class.cc (type_has_non_deleted_trivial_default_ctor): Create.
> >       * constexpr.cc (cxx_eval_call_expression): Start lifetime of
> >       union member before entering constructor.
> >       (cxx_eval_store_expression): Activate member for
> >       value-initialised union. Check for accessing inactive union
> >       member indirectly.
> >       * cp-tree.h (type_has_non_deleted_trivial_default_ctor):
> >       Forward declare.
> >
> > gcc/testsuite/ChangeLog:
> >
> >       * g++.dg/cpp1y/constexpr-89336-3.C: Fix union initialisation.
> >       * g++.dg/cpp1y/constexpr-union6.C: New test.
> >       * g++.dg/cpp2a/constexpr-union2.C: New test.
> >       * g++.dg/cpp2a/constexpr-union3.C: New test.
> >       * g++.dg/cpp2a/constexpr-union4.C: New test.
> >       * g++.dg/cpp2a/constexpr-union5.C: New test.
> >       * g++.dg/cpp2a/constexpr-union6.C: New test.
> >
> > Signed-off-by: Nathaniel Shead <nathanieloshead@gmail.com>
> > ---
> >  gcc/cp/call.cc                                |  11 +-
> >  gcc/cp/class.cc                               |   8 ++
> >  gcc/cp/constexpr.cc                           | 135 +++++++++++++-----
> >  gcc/cp/cp-tree.h                              |   1 +
> >  .../g++.dg/cpp1y/constexpr-89336-3.C          |   2 +-
> >  gcc/testsuite/g++.dg/cpp1y/constexpr-union6.C |  13 ++
> >  gcc/testsuite/g++.dg/cpp2a/constexpr-union2.C |  30 ++++
> >  gcc/testsuite/g++.dg/cpp2a/constexpr-union3.C |  45 ++++++
> >  gcc/testsuite/g++.dg/cpp2a/constexpr-union4.C |  29 ++++
> >  gcc/testsuite/g++.dg/cpp2a/constexpr-union5.C |  71 +++++++++
> >  gcc/testsuite/g++.dg/cpp2a/constexpr-union6.C |  43 ++++++
> >  11 files changed, 345 insertions(+), 43 deletions(-)
> >  create mode 100644 gcc/testsuite/g++.dg/cpp1y/constexpr-union6.C
> >  create mode 100644 gcc/testsuite/g++.dg/cpp2a/constexpr-union2.C
> >  create mode 100644 gcc/testsuite/g++.dg/cpp2a/constexpr-union3.C
> >  create mode 100644 gcc/testsuite/g++.dg/cpp2a/constexpr-union4.C
> >  create mode 100644 gcc/testsuite/g++.dg/cpp2a/constexpr-union5.C
> >  create mode 100644 gcc/testsuite/g++.dg/cpp2a/constexpr-union6.C
> >
> > diff --git a/gcc/cp/call.cc b/gcc/cp/call.cc
> > index e8dafbd8ba6..c1fb8807d3f 100644
> > --- a/gcc/cp/call.cc
> > +++ b/gcc/cp/call.cc
> > @@ -10330,10 +10330,7 @@ build_over_call (struct z_candidate *cand, int
> flags, tsubst_flags_t complain)
> >          && DECL_OVERLOADED_OPERATOR_IS (fn, NOP_EXPR)
> >          && trivial_fn_p (fn))
> >      {
> > -      /* Don't use cp_build_fold_indirect_ref, op= returns an lvalue
> even if
> > -      the object argument isn't one.  */
> > -      tree to = cp_build_indirect_ref (input_location, argarray[0],
> > -                                    RO_ARROW, complain);
> > +      tree to = cp_build_fold_indirect_ref (argarray[0]);
> >        tree type = TREE_TYPE (to);
> >        tree as_base = CLASSTYPE_AS_BASE (type);
> >        tree arg = argarray[1];
> > @@ -10341,7 +10338,11 @@ build_over_call (struct z_candidate *cand, int
> flags, tsubst_flags_t complain)
> >
> >        if (is_really_empty_class (type, /*ignore_vptr*/true))
> >       {
> > -       /* Avoid copying empty classes.  */
> > +       /* Avoid copying empty classes, but ensure op= returns an lvalue
> even
> > +          if the object argument isn't one. This isn't needed in other
> cases
> > +          since MODIFY_EXPR is always considered an lvalue.  */
> > +       to = cp_build_addr_expr (to, tf_none);
> > +       to = cp_build_indirect_ref (input_location, to, RO_ARROW,
> complain);
> >         val = build2 (COMPOUND_EXPR, type, arg, to);
> >         suppress_warning (val, OPT_Wunused);
> >       }
> > diff --git a/gcc/cp/class.cc b/gcc/cp/class.cc
> > index b71333af1f8..e31aeb8e68b 100644
> > --- a/gcc/cp/class.cc
> > +++ b/gcc/cp/class.cc
> > @@ -5688,6 +5688,14 @@ type_has_virtual_destructor (tree type)
> >    return (dtor && DECL_VIRTUAL_P (dtor));
> >  }
> >
> > +/* True iff class TYPE has a non-deleted trivial default
> > +   constructor.  */
> > +
> > +bool type_has_non_deleted_trivial_default_ctor (tree type)
> > +{
> > +  return TYPE_HAS_TRIVIAL_DFLT (type) && locate_ctor (type);
> > +}
> > +
> >  /* Returns true iff T, a class, has a move-assignment or
> >     move-constructor.  Does not lazily declare either.
> >     If USER_P is false, any move function will do.  If it is true, the
> > diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc
> > index a673a6022f1..8e6f60d03d2 100644
> > --- a/gcc/cp/constexpr.cc
> > +++ b/gcc/cp/constexpr.cc
> > @@ -3146,40 +3146,34 @@ cxx_eval_call_expression (const constexpr_ctx
> *ctx, tree t,
> >           cxx_set_object_constness (ctx, new_obj, /*readonly_p=*/false,
> >                                     non_constant_p, overflow_p);
> >
> > +       /* If this is a constructor, we are beginning the lifetime of the
> > +          object we are initializing.  */
> > +       if (new_obj
> > +           && DECL_CONSTRUCTOR_P (fun)
> > +           && TREE_CODE (new_obj) == COMPONENT_REF
> > +           && TREE_CODE (TREE_TYPE (TREE_OPERAND (new_obj, 0))) ==
> UNION_TYPE)
> > +         {
> > +           tree activate = build2 (INIT_EXPR, TREE_TYPE (new_obj),
> > +                                   new_obj,
> > +                                   build_constructor (TREE_TYPE
> (new_obj),
> > +                                                      NULL));
> > +           cxx_eval_constant_expression (ctx, activate,
> > +                                         lval, non_constant_p,
> overflow_p);
> > +           ggc_free (activate);
> > +         }
> > +
> >         tree jump_target = NULL_TREE;
> >         cxx_eval_constant_expression (&call_ctx, body,
> >                                       vc_discard, non_constant_p,
> overflow_p,
> >                                       &jump_target);
> >
> >         if (DECL_CONSTRUCTOR_P (fun))
> > -         {
> > -           /* This can be null for a subobject constructor call, in
> > -              which case what we care about is the initialization
> > -              side-effects rather than the value.  We could get at the
> > -              value by evaluating *this, but we don't bother; there's
> > -              no need to put such a call in the hash table.  */
> > -           result = lval ? ctx->object : ctx->ctor;
> > -
> > -           /* If we've just evaluated a subobject constructor call for
> an
> > -              empty union member, it might not have produced a side
> effect
> > -              that actually activated the union member.  So produce
> such a
> > -              side effect now to ensure the union appears initialized.
> */
> > -           if (!result && new_obj
> > -               && TREE_CODE (new_obj) == COMPONENT_REF
> > -               && TREE_CODE (TREE_TYPE
> > -                             (TREE_OPERAND (new_obj, 0))) == UNION_TYPE
> > -               && is_really_empty_class (TREE_TYPE (new_obj),
> > -                                         /*ignore_vptr*/false))
> > -             {
> > -               tree activate = build2 (MODIFY_EXPR, TREE_TYPE (new_obj),
> > -                                       new_obj,
> > -                                       build_constructor (TREE_TYPE
> (new_obj),
> > -                                                          NULL));
> > -               cxx_eval_constant_expression (ctx, activate, lval,
> > -                                             non_constant_p,
> overflow_p);
> > -               ggc_free (activate);
> > -             }
> > -         }
> > +         /* This can be null for a subobject constructor call, in
> > +            which case what we care about is the initialization
> > +            side-effects rather than the value.  We could get at the
> > +            value by evaluating *this, but we don't bother; there's
> > +            no need to put such a call in the hash table.  */
> > +         result = lval ? ctx->object : ctx->ctor;
> >         else if (VOID_TYPE_P (TREE_TYPE (res)))
> >           result = void_node;
> >         else
> > @@ -6127,6 +6121,14 @@ cxx_eval_store_expression (const constexpr_ctx
> *ctx, tree t,
> >                                                 mutable_p)
> >                    && const_object_being_modified == NULL_TREE)
> >             const_object_being_modified = probe;
> > +
> > +         /* Track named member accesses for unions to validate
> modifications
> > +            that change active member.  */
> > +         if (!evaluated && TREE_CODE (probe) == COMPONENT_REF)
> > +           vec_safe_push (refs, probe);
> > +         else
> > +           vec_safe_push (refs, NULL_TREE);
> > +
> >           vec_safe_push (refs, elt);
> >           vec_safe_push (refs, TREE_TYPE (probe));
> >           probe = ob;
> > @@ -6135,6 +6137,7 @@ cxx_eval_store_expression (const constexpr_ctx
> *ctx, tree t,
> >
> >       case REALPART_EXPR:
> >         gcc_assert (probe == target);
> > +       vec_safe_push (refs, NULL_TREE);
> >         vec_safe_push (refs, probe);
> >         vec_safe_push (refs, TREE_TYPE (probe));
> >         probe = TREE_OPERAND (probe, 0);
> > @@ -6142,6 +6145,7 @@ cxx_eval_store_expression (const constexpr_ctx
> *ctx, tree t,
> >
> >       case IMAGPART_EXPR:
> >         gcc_assert (probe == target);
> > +       vec_safe_push (refs, NULL_TREE);
> >         vec_safe_push (refs, probe);
> >         vec_safe_push (refs, TREE_TYPE (probe));
> >         probe = TREE_OPERAND (probe, 0);
> > @@ -6230,6 +6234,7 @@ cxx_eval_store_expression (const constexpr_ctx
> *ctx, tree t,
> >        enum tree_code code = TREE_CODE (type);
> >        tree reftype = refs->pop();
> >        tree index = refs->pop();
> > +      bool is_access_expr = refs->pop() != NULL_TREE;
> >
> >        if (code == COMPLEX_TYPE)
> >       {
> > @@ -6270,21 +6275,78 @@ cxx_eval_store_expression (const constexpr_ctx
> *ctx, tree t,
> >
> >        type = reftype;
> >
> > -      if (code == UNION_TYPE && CONSTRUCTOR_NELTS (*valp)
> > -       && CONSTRUCTOR_ELT (*valp, 0)->index != index)
> > +      /* Check for implicit change of active member for a union.  */
> > +      if (code == UNION_TYPE
> > +       && (TREE_CODE (t) == MODIFY_EXPR
> > +           /* Also check if initializations have implicit change of
> active
> > +              member earlier up the access chain.  */
> > +           || !refs->is_empty())
> > +       && (CONSTRUCTOR_NELTS (*valp) == 0
> > +           || CONSTRUCTOR_ELT (*valp, 0)->index != index))
> >       {
> > -       if (cxx_dialect < cxx20)
> > +       no_zero_init = true;
> > +
> > +       tree init = *valp;
> > +       if (TREE_CODE (t) == MODIFY_EXPR
> > +           && !CONSTRUCTOR_NO_CLEARING (*valp)
> > +           && CONSTRUCTOR_NELTS (*valp) == 0)
> > +         /* Modifying a value-initialized union, process the
> initialization
> > +            to ensure that the active member is properly set.  */
> > +         init = build_value_init (TREE_TYPE (*valp),
> tf_warning_or_error);
> > +
> > +       bool has_active_member = CONSTRUCTOR_NELTS (init) != 0;
> > +       tree inner = strip_array_types (reftype);
> > +
> > +       if (has_active_member && CONSTRUCTOR_ELT (init, 0)->index ==
> index)
> > +         /* After handling value-initialization, this wasn't actually a
> > +            change of active member like we initially thought.  */
> > +         no_zero_init = false;
> > +       else if (has_active_member && cxx_dialect < cxx20)
> >           {
> >             if (!ctx->quiet)
> >               error_at (cp_expr_loc_or_input_loc (t),
> >                         "change of the active member of a union "
> > -                       "from %qD to %qD",
> > -                       CONSTRUCTOR_ELT (*valp, 0)->index,
> > +                       "from %qD to %qD is not a constant expression "
> > +                       "before C++20",
> > +                       CONSTRUCTOR_ELT (init, 0)->index,
> >                         index);
> >             *non_constant_p = true;
> >           }
> > -       else if (TREE_CODE (t) == MODIFY_EXPR
> > -                && CONSTRUCTOR_NO_CLEARING (*valp))
> > +       else if (!is_access_expr
> > +                || (TREE_CODE (t) == MODIFY_EXPR
> > +                    && CLASS_TYPE_P (inner)
> > +                    && !type_has_non_deleted_trivial_default_ctor
> (inner)))
> > +         {
> > +           /* Diagnose changing active union member after initialization
> > +              without a valid member access expression, as described in
> > +              [class.union.general] p5.  */
> > +           if (!ctx->quiet)
> > +             {
> > +               if (has_active_member)
> > +                 error_at (cp_expr_loc_or_input_loc (t),
> > +                           "accessing %qD member instead of initialized
> "
> > +                           "%qD member in constant expression",
> > +                           index, CONSTRUCTOR_ELT (init, 0)->index);
> > +               else
> > +                 error_at (cp_expr_loc_or_input_loc (t),
> > +                           "accessing uninitialized member %qD",
> > +                           index);
> > +               if (is_access_expr)
> > +                 inform (DECL_SOURCE_LOCATION (index),
> > +                         "%qD does not implicitly begin its lifetime "
> > +                         "because %qT does not have a non-deleted "
> > +                         "trivial default constructor, use "
> > +                         "%<std::construct_at%> instead",
> > +                         index, inner);
> > +               else
> > +                 inform (DECL_SOURCE_LOCATION (index),
> > +                         "initializing %qD requires a member access "
> > +                         "expression as the left operand of the
> assignment",
> > +                         index);
> > +             }
> > +           *non_constant_p = true;
> > +         }
> > +       else if (has_active_member && CONSTRUCTOR_NO_CLEARING (init))
> >           {
> >             /* Diagnose changing the active union member while the union
> >                is in the process of being initialized.  */
> > @@ -6292,11 +6354,10 @@ cxx_eval_store_expression (const constexpr_ctx
> *ctx, tree t,
> >               error_at (cp_expr_loc_or_input_loc (t),
> >                         "change of the active member of a union "
> >                         "from %qD to %qD during initialization",
> > -                       CONSTRUCTOR_ELT (*valp, 0)->index,
> > +                       CONSTRUCTOR_ELT (init, 0)->index,
> >                         index);
> >             *non_constant_p = true;
> >           }
> > -       no_zero_init = true;
> >       }
> >
> >        ctors.safe_push (valp);
> > diff --git a/gcc/cp/cp-tree.h b/gcc/cp/cp-tree.h
> > index fd6bf9fd4a0..40aa9f0be2b 100644
> > --- a/gcc/cp/cp-tree.h
> > +++ b/gcc/cp/cp-tree.h
> > @@ -6815,6 +6815,7 @@ extern bool
> trivial_default_constructor_is_constexpr (tree);
> >  extern bool type_has_constexpr_default_constructor (tree);
> >  extern bool type_has_constexpr_destructor    (tree);
> >  extern bool type_has_virtual_destructor              (tree);
> > +extern bool type_has_non_deleted_trivial_default_ctor (tree);
> >  extern bool classtype_has_move_assign_or_move_ctor_p (tree, bool
> user_declared);
> >  extern bool classtype_has_non_deleted_move_ctor (tree);
> >  extern tree classtype_has_depr_implicit_copy (tree);
> > diff --git a/gcc/testsuite/g++.dg/cpp1y/constexpr-89336-3.C
> b/gcc/testsuite/g++.dg/cpp1y/constexpr-89336-3.C
> > index 9d370ddefcf..6a60966f6f8 100644
> > --- a/gcc/testsuite/g++.dg/cpp1y/constexpr-89336-3.C
> > +++ b/gcc/testsuite/g++.dg/cpp1y/constexpr-89336-3.C
> > @@ -18,7 +18,7 @@ constexpr int
> >  bar ()
> >  {
> >    union U { int a[5]; long b; };
> > -  union V { union U u; short v; };
> > +  union V { short v; union U u; };
> >    V w {};
> >    w.v = 5;
> >    w.u.a[3] = w.u.a[1] = w.v;         // { dg-error "change of the
> active member of a union from" "" { target c++17_down } }
> > diff --git a/gcc/testsuite/g++.dg/cpp1y/constexpr-union6.C
> b/gcc/testsuite/g++.dg/cpp1y/constexpr-union6.C
> > new file mode 100644
> > index 00000000000..ff7ebf19f89
> > --- /dev/null
> > +++ b/gcc/testsuite/g++.dg/cpp1y/constexpr-union6.C
> > @@ -0,0 +1,13 @@
> > +// { dg-do compile { target c++14 } }
> > +
> > +union U {
> > +  int a;
> > +  float b;
> > +};
> > +
> > +constexpr bool foo() {
> > +  U u {};
> > +  u.b = 1.0f;  // { dg-error "change of the active member" "" { target
> c++17_down } }
> > +  return u.b == 1.0f;
> > +}
> > +constexpr bool x = foo();
> > diff --git a/gcc/testsuite/g++.dg/cpp2a/constexpr-union2.C
> b/gcc/testsuite/g++.dg/cpp2a/constexpr-union2.C
> > new file mode 100644
> > index 00000000000..1712395d7e7
> > --- /dev/null
> > +++ b/gcc/testsuite/g++.dg/cpp2a/constexpr-union2.C
> > @@ -0,0 +1,30 @@
> > +// PR c++/101631
> > +// { dg-do compile { target c++20 } }
> > +
> > +struct sso {
> > +  union {
> > +    int buf[10];
> > +    int* alloc;
> > +  };
> > +};
> > +
> > +constexpr bool direct() {
> > +  sso val;
> > +  val.alloc = nullptr;
> > +  val.buf[5] = 42;
> > +  return true;
> > +}
> > +constexpr bool ok = direct();
> > +
> > +
> > +constexpr void perform_assignment(int& left, int right) noexcept {
> > +  left = right;  // { dg-error "accessing .+ member instead of
> initialized" }
> > +}
> > +
> > +constexpr bool indirect() {
> > +  sso val;
> > +  val.alloc = nullptr;
> > +  perform_assignment(val.buf[5], 42);  // { dg-message "in .constexpr.
> expansion" }
> > +  return true;
> > +}
> > +constexpr bool err = indirect();  // { dg-message "in .constexpr.
> expansion" }
> > diff --git a/gcc/testsuite/g++.dg/cpp2a/constexpr-union3.C
> b/gcc/testsuite/g++.dg/cpp2a/constexpr-union3.C
> > new file mode 100644
> > index 00000000000..6d30bb2498f
> > --- /dev/null
> > +++ b/gcc/testsuite/g++.dg/cpp2a/constexpr-union3.C
> > @@ -0,0 +1,45 @@
> > +// { dg-do compile { target c++20 } }
> > +
> > +struct S
> > +{
> > +  union {
> > +    char buf[8];
> > +    char* ptr;
> > +  };
> > +  unsigned len;
> > +
> > +  constexpr S(const char* s, unsigned n)
> > +  {
> > +    char* p;
> > +    if (n > 7)
> > +      p = ptr = new char[n+1];
> > +    else
> > +      p = buf;
> > +    for (len = 0; len < n; ++len)
> > +      p[len] = s[len];  // { dg-error "accessing uninitialized member" }
> > +    p[len] = '\0';
> > +  }
> > +
> > +  constexpr ~S()
> > +  {
> > +    if (len > 7)
> > +      delete[] ptr;
> > +  }
> > +};
> > +
> > +constexpr bool test1()
> > +{
> > +  S s("test", 4);  // { dg-message "in .constexpr. expansion" }
> > +  return true;
> > +}
> > +
> > +constexpr bool a = test1();  // { dg-message "in .constexpr. expansion"
> }
> > +
> > +
> > +constexpr bool test2()
> > +{
> > +  S s("hello world", 11);
> > +  return true;
> > +}
> > +
> > +constexpr bool b = test2();
> > diff --git a/gcc/testsuite/g++.dg/cpp2a/constexpr-union4.C
> b/gcc/testsuite/g++.dg/cpp2a/constexpr-union4.C
> > new file mode 100644
> > index 00000000000..429ab203ac2
> > --- /dev/null
> > +++ b/gcc/testsuite/g++.dg/cpp2a/constexpr-union4.C
> > @@ -0,0 +1,29 @@
> > +// { dg-do compile { target c++20 } }
> > +
> > +// from [class.union.general] p5
> > +
> > +union A { int x; int y[4]; };
> > +struct B { A a; };
> > +union C { B b; int k; };
> > +constexpr int f() {
> > +  C c;                  // does not start lifetime of any union member
> > +  c.b.a.y[3] = 4;       // OK, S(c.b.a.y[3]) contains c.b and c.b.a.y;
> > +                        // creates objects to hold union members c.b
> and c.b.a.y
> > +  return c.b.a.y[3];    // OK, c.b.a.y refers to newly created object
> (see [basic.life])
> > +}
> > +constexpr int a = f();
> > +
> > +struct X { const int a; int b; };
> > +union Y { X x; int k; };// { dg-message "does not implicitly begin its
> lifetime" }
> > +constexpr int g() {
> > +  Y y = { { 1, 2 } };   // OK, y.x is active union member ([class.mem])
> > +  int n = y.x.a;
> > +  y.k = 4;              // OK, ends lifetime of y.x, y.k is active
> member of union
> > +
> > +  y.x.b = n;            // { dg-error "accessing .* member instead of
> initialized .* member" }
> > +                        // undefined behavior: y.x.b modified outside
> its lifetime,
> > +                        // S(y.x.b) is empty because X's default
> constructor is deleted,
> > +                        // so union member y.x's lifetime does not
> implicitly start
> > +  return 0;
> > +}
> > +constexpr int b = g();  // { dg-message "in .constexpr. expansion" }
> > diff --git a/gcc/testsuite/g++.dg/cpp2a/constexpr-union5.C
> b/gcc/testsuite/g++.dg/cpp2a/constexpr-union5.C
> > new file mode 100644
> > index 00000000000..c94d7cc0fc9
> > --- /dev/null
> > +++ b/gcc/testsuite/g++.dg/cpp2a/constexpr-union5.C
> > @@ -0,0 +1,71 @@
> > +// { dg-do compile { target c++20 } }
> > +
> > +union U { int a; int b; int c[2]; };
> > +
> > +constexpr int test1() {
> > +  U u;
> > +  u.a = 10;
> > +  *&u.b = 20;  // { dg-error "accessing" }
> > +  return u.b;
> > +}
> > +constexpr int x1 = test1();  // { dg-message "in .constexpr. expansion"
> }
> > +
> > +constexpr int test2() {
> > +  U u;
> > +  u.a = 10;
> > +  (0, u.b) = 20;  // { dg-error "accessing" }
> > +  return u.b;
> > +}
> > +constexpr int x2 = test2();  // { dg-message "in .constexpr. expansion"
> }
> > +
> > +constexpr int test3() {
> > +  U u;
> > +  u.a = 0;
> > +  int* p = &u.b;
> > +  p[u.a] = 10;  // { dg-error "accessing" }
> > +  return u.b;
> > +}
> > +constexpr int x3 = test3();  // { dg-message "in .constexpr. expansion"
> }
> > +
> > +constexpr int test4() {
> > +  U u;
> > +  u.a = 0;
> > +  int* p = &u.b;
> > +  u.a[p] = 10;  // { dg-error "accessing" }
> > +  return u.b;
> > +}
> > +constexpr int x4 = test4();  // { dg-message "in .constexpr. expansion"
> }
> > +
> > +struct S { U u[10]; };
> > +constexpr int test5() {
> > +  S s;
> > +  s.u[4].a = 10;
> > +  6[s.u].b = 15;
> > +  return 4[s.u].a + s.u[6].b;
> > +}
> > +static_assert(test5() == 25);
> > +
> > +constexpr int test6() {
> > +  U u;
> > +  u.a = 5;
> > +  u.c[0] = 3;
> > +  1[u.c] = 8;
> > +  return 1[u.c] + u.c[0];
> > +}
> > +static_assert(test6() == 11);
> > +
> > +constexpr int test7() {
> > +  U u {};  // value initialisation initialises first member
> > +  int* p = &u.a;
> > +  *p = 10;
> > +  return *p;
> > +}
> > +constexpr int x7 = test7();
> > +
> > +constexpr int test8() {
> > +  U u;  // default initialisation leaves no member initialised
> > +  int* p = &u.a;
> > +  *p = 10;  // { dg-error "accessing" }
> > +  return *p;
> > +}
> > +constexpr int x8 = test8();  // { dg-message "in .constexpr. expansion"
> }
> > diff --git a/gcc/testsuite/g++.dg/cpp2a/constexpr-union6.C
> b/gcc/testsuite/g++.dg/cpp2a/constexpr-union6.C
> > new file mode 100644
> > index 00000000000..7c37716892c
> > --- /dev/null
> > +++ b/gcc/testsuite/g++.dg/cpp2a/constexpr-union6.C
> > @@ -0,0 +1,43 @@
> > +// { dg-do compile { target c++20 } }
> > +// PR c++/102286
> > +
> > +#include "construct_at.h"
> > +
> > +struct S { const int a; int b; };
> > +union U { int k; S s; };
> > +
> > +constexpr int test1() {
> > +  U u {};
> > +  std::construct_at(&u.s, S{ 1, 2 });
> > +  return u.s.b;
> > +}
> > +static_assert(test1() == 2);
> > +
> > +constexpr int test2() {
> > +  U u {};
> > +  int* p = &u.s.b;
> > +  std::construct_at(p, 5);  // { dg-message "in .constexpr. expansion" }
> > +  return u.s.b;
> > +}
> > +constexpr int x2 = test2();  // { dg-message "in .constexpr. expansion"
> }
> > +
> > +struct S2 { int a; int b; };
> > +union U2 { int k; S2 s; };
> > +constexpr int test3() {
> > +  U2 u;
> > +  int* p = &u.s.b;
> > +  std::construct_at(p, 8);  // { dg-message "in .constexpr. expansion" }
> > +  return u.s.b;
> > +};
> > +constexpr int x3 = test3();  // { dg-message "in .constexpr. expansion"
> }
> > +
> > +constexpr int test4() {
> > +  union {
> > +    int data[1];
> > +  } u;
> > +  std::construct_at(u.data, 0);  // { dg-message "in .constexpr.
> expansion" }
> > +  return 0;
> > +}
> > +constexpr int x4 = test4();  // { dg-message "in .constexpr. expansion"
> }
> > +
> > +// { dg-error "accessing uninitialized member" "" { target *-*-* } 0 }
> > --
> > 2.41.0
> >
>

[PATCH] libstdc++: Ensure active union member is correctly set

[Nathaniel Shead]

On Sat, Sep 23, 2023 at 07:40:48AM +0100, Jonathan Wakely wrote:
> On Sat, 23 Sept 2023, 01:39 Nathaniel Shead via Libstdc++, <
> libstdc++@gcc.gnu.org> wrote:
> 
> > Now that bootstrap has finished, I have gotten regressions in the
> > following libstdc++ tests:
> >
> > Running libstdc++:libstdc++-dg/conformance.exp ...
> > FAIL: 20_util/bitset/access/constexpr.cc -std=gnu++23 (test for excess
> > errors)
> > FAIL: 20_util/bitset/access/constexpr.cc -std=gnu++26 (test for excess
> > errors)
> > FAIL: 20_util/variant/constexpr.cc -std=gnu++20 (test for excess errors)
> > FAIL: 20_util/variant/constexpr.cc -std=gnu++26 (test for excess errors)
> > FAIL: 21_strings/basic_string/cons/char/constexpr.cc -std=gnu++20 (test
> > for excess errors)
> > FAIL: 21_strings/basic_string/cons/char/constexpr.cc -std=gnu++26 (test
> > for excess errors)
> > FAIL: 21_strings/basic_string/cons/wchar_t/constexpr.cc -std=gnu++20 (test
> > for excess errors)
> > FAIL: 21_strings/basic_string/cons/wchar_t/constexpr.cc -std=gnu++26 (test
> > for excess errors)
> > FAIL: 21_strings/basic_string/modifiers/swap/constexpr-wchar_t.cc
> > -std=gnu++20 (test for excess errors)
> > FAIL: 21_strings/basic_string/modifiers/swap/constexpr-wchar_t.cc
> > -std=gnu++26 (test for excess errors)
> > FAIL: 21_strings/basic_string/modifiers/swap/constexpr.cc -std=gnu++20
> > (test for excess errors)
> > FAIL: 21_strings/basic_string/modifiers/swap/constexpr.cc -std=gnu++26
> > (test for excess errors)
> > FAIL: std/ranges/adaptors/join_with/1.cc -std=gnu++23 (test for excess
> > errors)
> > UNRESOLVED: std/ranges/adaptors/join_with/1.cc -std=gnu++23 compilation
> > failed to produce executable
> > FAIL: std/ranges/adaptors/join_with/1.cc -std=gnu++26 (test for excess
> > errors)
> > UNRESOLVED: std/ranges/adaptors/join_with/1.cc -std=gnu++26 compilation
> > failed to produce executable
> >
> > On investigation though it looks like the issue might be with libstdc++
> > rather than the patch itself; running the failing tests using clang with
> > libstdc++ also produces similar errors, and my reading of the code
> > suggests that this is correct.
> >
> > What's the way forward here? Should I look at creating a patch to fix
> > the libstdc++ issues before resubmitting this patch for the C++
> > frontend? Or should I submit a version of this patch without the
> > `std::construct_at` changes and wait till libstdc++ gets fixed for that?
> >
> 
> I think we should fix libstdc++. There are probably only a few places that
> need a fix, which cause all those failures.
> 
> I can help with those fixes. I'll look into it after the weekend.
> 

Thanks. I did end up getting a chance to look at it earlier today, and
with the following patch I had no regressions when applying the frontend
changes. Bootstrapped and regtested on x86_64-pc-linux-gnu.

-- >8 --

This patch ensures that the union members for std::string and
std::variant are always properly set when a change occurs.

libstdc++-v3/ChangeLog:

	* include/bits/basic_string.h: (basic_string(basic_string&&)):
	Activate _M_local_buf when needed.
	(basic_string(basic_string&&, const _Alloc&)): Likewise.
	* include/bits/basic_string.tcc: (basic_string::swap): Likewise.
	* include/std/variant: (__detail::__variant::__construct_n): New.
	(__detail::_variant::__emplace): Use __construct_n.

Signed-off-by: Nathaniel Shead <nathanieloshead@gmail.com>
---
 libstdc++-v3/include/bits/basic_string.h   |  7 +++--
 libstdc++-v3/include/bits/basic_string.tcc |  8 +++---
 libstdc++-v3/include/std/variant           | 32 ++++++++++++++++++++--
 3 files changed, 38 insertions(+), 9 deletions(-)

diff --git a/libstdc++-v3/include/bits/basic_string.h b/libstdc++-v3/include/bits/basic_string.h
index 09fd62afa66..7c342879827 100644
--- a/libstdc++-v3/include/bits/basic_string.h
+++ b/libstdc++-v3/include/bits/basic_string.h
@@ -678,7 +678,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
       {
 	if (__str._M_is_local())
 	  {
-	    traits_type::copy(_M_local_buf, __str._M_local_buf,
+	    traits_type::copy(_M_use_local_data(), __str._M_local_buf,
 			      __str.length() + 1);
 	  }
 	else
@@ -691,7 +691,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
 	// basic_stringbuf relies on writing into unallocated capacity so
 	// we mess up the contents if we put a '\0' in the string.
 	_M_length(__str.length());
-	__str._M_data(__str._M_local_data());
+	__str._M_data(__str._M_use_local_data());
 	__str._M_set_length(0);
       }
 
@@ -717,6 +717,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
       {
 	if (__str._M_is_local())
 	  {
+	    _M_use_local_data();
 	    traits_type::copy(_M_local_buf, __str._M_local_buf,
 			      __str.length() + 1);
 	    _M_length(__str.length());
@@ -728,7 +729,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
 	    _M_data(__str._M_data());
 	    _M_length(__str.length());
 	    _M_capacity(__str._M_allocated_capacity);
-	    __str._M_data(__str._M_local_buf);
+	    __str._M_data(__str._M_use_local_data());
 	    __str._M_set_length(0);
 	  }
 	else
diff --git a/libstdc++-v3/include/bits/basic_string.tcc b/libstdc++-v3/include/bits/basic_string.tcc
index 104a517f794..ee6e57da555 100644
--- a/libstdc++-v3/include/bits/basic_string.tcc
+++ b/libstdc++-v3/include/bits/basic_string.tcc
@@ -79,7 +79,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	      }
 	    else if (__s.length())
 	      {
-		traits_type::copy(_M_local_buf, __s._M_local_buf,
+		traits_type::copy(_M_use_local_data(), __s._M_local_buf,
 				  __s.length() + 1);
 		_M_length(__s.length());
 		__s._M_set_length(0);
@@ -87,7 +87,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	      }
 	    else if (length())
 	      {
-		traits_type::copy(__s._M_local_buf, _M_local_buf,
+		traits_type::copy(__s._M_use_local_data(), _M_local_buf,
 				  length() + 1);
 		__s._M_length(length());
 		_M_set_length(0);
@@ -97,7 +97,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	else
 	  {
 	    const size_type __tmp_capacity = __s._M_allocated_capacity;
-	    traits_type::copy(__s._M_local_buf, _M_local_buf,
+	    traits_type::copy(__s._M_use_local_data(), _M_local_buf,
 			      length() + 1);
 	    _M_data(__s._M_data());
 	    __s._M_data(__s._M_local_buf);
@@ -108,7 +108,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	  const size_type __tmp_capacity = _M_allocated_capacity;
 	  if (__s._M_is_local())
 	    {
-	      traits_type::copy(_M_local_buf, __s._M_local_buf,
+	      traits_type::copy(_M_use_local_data(), __s._M_local_buf,
 				__s.length() + 1);
 	      __s._M_data(_M_data());
 	      _M_data(_M_local_buf);
diff --git a/libstdc++-v3/include/std/variant b/libstdc++-v3/include/std/variant
index c0e41740dcf..7f24e760bb1 100644
--- a/libstdc++-v3/include/std/variant
+++ b/libstdc++-v3/include/std/variant
@@ -320,6 +320,33 @@ namespace __variant
     __get(_Variant&& __v) noexcept
     { return __variant::__get_n<_Np>(std::forward<_Variant>(__v)._M_u); }
 
+  // Gets the _Uninitialized to construct into for __u.
+  template<size_t _Np, typename _Union>
+    constexpr decltype(auto)
+    __construct_n(_Union& __u) noexcept
+    {
+      if constexpr (_Np == 0)
+	return &__u._M_first;
+      else if constexpr (_Np == 1)
+	{
+	  std::_Construct(&__u._M_rest);
+	  return &__u._M_rest._M_first;
+	}
+      else if constexpr (_Np == 2)
+	{
+	  std::_Construct(&__u._M_rest);
+	  std::_Construct(&__u._M_rest._M_rest);
+	  return &__u._M_rest._M_rest._M_first;
+	}
+      else
+	{
+	  std::_Construct(&__u._M_rest);
+	  std::_Construct(&__u._M_rest._M_rest);
+	  std::_Construct(&__u._M_rest._M_rest._M_rest);
+	  return __variant::__construct_n<_Np - 3>(__u._M_rest._M_rest._M_rest);
+	}
+    }
+
   template<typename... _Types>
     struct _Traits
     {
@@ -536,8 +563,9 @@ namespace __variant
     __emplace(_Variant_storage<_Triv, _Types...>& __v, _Args&&... __args)
     {
       __v._M_reset();
-      auto* __addr = std::__addressof(__variant::__get_n<_Np>(__v._M_u));
-      std::_Construct(__addr, std::forward<_Args>(__args)...);
+      auto* __addr = __variant::__construct_n<_Np>(__v._M_u);
+      std::_Construct(__addr, in_place_index<0>,
+		      std::forward<_Args>(__args)...);
       // Construction didn't throw, so can set the new index now:
       __v._M_index = _Np;
     }
-- 
2.41.0

Re: [PATCH] libstdc++: Ensure active union member is correctly set

[Jonathan Wakely]

[-- Attachment #1: Type: text/plain, Size: 9471 bytes --]

On Sat, 23 Sept 2023, 08:30 Nathaniel Shead, <nathanieloshead@gmail.com>
wrote:

> On Sat, Sep 23, 2023 at 07:40:48AM +0100, Jonathan Wakely wrote:
> > On Sat, 23 Sept 2023, 01:39 Nathaniel Shead via Libstdc++, <
> > libstdc++@gcc.gnu.org> wrote:
> >
> > > Now that bootstrap has finished, I have gotten regressions in the
> > > following libstdc++ tests:
> > >
> > > Running libstdc++:libstdc++-dg/conformance.exp ...
> > > FAIL: 20_util/bitset/access/constexpr.cc -std=gnu++23 (test for excess
> > > errors)
> > > FAIL: 20_util/bitset/access/constexpr.cc -std=gnu++26 (test for excess
> > > errors)
> > > FAIL: 20_util/variant/constexpr.cc -std=gnu++20 (test for excess
> errors)
> > > FAIL: 20_util/variant/constexpr.cc -std=gnu++26 (test for excess
> errors)
> > > FAIL: 21_strings/basic_string/cons/char/constexpr.cc -std=gnu++20 (test
> > > for excess errors)
> > > FAIL: 21_strings/basic_string/cons/char/constexpr.cc -std=gnu++26 (test
> > > for excess errors)
> > > FAIL: 21_strings/basic_string/cons/wchar_t/constexpr.cc -std=gnu++20
> (test
> > > for excess errors)
> > > FAIL: 21_strings/basic_string/cons/wchar_t/constexpr.cc -std=gnu++26
> (test
> > > for excess errors)
> > > FAIL: 21_strings/basic_string/modifiers/swap/constexpr-wchar_t.cc
> > > -std=gnu++20 (test for excess errors)
> > > FAIL: 21_strings/basic_string/modifiers/swap/constexpr-wchar_t.cc
> > > -std=gnu++26 (test for excess errors)
> > > FAIL: 21_strings/basic_string/modifiers/swap/constexpr.cc -std=gnu++20
> > > (test for excess errors)
> > > FAIL: 21_strings/basic_string/modifiers/swap/constexpr.cc -std=gnu++26
> > > (test for excess errors)
> > > FAIL: std/ranges/adaptors/join_with/1.cc -std=gnu++23 (test for excess
> > > errors)
> > > UNRESOLVED: std/ranges/adaptors/join_with/1.cc -std=gnu++23 compilation
> > > failed to produce executable
> > > FAIL: std/ranges/adaptors/join_with/1.cc -std=gnu++26 (test for excess
> > > errors)
> > > UNRESOLVED: std/ranges/adaptors/join_with/1.cc -std=gnu++26 compilation
> > > failed to produce executable
> > >
> > > On investigation though it looks like the issue might be with libstdc++
> > > rather than the patch itself; running the failing tests using clang
> with
> > > libstdc++ also produces similar errors, and my reading of the code
> > > suggests that this is correct.
> > >
> > > What's the way forward here? Should I look at creating a patch to fix
> > > the libstdc++ issues before resubmitting this patch for the C++
> > > frontend? Or should I submit a version of this patch without the
> > > `std::construct_at` changes and wait till libstdc++ gets fixed for
> that?
> > >
> >
> > I think we should fix libstdc++. There are probably only a few places
> that
> > need a fix, which cause all those failures.
> >
> > I can help with those fixes. I'll look into it after the weekend.
> >
>
> Thanks. I did end up getting a chance to look at it earlier today, and
> with the following patch I had no regressions when applying the frontend
> changes. Bootstrapped and regtested on x86_64-pc-linux-gnu.
>

Nice, thanks! This looks good at first glance, I'll review it properly ASAP.



> -- >8 --
>
> This patch ensures that the union members for std::string and
> std::variant are always properly set when a change occurs.
>
> libstdc++-v3/ChangeLog:
>
>         * include/bits/basic_string.h: (basic_string(basic_string&&)):
>         Activate _M_local_buf when needed.
>         (basic_string(basic_string&&, const _Alloc&)): Likewise.
>         * include/bits/basic_string.tcc: (basic_string::swap): Likewise.
>         * include/std/variant: (__detail::__variant::__construct_n): New.
>         (__detail::_variant::__emplace): Use __construct_n.
>
> Signed-off-by: Nathaniel Shead <nathanieloshead@gmail.com>
> ---
>  libstdc++-v3/include/bits/basic_string.h   |  7 +++--
>  libstdc++-v3/include/bits/basic_string.tcc |  8 +++---
>  libstdc++-v3/include/std/variant           | 32 ++++++++++++++++++++--
>  3 files changed, 38 insertions(+), 9 deletions(-)
>
> diff --git a/libstdc++-v3/include/bits/basic_string.h
> b/libstdc++-v3/include/bits/basic_string.h
> index 09fd62afa66..7c342879827 100644
> --- a/libstdc++-v3/include/bits/basic_string.h
> +++ b/libstdc++-v3/include/bits/basic_string.h
> @@ -678,7 +678,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
>        {
>         if (__str._M_is_local())
>           {
> -           traits_type::copy(_M_local_buf, __str._M_local_buf,
> +           traits_type::copy(_M_use_local_data(), __str._M_local_buf,
>                               __str.length() + 1);
>           }
>         else
> @@ -691,7 +691,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
>         // basic_stringbuf relies on writing into unallocated capacity so
>         // we mess up the contents if we put a '\0' in the string.
>         _M_length(__str.length());
> -       __str._M_data(__str._M_local_data());
> +       __str._M_data(__str._M_use_local_data());
>         __str._M_set_length(0);
>        }
>
> @@ -717,6 +717,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
>        {
>         if (__str._M_is_local())
>           {
> +           _M_use_local_data();
>             traits_type::copy(_M_local_buf, __str._M_local_buf,
>                               __str.length() + 1);
>             _M_length(__str.length());
> @@ -728,7 +729,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
>             _M_data(__str._M_data());
>             _M_length(__str.length());
>             _M_capacity(__str._M_allocated_capacity);
> -           __str._M_data(__str._M_local_buf);
> +           __str._M_data(__str._M_use_local_data());
>             __str._M_set_length(0);
>           }
>         else
> diff --git a/libstdc++-v3/include/bits/basic_string.tcc
> b/libstdc++-v3/include/bits/basic_string.tcc
> index 104a517f794..ee6e57da555 100644
> --- a/libstdc++-v3/include/bits/basic_string.tcc
> +++ b/libstdc++-v3/include/bits/basic_string.tcc
> @@ -79,7 +79,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>               }
>             else if (__s.length())
>               {
> -               traits_type::copy(_M_local_buf, __s._M_local_buf,
> +               traits_type::copy(_M_use_local_data(), __s._M_local_buf,
>                                   __s.length() + 1);
>                 _M_length(__s.length());
>                 __s._M_set_length(0);
> @@ -87,7 +87,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>               }
>             else if (length())
>               {
> -               traits_type::copy(__s._M_local_buf, _M_local_buf,
> +               traits_type::copy(__s._M_use_local_data(), _M_local_buf,
>                                   length() + 1);
>                 __s._M_length(length());
>                 _M_set_length(0);
> @@ -97,7 +97,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>         else
>           {
>             const size_type __tmp_capacity = __s._M_allocated_capacity;
> -           traits_type::copy(__s._M_local_buf, _M_local_buf,
> +           traits_type::copy(__s._M_use_local_data(), _M_local_buf,
>                               length() + 1);
>             _M_data(__s._M_data());
>             __s._M_data(__s._M_local_buf);
> @@ -108,7 +108,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>           const size_type __tmp_capacity = _M_allocated_capacity;
>           if (__s._M_is_local())
>             {
> -             traits_type::copy(_M_local_buf, __s._M_local_buf,
> +             traits_type::copy(_M_use_local_data(), __s._M_local_buf,
>                                 __s.length() + 1);
>               __s._M_data(_M_data());
>               _M_data(_M_local_buf);
> diff --git a/libstdc++-v3/include/std/variant
> b/libstdc++-v3/include/std/variant
> index c0e41740dcf..7f24e760bb1 100644
> --- a/libstdc++-v3/include/std/variant
> +++ b/libstdc++-v3/include/std/variant
> @@ -320,6 +320,33 @@ namespace __variant
>      __get(_Variant&& __v) noexcept
>      { return __variant::__get_n<_Np>(std::forward<_Variant>(__v)._M_u); }
>
> +  // Gets the _Uninitialized to construct into for __u.
> +  template<size_t _Np, typename _Union>
> +    constexpr decltype(auto)
> +    __construct_n(_Union& __u) noexcept
> +    {
> +      if constexpr (_Np == 0)
> +       return &__u._M_first;
> +      else if constexpr (_Np == 1)
> +       {
> +         std::_Construct(&__u._M_rest);
> +         return &__u._M_rest._M_first;
> +       }
> +      else if constexpr (_Np == 2)
> +       {
> +         std::_Construct(&__u._M_rest);
> +         std::_Construct(&__u._M_rest._M_rest);
> +         return &__u._M_rest._M_rest._M_first;
> +       }
> +      else
> +       {
> +         std::_Construct(&__u._M_rest);
> +         std::_Construct(&__u._M_rest._M_rest);
> +         std::_Construct(&__u._M_rest._M_rest._M_rest);
> +         return __variant::__construct_n<_Np -
> 3>(__u._M_rest._M_rest._M_rest);
> +       }
> +    }
> +
>    template<typename... _Types>
>      struct _Traits
>      {
> @@ -536,8 +563,9 @@ namespace __variant
>      __emplace(_Variant_storage<_Triv, _Types...>& __v, _Args&&... __args)
>      {
>        __v._M_reset();
> -      auto* __addr = std::__addressof(__variant::__get_n<_Np>(__v._M_u));
> -      std::_Construct(__addr, std::forward<_Args>(__args)...);
> +      auto* __addr = __variant::__construct_n<_Np>(__v._M_u);
> +      std::_Construct(__addr, in_place_index<0>,
> +                     std::forward<_Args>(__args)...);
>        // Construction didn't throw, so can set the new index now:
>        __v._M_index = _Np;
>      }
> --
> 2.41.0
>
>

Re: [PATCH] libstdc++: Ensure active union member is correctly set

[Jonathan Wakely]

On Sat, 23 Sept 2023 at 08:30, Nathaniel Shead via Libstdc++
<libstdc++@gcc.gnu.org> wrote:
>
> On Sat, Sep 23, 2023 at 07:40:48AM +0100, Jonathan Wakely wrote:
> > On Sat, 23 Sept 2023, 01:39 Nathaniel Shead via Libstdc++, <
> > libstdc++@gcc.gnu.org> wrote:
> >
> > > Now that bootstrap has finished, I have gotten regressions in the
> > > following libstdc++ tests:
> > >
> > > Running libstdc++:libstdc++-dg/conformance.exp ...
> > > FAIL: 20_util/bitset/access/constexpr.cc -std=gnu++23 (test for excess
> > > errors)
> > > FAIL: 20_util/bitset/access/constexpr.cc -std=gnu++26 (test for excess
> > > errors)
> > > FAIL: 20_util/variant/constexpr.cc -std=gnu++20 (test for excess errors)
> > > FAIL: 20_util/variant/constexpr.cc -std=gnu++26 (test for excess errors)
> > > FAIL: 21_strings/basic_string/cons/char/constexpr.cc -std=gnu++20 (test
> > > for excess errors)
> > > FAIL: 21_strings/basic_string/cons/char/constexpr.cc -std=gnu++26 (test
> > > for excess errors)
> > > FAIL: 21_strings/basic_string/cons/wchar_t/constexpr.cc -std=gnu++20 (test
> > > for excess errors)
> > > FAIL: 21_strings/basic_string/cons/wchar_t/constexpr.cc -std=gnu++26 (test
> > > for excess errors)
> > > FAIL: 21_strings/basic_string/modifiers/swap/constexpr-wchar_t.cc
> > > -std=gnu++20 (test for excess errors)
> > > FAIL: 21_strings/basic_string/modifiers/swap/constexpr-wchar_t.cc
> > > -std=gnu++26 (test for excess errors)
> > > FAIL: 21_strings/basic_string/modifiers/swap/constexpr.cc -std=gnu++20
> > > (test for excess errors)
> > > FAIL: 21_strings/basic_string/modifiers/swap/constexpr.cc -std=gnu++26
> > > (test for excess errors)
> > > FAIL: std/ranges/adaptors/join_with/1.cc -std=gnu++23 (test for excess
> > > errors)
> > > UNRESOLVED: std/ranges/adaptors/join_with/1.cc -std=gnu++23 compilation
> > > failed to produce executable
> > > FAIL: std/ranges/adaptors/join_with/1.cc -std=gnu++26 (test for excess
> > > errors)
> > > UNRESOLVED: std/ranges/adaptors/join_with/1.cc -std=gnu++26 compilation
> > > failed to produce executable
> > >
> > > On investigation though it looks like the issue might be with libstdc++
> > > rather than the patch itself; running the failing tests using clang with
> > > libstdc++ also produces similar errors, and my reading of the code
> > > suggests that this is correct.
> > >
> > > What's the way forward here? Should I look at creating a patch to fix
> > > the libstdc++ issues before resubmitting this patch for the C++
> > > frontend? Or should I submit a version of this patch without the
> > > `std::construct_at` changes and wait till libstdc++ gets fixed for that?
> > >
> >
> > I think we should fix libstdc++. There are probably only a few places that
> > need a fix, which cause all those failures.
> >
> > I can help with those fixes. I'll look into it after the weekend.
> >
>
> Thanks. I did end up getting a chance to look at it earlier today, and
> with the following patch I had no regressions when applying the frontend
> changes. Bootstrapped and regtested on x86_64-pc-linux-gnu.
>
> -- >8 --
>
> This patch ensures that the union members for std::string and
> std::variant are always properly set when a change occurs.
>
> libstdc++-v3/ChangeLog:
>
>         * include/bits/basic_string.h: (basic_string(basic_string&&)):
>         Activate _M_local_buf when needed.
>         (basic_string(basic_string&&, const _Alloc&)): Likewise.
>         * include/bits/basic_string.tcc: (basic_string::swap): Likewise.
>         * include/std/variant: (__detail::__variant::__construct_n): New.
>         (__detail::_variant::__emplace): Use __construct_n.
>
> Signed-off-by: Nathaniel Shead <nathanieloshead@gmail.com>
> ---
>  libstdc++-v3/include/bits/basic_string.h   |  7 +++--
>  libstdc++-v3/include/bits/basic_string.tcc |  8 +++---
>  libstdc++-v3/include/std/variant           | 32 ++++++++++++++++++++--
>  3 files changed, 38 insertions(+), 9 deletions(-)
>
> diff --git a/libstdc++-v3/include/bits/basic_string.h b/libstdc++-v3/include/bits/basic_string.h
> index 09fd62afa66..7c342879827 100644
> --- a/libstdc++-v3/include/bits/basic_string.h
> +++ b/libstdc++-v3/include/bits/basic_string.h
> @@ -678,7 +678,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
>        {
>         if (__str._M_is_local())
>           {
> -           traits_type::copy(_M_local_buf, __str._M_local_buf,
> +           traits_type::copy(_M_use_local_data(), __str._M_local_buf,
>                               __str.length() + 1);
>           }
>         else
> @@ -691,7 +691,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
>         // basic_stringbuf relies on writing into unallocated capacity so
>         // we mess up the contents if we put a '\0' in the string.
>         _M_length(__str.length());
> -       __str._M_data(__str._M_local_data());
> +       __str._M_data(__str._M_use_local_data());
>         __str._M_set_length(0);
>        }
>
> @@ -717,6 +717,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
>        {
>         if (__str._M_is_local())
>           {
> +           _M_use_local_data();

Lets add a cast to void to make it clear we're intentionally
discarding the return value here, and only calling it for its
constexpr "side effects":

    (void) _M_use_local_data();


>             traits_type::copy(_M_local_buf, __str._M_local_buf,
>                               __str.length() + 1);
>             _M_length(__str.length());
> @@ -728,7 +729,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
>             _M_data(__str._M_data());
>             _M_length(__str.length());
>             _M_capacity(__str._M_allocated_capacity);
> -           __str._M_data(__str._M_local_buf);
> +           __str._M_data(__str._M_use_local_data());
>             __str._M_set_length(0);
>           }
>         else
> diff --git a/libstdc++-v3/include/bits/basic_string.tcc b/libstdc++-v3/include/bits/basic_string.tcc
> index 104a517f794..ee6e57da555 100644
> --- a/libstdc++-v3/include/bits/basic_string.tcc
> +++ b/libstdc++-v3/include/bits/basic_string.tcc
> @@ -79,7 +79,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>               }
>             else if (__s.length())
>               {
> -               traits_type::copy(_M_local_buf, __s._M_local_buf,
> +               traits_type::copy(_M_use_local_data(), __s._M_local_buf,

I think we should call _M_use_local_data() before calling
traits_type::copy, as you did for
basic_string(basic_string&& __str, const _Alloc& __a)
above.

The problem is that _M_use_local_data() returns the allocator's
pointer type, but traits_type::copy expects char_type*, and those
might not be the same type.
So:

@@ -79,6 +79,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
             }
           else if (__s.length())
             {
+               (void) _M_use_local_data();
               traits_type::copy(_M_local_buf, __s._M_local_buf,
                                 __s.length() + 1);
               _M_length(__s.length());



>                                   __s.length() + 1);
>                 _M_length(__s.length());
>                 __s._M_set_length(0);
> @@ -87,7 +87,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>               }
>             else if (length())
>               {
> -               traits_type::copy(__s._M_local_buf, _M_local_buf,
> +               traits_type::copy(__s._M_use_local_data(), _M_local_buf,

Same here, for __s._M_use_local_data()

>                                   length() + 1);
>                 __s._M_length(length());
>                 _M_set_length(0);
> @@ -97,7 +97,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>         else
>           {
>             const size_type __tmp_capacity = __s._M_allocated_capacity;
> -           traits_type::copy(__s._M_local_buf, _M_local_buf,
> +           traits_type::copy(__s._M_use_local_data(), _M_local_buf,

And again.

>                               length() + 1);
>             _M_data(__s._M_data());
>             __s._M_data(__s._M_local_buf);
> @@ -108,7 +108,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>           const size_type __tmp_capacity = _M_allocated_capacity;
>           if (__s._M_is_local())
>             {
> -             traits_type::copy(_M_local_buf, __s._M_local_buf,
> +             traits_type::copy(_M_use_local_data(), __s._M_local_buf,

And again.

>                                 __s.length() + 1);
>               __s._M_data(_M_data());
>               _M_data(_M_local_buf);
> diff --git a/libstdc++-v3/include/std/variant b/libstdc++-v3/include/std/variant
> index c0e41740dcf..7f24e760bb1 100644
> --- a/libstdc++-v3/include/std/variant
> +++ b/libstdc++-v3/include/std/variant
> @@ -320,6 +320,33 @@ namespace __variant
>      __get(_Variant&& __v) noexcept
>      { return __variant::__get_n<_Np>(std::forward<_Variant>(__v)._M_u); }
>
> +  // Gets the _Uninitialized to construct into for __u.
> +  template<size_t _Np, typename _Union>
> +    constexpr decltype(auto)
> +    __construct_n(_Union& __u) noexcept
> +    {
> +      if constexpr (_Np == 0)
> +       return &__u._M_first;
> +      else if constexpr (_Np == 1)
> +       {
> +         std::_Construct(&__u._M_rest);
> +         return &__u._M_rest._M_first;
> +       }
> +      else if constexpr (_Np == 2)
> +       {
> +         std::_Construct(&__u._M_rest);
> +         std::_Construct(&__u._M_rest._M_rest);
> +         return &__u._M_rest._M_rest._M_first;
> +       }
> +      else
> +       {
> +         std::_Construct(&__u._M_rest);
> +         std::_Construct(&__u._M_rest._M_rest);
> +         std::_Construct(&__u._M_rest._M_rest._M_rest);
> +         return __variant::__construct_n<_Np - 3>(__u._M_rest._M_rest._M_rest);
> +       }
> +    }

This is nice, thanks for optimizing it to reduce the recursion depth.

> +
>    template<typename... _Types>
>      struct _Traits
>      {
> @@ -536,8 +563,9 @@ namespace __variant
>      __emplace(_Variant_storage<_Triv, _Types...>& __v, _Args&&... __args)
>      {
>        __v._M_reset();
> -      auto* __addr = std::__addressof(__variant::__get_n<_Np>(__v._M_u));
> -      std::_Construct(__addr, std::forward<_Args>(__args)...);
> +      auto* __addr = __variant::__construct_n<_Np>(__v._M_u);
> +      std::_Construct(__addr, in_place_index<0>,
> +                     std::forward<_Args>(__args)...);
>        // Construction didn't throw, so can set the new index now:
>        __v._M_index = _Np;
>      }
> --
> 2.41.0
>

Re: [PATCH] libstdc++: Ensure active union member is correctly set

[Nathaniel Shead]

On Wed, Sep 27, 2023 at 03:13:35PM +0100, Jonathan Wakely wrote:
> On Sat, 23 Sept 2023 at 08:30, Nathaniel Shead via Libstdc++
> <libstdc++@gcc.gnu.org> wrote:
> >
> > On Sat, Sep 23, 2023 at 07:40:48AM +0100, Jonathan Wakely wrote:
> > > On Sat, 23 Sept 2023, 01:39 Nathaniel Shead via Libstdc++, <
> > > libstdc++@gcc.gnu.org> wrote:
> > >
> > > > Now that bootstrap has finished, I have gotten regressions in the
> > > > following libstdc++ tests:
> > > >
> > > > Running libstdc++:libstdc++-dg/conformance.exp ...
> > > > FAIL: 20_util/bitset/access/constexpr.cc -std=gnu++23 (test for excess
> > > > errors)
> > > > FAIL: 20_util/bitset/access/constexpr.cc -std=gnu++26 (test for excess
> > > > errors)
> > > > FAIL: 20_util/variant/constexpr.cc -std=gnu++20 (test for excess errors)
> > > > FAIL: 20_util/variant/constexpr.cc -std=gnu++26 (test for excess errors)
> > > > FAIL: 21_strings/basic_string/cons/char/constexpr.cc -std=gnu++20 (test
> > > > for excess errors)
> > > > FAIL: 21_strings/basic_string/cons/char/constexpr.cc -std=gnu++26 (test
> > > > for excess errors)
> > > > FAIL: 21_strings/basic_string/cons/wchar_t/constexpr.cc -std=gnu++20 (test
> > > > for excess errors)
> > > > FAIL: 21_strings/basic_string/cons/wchar_t/constexpr.cc -std=gnu++26 (test
> > > > for excess errors)
> > > > FAIL: 21_strings/basic_string/modifiers/swap/constexpr-wchar_t.cc
> > > > -std=gnu++20 (test for excess errors)
> > > > FAIL: 21_strings/basic_string/modifiers/swap/constexpr-wchar_t.cc
> > > > -std=gnu++26 (test for excess errors)
> > > > FAIL: 21_strings/basic_string/modifiers/swap/constexpr.cc -std=gnu++20
> > > > (test for excess errors)
> > > > FAIL: 21_strings/basic_string/modifiers/swap/constexpr.cc -std=gnu++26
> > > > (test for excess errors)
> > > > FAIL: std/ranges/adaptors/join_with/1.cc -std=gnu++23 (test for excess
> > > > errors)
> > > > UNRESOLVED: std/ranges/adaptors/join_with/1.cc -std=gnu++23 compilation
> > > > failed to produce executable
> > > > FAIL: std/ranges/adaptors/join_with/1.cc -std=gnu++26 (test for excess
> > > > errors)
> > > > UNRESOLVED: std/ranges/adaptors/join_with/1.cc -std=gnu++26 compilation
> > > > failed to produce executable
> > > >
> > > > On investigation though it looks like the issue might be with libstdc++
> > > > rather than the patch itself; running the failing tests using clang with
> > > > libstdc++ also produces similar errors, and my reading of the code
> > > > suggests that this is correct.
> > > >
> > > > What's the way forward here? Should I look at creating a patch to fix
> > > > the libstdc++ issues before resubmitting this patch for the C++
> > > > frontend? Or should I submit a version of this patch without the
> > > > `std::construct_at` changes and wait till libstdc++ gets fixed for that?
> > > >
> > >
> > > I think we should fix libstdc++. There are probably only a few places that
> > > need a fix, which cause all those failures.
> > >
> > > I can help with those fixes. I'll look into it after the weekend.
> > >
> >
> > Thanks. I did end up getting a chance to look at it earlier today, and
> > with the following patch I had no regressions when applying the frontend
> > changes. Bootstrapped and regtested on x86_64-pc-linux-gnu.
> >
> > -- >8 --
> >
> > This patch ensures that the union members for std::string and
> > std::variant are always properly set when a change occurs.
> >
> > libstdc++-v3/ChangeLog:
> >
> >         * include/bits/basic_string.h: (basic_string(basic_string&&)):
> >         Activate _M_local_buf when needed.
> >         (basic_string(basic_string&&, const _Alloc&)): Likewise.
> >         * include/bits/basic_string.tcc: (basic_string::swap): Likewise.
> >         * include/std/variant: (__detail::__variant::__construct_n): New.
> >         (__detail::_variant::__emplace): Use __construct_n.
> >
> > Signed-off-by: Nathaniel Shead <nathanieloshead@gmail.com>
> > ---
> >  libstdc++-v3/include/bits/basic_string.h   |  7 +++--
> >  libstdc++-v3/include/bits/basic_string.tcc |  8 +++---
> >  libstdc++-v3/include/std/variant           | 32 ++++++++++++++++++++--
> >  3 files changed, 38 insertions(+), 9 deletions(-)
> >
> > diff --git a/libstdc++-v3/include/bits/basic_string.h b/libstdc++-v3/include/bits/basic_string.h
> > index 09fd62afa66..7c342879827 100644
> > --- a/libstdc++-v3/include/bits/basic_string.h
> > +++ b/libstdc++-v3/include/bits/basic_string.h
> > @@ -678,7 +678,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
> >        {
> >         if (__str._M_is_local())
> >           {
> > -           traits_type::copy(_M_local_buf, __str._M_local_buf,
> > +           traits_type::copy(_M_use_local_data(), __str._M_local_buf,
> >                               __str.length() + 1);
> >           }
> >         else
> > @@ -691,7 +691,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
> >         // basic_stringbuf relies on writing into unallocated capacity so
> >         // we mess up the contents if we put a '\0' in the string.
> >         _M_length(__str.length());
> > -       __str._M_data(__str._M_local_data());
> > +       __str._M_data(__str._M_use_local_data());
> >         __str._M_set_length(0);
> >        }
> >
> > @@ -717,6 +717,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
> >        {
> >         if (__str._M_is_local())
> >           {
> > +           _M_use_local_data();
> 
> Lets add a cast to void to make it clear we're intentionally
> discarding the return value here, and only calling it for its
> constexpr "side effects":
> 
>     (void) _M_use_local_data();
> 
> 
> >             traits_type::copy(_M_local_buf, __str._M_local_buf,
> >                               __str.length() + 1);
> >             _M_length(__str.length());
> > @@ -728,7 +729,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
> >             _M_data(__str._M_data());
> >             _M_length(__str.length());
> >             _M_capacity(__str._M_allocated_capacity);
> > -           __str._M_data(__str._M_local_buf);
> > +           __str._M_data(__str._M_use_local_data());
> >             __str._M_set_length(0);
> >           }
> >         else
> > diff --git a/libstdc++-v3/include/bits/basic_string.tcc b/libstdc++-v3/include/bits/basic_string.tcc
> > index 104a517f794..ee6e57da555 100644
> > --- a/libstdc++-v3/include/bits/basic_string.tcc
> > +++ b/libstdc++-v3/include/bits/basic_string.tcc
> > @@ -79,7 +79,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> >               }
> >             else if (__s.length())
> >               {
> > -               traits_type::copy(_M_local_buf, __s._M_local_buf,
> > +               traits_type::copy(_M_use_local_data(), __s._M_local_buf,
> 
> I think we should call _M_use_local_data() before calling
> traits_type::copy, as you did for
> basic_string(basic_string&& __str, const _Alloc& __a)
> above.
> 
> The problem is that _M_use_local_data() returns the allocator's
> pointer type, but traits_type::copy expects char_type*, and those
> might not be the same type.
> So:
> 
> @@ -79,6 +79,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>              }
>            else if (__s.length())
>              {
> +               (void) _M_use_local_data();
>                traits_type::copy(_M_local_buf, __s._M_local_buf,
>                                  __s.length() + 1);
>                _M_length(__s.length());
> 
> 
> 
> >                                   __s.length() + 1);
> >                 _M_length(__s.length());
> >                 __s._M_set_length(0);
> > @@ -87,7 +87,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> >               }
> >             else if (length())
> >               {
> > -               traits_type::copy(__s._M_local_buf, _M_local_buf,
> > +               traits_type::copy(__s._M_use_local_data(), _M_local_buf,
> 
> Same here, for __s._M_use_local_data()
> 
> >                                   length() + 1);
> >                 __s._M_length(length());
> >                 _M_set_length(0);
> > @@ -97,7 +97,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> >         else
> >           {
> >             const size_type __tmp_capacity = __s._M_allocated_capacity;
> > -           traits_type::copy(__s._M_local_buf, _M_local_buf,
> > +           traits_type::copy(__s._M_use_local_data(), _M_local_buf,
> 
> And again.
> 
> >                               length() + 1);
> >             _M_data(__s._M_data());
> >             __s._M_data(__s._M_local_buf);
> > @@ -108,7 +108,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> >           const size_type __tmp_capacity = _M_allocated_capacity;
> >           if (__s._M_is_local())
> >             {
> > -             traits_type::copy(_M_local_buf, __s._M_local_buf,
> > +             traits_type::copy(_M_use_local_data(), __s._M_local_buf,
> 
> And again.
> 
> >                                 __s.length() + 1);
> >               __s._M_data(_M_data());
> >               _M_data(_M_local_buf);
> > diff --git a/libstdc++-v3/include/std/variant b/libstdc++-v3/include/std/variant
> > index c0e41740dcf..7f24e760bb1 100644
> > --- a/libstdc++-v3/include/std/variant
> > +++ b/libstdc++-v3/include/std/variant
> > @@ -320,6 +320,33 @@ namespace __variant
> >      __get(_Variant&& __v) noexcept
> >      { return __variant::__get_n<_Np>(std::forward<_Variant>(__v)._M_u); }
> >
> > +  // Gets the _Uninitialized to construct into for __u.
> > +  template<size_t _Np, typename _Union>
> > +    constexpr decltype(auto)
> > +    __construct_n(_Union& __u) noexcept
> > +    {
> > +      if constexpr (_Np == 0)
> > +       return &__u._M_first;
> > +      else if constexpr (_Np == 1)
> > +       {
> > +         std::_Construct(&__u._M_rest);
> > +         return &__u._M_rest._M_first;
> > +       }
> > +      else if constexpr (_Np == 2)
> > +       {
> > +         std::_Construct(&__u._M_rest);
> > +         std::_Construct(&__u._M_rest._M_rest);
> > +         return &__u._M_rest._M_rest._M_first;
> > +       }
> > +      else
> > +       {
> > +         std::_Construct(&__u._M_rest);
> > +         std::_Construct(&__u._M_rest._M_rest);
> > +         std::_Construct(&__u._M_rest._M_rest._M_rest);
> > +         return __variant::__construct_n<_Np - 3>(__u._M_rest._M_rest._M_rest);
> > +       }
> > +    }
> 
> This is nice, thanks for optimizing it to reduce the recursion depth.
> 
> > +
> >    template<typename... _Types>
> >      struct _Traits
> >      {
> > @@ -536,8 +563,9 @@ namespace __variant
> >      __emplace(_Variant_storage<_Triv, _Types...>& __v, _Args&&... __args)
> >      {
> >        __v._M_reset();
> > -      auto* __addr = std::__addressof(__variant::__get_n<_Np>(__v._M_u));
> > -      std::_Construct(__addr, std::forward<_Args>(__args)...);
> > +      auto* __addr = __variant::__construct_n<_Np>(__v._M_u);
> > +      std::_Construct(__addr, in_place_index<0>,
> > +                     std::forward<_Args>(__args)...);
> >        // Construction didn't throw, so can set the new index now:
> >        __v._M_index = _Np;
> >      }
> > --
> > 2.41.0
> >
> 

Thanks for the comments, here's an updated version of the patch.
Bootstrapped and regtested on x86_64-pc-linux-gnu.

I'll note that there are some existing calls to `_M_use_local_data()`
already used only for their side effects without a cast to void, e.g.

      /**
       *  @brief  Default constructor creates an empty string.
       */
      _GLIBCXX20_CONSTEXPR
      basic_string()
      _GLIBCXX_NOEXCEPT_IF(is_nothrow_default_constructible<_Alloc>::value)
      : _M_dataplus(_M_local_data())
      {
	_M_use_local_data();
	_M_set_length(0);
      }

I haven't updated these, but should this be changed for consistency?

-- >8 --

This patch ensures that the union members for std::string and
std::variant are always properly set when a change occurs.

libstdc++-v3/ChangeLog:

	* include/bits/basic_string.h: (basic_string(basic_string&&)):
	Activate _M_local_buf when needed.
	(basic_string(basic_string&&, const _Alloc&)): Likewise.
	* include/bits/basic_string.tcc: (basic_string::swap): Likewise.
	* include/std/variant: (__detail::__variant::__construct_n): New.
	(__detail::__variant::__emplace): Use __construct_n.

Signed-off-by: Nathaniel Shead <nathanieloshead@gmail.com>
---
 libstdc++-v3/include/bits/basic_string.h   |  6 ++--
 libstdc++-v3/include/bits/basic_string.tcc |  4 +++
 libstdc++-v3/include/std/variant           | 32 ++++++++++++++++++++--
 3 files changed, 38 insertions(+), 4 deletions(-)

diff --git a/libstdc++-v3/include/bits/basic_string.h b/libstdc++-v3/include/bits/basic_string.h
index 09fd62afa66..4f94cd967cf 100644
--- a/libstdc++-v3/include/bits/basic_string.h
+++ b/libstdc++-v3/include/bits/basic_string.h
@@ -678,6 +678,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
       {
 	if (__str._M_is_local())
 	  {
+	    (void)_M_use_local_data();
 	    traits_type::copy(_M_local_buf, __str._M_local_buf,
 			      __str.length() + 1);
 	  }
@@ -691,7 +692,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
 	// basic_stringbuf relies on writing into unallocated capacity so
 	// we mess up the contents if we put a '\0' in the string.
 	_M_length(__str.length());
-	__str._M_data(__str._M_local_data());
+	__str._M_data(__str._M_use_local_data());
 	__str._M_set_length(0);
       }
 
@@ -717,6 +718,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
       {
 	if (__str._M_is_local())
 	  {
+	    (void)_M_use_local_data();
 	    traits_type::copy(_M_local_buf, __str._M_local_buf,
 			      __str.length() + 1);
 	    _M_length(__str.length());
@@ -728,7 +730,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
 	    _M_data(__str._M_data());
 	    _M_length(__str.length());
 	    _M_capacity(__str._M_allocated_capacity);
-	    __str._M_data(__str._M_local_buf);
+	    __str._M_data(__str._M_use_local_data());
 	    __str._M_set_length(0);
 	  }
 	else
diff --git a/libstdc++-v3/include/bits/basic_string.tcc b/libstdc++-v3/include/bits/basic_string.tcc
index 104a517f794..4bc98f2aea7 100644
--- a/libstdc++-v3/include/bits/basic_string.tcc
+++ b/libstdc++-v3/include/bits/basic_string.tcc
@@ -79,6 +79,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	      }
 	    else if (__s.length())
 	      {
+		(void)_M_use_local_data();
 		traits_type::copy(_M_local_buf, __s._M_local_buf,
 				  __s.length() + 1);
 		_M_length(__s.length());
@@ -87,6 +88,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	      }
 	    else if (length())
 	      {
+		(void)__s._M_use_local_data();
 		traits_type::copy(__s._M_local_buf, _M_local_buf,
 				  length() + 1);
 		__s._M_length(length());
@@ -97,6 +99,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	else
 	  {
 	    const size_type __tmp_capacity = __s._M_allocated_capacity;
+	    (void)__s._M_use_local_data();
 	    traits_type::copy(__s._M_local_buf, _M_local_buf,
 			      length() + 1);
 	    _M_data(__s._M_data());
@@ -108,6 +111,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	  const size_type __tmp_capacity = _M_allocated_capacity;
 	  if (__s._M_is_local())
 	    {
+	      (void)_M_use_local_data();
 	      traits_type::copy(_M_local_buf, __s._M_local_buf,
 				__s.length() + 1);
 	      __s._M_data(_M_data());
diff --git a/libstdc++-v3/include/std/variant b/libstdc++-v3/include/std/variant
index c0e41740dcf..7f24e760bb1 100644
--- a/libstdc++-v3/include/std/variant
+++ b/libstdc++-v3/include/std/variant
@@ -320,6 +320,33 @@ namespace __variant
     __get(_Variant&& __v) noexcept
     { return __variant::__get_n<_Np>(std::forward<_Variant>(__v)._M_u); }
 
+  // Gets the _Uninitialized to construct into for __u.
+  template<size_t _Np, typename _Union>
+    constexpr decltype(auto)
+    __construct_n(_Union& __u) noexcept
+    {
+      if constexpr (_Np == 0)
+	return &__u._M_first;
+      else if constexpr (_Np == 1)
+	{
+	  std::_Construct(&__u._M_rest);
+	  return &__u._M_rest._M_first;
+	}
+      else if constexpr (_Np == 2)
+	{
+	  std::_Construct(&__u._M_rest);
+	  std::_Construct(&__u._M_rest._M_rest);
+	  return &__u._M_rest._M_rest._M_first;
+	}
+      else
+	{
+	  std::_Construct(&__u._M_rest);
+	  std::_Construct(&__u._M_rest._M_rest);
+	  std::_Construct(&__u._M_rest._M_rest._M_rest);
+	  return __variant::__construct_n<_Np - 3>(__u._M_rest._M_rest._M_rest);
+	}
+    }
+
   template<typename... _Types>
     struct _Traits
     {
@@ -536,8 +563,9 @@ namespace __variant
     __emplace(_Variant_storage<_Triv, _Types...>& __v, _Args&&... __args)
     {
       __v._M_reset();
-      auto* __addr = std::__addressof(__variant::__get_n<_Np>(__v._M_u));
-      std::_Construct(__addr, std::forward<_Args>(__args)...);
+      auto* __addr = __variant::__construct_n<_Np>(__v._M_u);
+      std::_Construct(__addr, in_place_index<0>,
+		      std::forward<_Args>(__args)...);
       // Construction didn't throw, so can set the new index now:
       __v._M_index = _Np;
     }
-- 
2.41.0

Re: [PATCH] libstdc++: Ensure active union member is correctly set

[Jonathan Wakely]

On Fri, 29 Sept 2023 at 00:25, Nathaniel Shead
<nathanieloshead@gmail.com> wrote:
>
> On Wed, Sep 27, 2023 at 03:13:35PM +0100, Jonathan Wakely wrote:
> > On Sat, 23 Sept 2023 at 08:30, Nathaniel Shead via Libstdc++
> > <libstdc++@gcc.gnu.org> wrote:
> > >
> > > On Sat, Sep 23, 2023 at 07:40:48AM +0100, Jonathan Wakely wrote:
> > > > On Sat, 23 Sept 2023, 01:39 Nathaniel Shead via Libstdc++, <
> > > > libstdc++@gcc.gnu.org> wrote:
> > > >
> > > > > Now that bootstrap has finished, I have gotten regressions in the
> > > > > following libstdc++ tests:
> > > > >
> > > > > Running libstdc++:libstdc++-dg/conformance.exp ...
> > > > > FAIL: 20_util/bitset/access/constexpr.cc -std=gnu++23 (test for excess
> > > > > errors)
> > > > > FAIL: 20_util/bitset/access/constexpr.cc -std=gnu++26 (test for excess
> > > > > errors)
> > > > > FAIL: 20_util/variant/constexpr.cc -std=gnu++20 (test for excess errors)
> > > > > FAIL: 20_util/variant/constexpr.cc -std=gnu++26 (test for excess errors)
> > > > > FAIL: 21_strings/basic_string/cons/char/constexpr.cc -std=gnu++20 (test
> > > > > for excess errors)
> > > > > FAIL: 21_strings/basic_string/cons/char/constexpr.cc -std=gnu++26 (test
> > > > > for excess errors)
> > > > > FAIL: 21_strings/basic_string/cons/wchar_t/constexpr.cc -std=gnu++20 (test
> > > > > for excess errors)
> > > > > FAIL: 21_strings/basic_string/cons/wchar_t/constexpr.cc -std=gnu++26 (test
> > > > > for excess errors)
> > > > > FAIL: 21_strings/basic_string/modifiers/swap/constexpr-wchar_t.cc
> > > > > -std=gnu++20 (test for excess errors)
> > > > > FAIL: 21_strings/basic_string/modifiers/swap/constexpr-wchar_t.cc
> > > > > -std=gnu++26 (test for excess errors)
> > > > > FAIL: 21_strings/basic_string/modifiers/swap/constexpr.cc -std=gnu++20
> > > > > (test for excess errors)
> > > > > FAIL: 21_strings/basic_string/modifiers/swap/constexpr.cc -std=gnu++26
> > > > > (test for excess errors)
> > > > > FAIL: std/ranges/adaptors/join_with/1.cc -std=gnu++23 (test for excess
> > > > > errors)
> > > > > UNRESOLVED: std/ranges/adaptors/join_with/1.cc -std=gnu++23 compilation
> > > > > failed to produce executable
> > > > > FAIL: std/ranges/adaptors/join_with/1.cc -std=gnu++26 (test for excess
> > > > > errors)
> > > > > UNRESOLVED: std/ranges/adaptors/join_with/1.cc -std=gnu++26 compilation
> > > > > failed to produce executable
> > > > >
> > > > > On investigation though it looks like the issue might be with libstdc++
> > > > > rather than the patch itself; running the failing tests using clang with
> > > > > libstdc++ also produces similar errors, and my reading of the code
> > > > > suggests that this is correct.
> > > > >
> > > > > What's the way forward here? Should I look at creating a patch to fix
> > > > > the libstdc++ issues before resubmitting this patch for the C++
> > > > > frontend? Or should I submit a version of this patch without the
> > > > > `std::construct_at` changes and wait till libstdc++ gets fixed for that?
> > > > >
> > > >
> > > > I think we should fix libstdc++. There are probably only a few places that
> > > > need a fix, which cause all those failures.
> > > >
> > > > I can help with those fixes. I'll look into it after the weekend.
> > > >
> > >
> > > Thanks. I did end up getting a chance to look at it earlier today, and
> > > with the following patch I had no regressions when applying the frontend
> > > changes. Bootstrapped and regtested on x86_64-pc-linux-gnu.
> > >
> > > -- >8 --
> > >
> > > This patch ensures that the union members for std::string and
> > > std::variant are always properly set when a change occurs.
> > >
> > > libstdc++-v3/ChangeLog:
> > >
> > >         * include/bits/basic_string.h: (basic_string(basic_string&&)):
> > >         Activate _M_local_buf when needed.
> > >         (basic_string(basic_string&&, const _Alloc&)): Likewise.
> > >         * include/bits/basic_string.tcc: (basic_string::swap): Likewise.
> > >         * include/std/variant: (__detail::__variant::__construct_n): New.
> > >         (__detail::_variant::__emplace): Use __construct_n.
> > >
> > > Signed-off-by: Nathaniel Shead <nathanieloshead@gmail.com>
> > > ---
> > >  libstdc++-v3/include/bits/basic_string.h   |  7 +++--
> > >  libstdc++-v3/include/bits/basic_string.tcc |  8 +++---
> > >  libstdc++-v3/include/std/variant           | 32 ++++++++++++++++++++--
> > >  3 files changed, 38 insertions(+), 9 deletions(-)
> > >
> > > diff --git a/libstdc++-v3/include/bits/basic_string.h b/libstdc++-v3/include/bits/basic_string.h
> > > index 09fd62afa66..7c342879827 100644
> > > --- a/libstdc++-v3/include/bits/basic_string.h
> > > +++ b/libstdc++-v3/include/bits/basic_string.h
> > > @@ -678,7 +678,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
> > >        {
> > >         if (__str._M_is_local())
> > >           {
> > > -           traits_type::copy(_M_local_buf, __str._M_local_buf,
> > > +           traits_type::copy(_M_use_local_data(), __str._M_local_buf,
> > >                               __str.length() + 1);
> > >           }
> > >         else
> > > @@ -691,7 +691,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
> > >         // basic_stringbuf relies on writing into unallocated capacity so
> > >         // we mess up the contents if we put a '\0' in the string.
> > >         _M_length(__str.length());
> > > -       __str._M_data(__str._M_local_data());
> > > +       __str._M_data(__str._M_use_local_data());
> > >         __str._M_set_length(0);
> > >        }
> > >
> > > @@ -717,6 +717,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
> > >        {
> > >         if (__str._M_is_local())
> > >           {
> > > +           _M_use_local_data();
> >
> > Lets add a cast to void to make it clear we're intentionally
> > discarding the return value here, and only calling it for its
> > constexpr "side effects":
> >
> >     (void) _M_use_local_data();
> >
> >
> > >             traits_type::copy(_M_local_buf, __str._M_local_buf,
> > >                               __str.length() + 1);
> > >             _M_length(__str.length());
> > > @@ -728,7 +729,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
> > >             _M_data(__str._M_data());
> > >             _M_length(__str.length());
> > >             _M_capacity(__str._M_allocated_capacity);
> > > -           __str._M_data(__str._M_local_buf);
> > > +           __str._M_data(__str._M_use_local_data());
> > >             __str._M_set_length(0);
> > >           }
> > >         else
> > > diff --git a/libstdc++-v3/include/bits/basic_string.tcc b/libstdc++-v3/include/bits/basic_string.tcc
> > > index 104a517f794..ee6e57da555 100644
> > > --- a/libstdc++-v3/include/bits/basic_string.tcc
> > > +++ b/libstdc++-v3/include/bits/basic_string.tcc
> > > @@ -79,7 +79,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> > >               }
> > >             else if (__s.length())
> > >               {
> > > -               traits_type::copy(_M_local_buf, __s._M_local_buf,
> > > +               traits_type::copy(_M_use_local_data(), __s._M_local_buf,
> >
> > I think we should call _M_use_local_data() before calling
> > traits_type::copy, as you did for
> > basic_string(basic_string&& __str, const _Alloc& __a)
> > above.
> >
> > The problem is that _M_use_local_data() returns the allocator's
> > pointer type, but traits_type::copy expects char_type*, and those
> > might not be the same type.
> > So:
> >
> > @@ -79,6 +79,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> >              }
> >            else if (__s.length())
> >              {
> > +               (void) _M_use_local_data();
> >                traits_type::copy(_M_local_buf, __s._M_local_buf,
> >                                  __s.length() + 1);
> >                _M_length(__s.length());
> >
> >
> >
> > >                                   __s.length() + 1);
> > >                 _M_length(__s.length());
> > >                 __s._M_set_length(0);
> > > @@ -87,7 +87,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> > >               }
> > >             else if (length())
> > >               {
> > > -               traits_type::copy(__s._M_local_buf, _M_local_buf,
> > > +               traits_type::copy(__s._M_use_local_data(), _M_local_buf,
> >
> > Same here, for __s._M_use_local_data()
> >
> > >                                   length() + 1);
> > >                 __s._M_length(length());
> > >                 _M_set_length(0);
> > > @@ -97,7 +97,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> > >         else
> > >           {
> > >             const size_type __tmp_capacity = __s._M_allocated_capacity;
> > > -           traits_type::copy(__s._M_local_buf, _M_local_buf,
> > > +           traits_type::copy(__s._M_use_local_data(), _M_local_buf,
> >
> > And again.
> >
> > >                               length() + 1);
> > >             _M_data(__s._M_data());
> > >             __s._M_data(__s._M_local_buf);
> > > @@ -108,7 +108,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> > >           const size_type __tmp_capacity = _M_allocated_capacity;
> > >           if (__s._M_is_local())
> > >             {
> > > -             traits_type::copy(_M_local_buf, __s._M_local_buf,
> > > +             traits_type::copy(_M_use_local_data(), __s._M_local_buf,
> >
> > And again.
> >
> > >                                 __s.length() + 1);
> > >               __s._M_data(_M_data());
> > >               _M_data(_M_local_buf);
> > > diff --git a/libstdc++-v3/include/std/variant b/libstdc++-v3/include/std/variant
> > > index c0e41740dcf..7f24e760bb1 100644
> > > --- a/libstdc++-v3/include/std/variant
> > > +++ b/libstdc++-v3/include/std/variant
> > > @@ -320,6 +320,33 @@ namespace __variant
> > >      __get(_Variant&& __v) noexcept
> > >      { return __variant::__get_n<_Np>(std::forward<_Variant>(__v)._M_u); }
> > >
> > > +  // Gets the _Uninitialized to construct into for __u.
> > > +  template<size_t _Np, typename _Union>
> > > +    constexpr decltype(auto)
> > > +    __construct_n(_Union& __u) noexcept
> > > +    {
> > > +      if constexpr (_Np == 0)
> > > +       return &__u._M_first;
> > > +      else if constexpr (_Np == 1)
> > > +       {
> > > +         std::_Construct(&__u._M_rest);
> > > +         return &__u._M_rest._M_first;
> > > +       }
> > > +      else if constexpr (_Np == 2)
> > > +       {
> > > +         std::_Construct(&__u._M_rest);
> > > +         std::_Construct(&__u._M_rest._M_rest);
> > > +         return &__u._M_rest._M_rest._M_first;
> > > +       }
> > > +      else
> > > +       {
> > > +         std::_Construct(&__u._M_rest);
> > > +         std::_Construct(&__u._M_rest._M_rest);
> > > +         std::_Construct(&__u._M_rest._M_rest._M_rest);
> > > +         return __variant::__construct_n<_Np - 3>(__u._M_rest._M_rest._M_rest);
> > > +       }
> > > +    }
> >
> > This is nice, thanks for optimizing it to reduce the recursion depth.
> >
> > > +
> > >    template<typename... _Types>
> > >      struct _Traits
> > >      {
> > > @@ -536,8 +563,9 @@ namespace __variant
> > >      __emplace(_Variant_storage<_Triv, _Types...>& __v, _Args&&... __args)
> > >      {
> > >        __v._M_reset();
> > > -      auto* __addr = std::__addressof(__variant::__get_n<_Np>(__v._M_u));
> > > -      std::_Construct(__addr, std::forward<_Args>(__args)...);
> > > +      auto* __addr = __variant::__construct_n<_Np>(__v._M_u);
> > > +      std::_Construct(__addr, in_place_index<0>,
> > > +                     std::forward<_Args>(__args)...);
> > >        // Construction didn't throw, so can set the new index now:
> > >        __v._M_index = _Np;
> > >      }
> > > --
> > > 2.41.0
> > >
> >
>
> Thanks for the comments, here's an updated version of the patch.
> Bootstrapped and regtested on x86_64-pc-linux-gnu.

Great, I'll get this committed today - thanks!

>
> I'll note that there are some existing calls to `_M_use_local_data()`
> already used only for their side effects without a cast to void, e.g.
>
>       /**
>        *  @brief  Default constructor creates an empty string.
>        */
>       _GLIBCXX20_CONSTEXPR
>       basic_string()
>       _GLIBCXX_NOEXCEPT_IF(is_nothrow_default_constructible<_Alloc>::value)
>       : _M_dataplus(_M_local_data())
>       {
>         _M_use_local_data();
>         _M_set_length(0);
>       }
>
> I haven't updated these, but should this be changed for consistency?

Yes, good idea. I can do that.

Thanks again for fixing these. I think this might fix some bug reports
about clang rejecting our std::string in constant expressions, so I'll
check those.


>
> -- >8 --
>
> This patch ensures that the union members for std::string and
> std::variant are always properly set when a change occurs.
>
> libstdc++-v3/ChangeLog:
>
>         * include/bits/basic_string.h: (basic_string(basic_string&&)):
>         Activate _M_local_buf when needed.
>         (basic_string(basic_string&&, const _Alloc&)): Likewise.
>         * include/bits/basic_string.tcc: (basic_string::swap): Likewise.
>         * include/std/variant: (__detail::__variant::__construct_n): New.
>         (__detail::__variant::__emplace): Use __construct_n.
>
> Signed-off-by: Nathaniel Shead <nathanieloshead@gmail.com>
> ---
>  libstdc++-v3/include/bits/basic_string.h   |  6 ++--
>  libstdc++-v3/include/bits/basic_string.tcc |  4 +++
>  libstdc++-v3/include/std/variant           | 32 ++++++++++++++++++++--
>  3 files changed, 38 insertions(+), 4 deletions(-)
>
> diff --git a/libstdc++-v3/include/bits/basic_string.h b/libstdc++-v3/include/bits/basic_string.h
> index 09fd62afa66..4f94cd967cf 100644
> --- a/libstdc++-v3/include/bits/basic_string.h
> +++ b/libstdc++-v3/include/bits/basic_string.h
> @@ -678,6 +678,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
>        {
>         if (__str._M_is_local())
>           {
> +           (void)_M_use_local_data();
>             traits_type::copy(_M_local_buf, __str._M_local_buf,
>                               __str.length() + 1);
>           }
> @@ -691,7 +692,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
>         // basic_stringbuf relies on writing into unallocated capacity so
>         // we mess up the contents if we put a '\0' in the string.
>         _M_length(__str.length());
> -       __str._M_data(__str._M_local_data());
> +       __str._M_data(__str._M_use_local_data());
>         __str._M_set_length(0);
>        }
>
> @@ -717,6 +718,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
>        {
>         if (__str._M_is_local())
>           {
> +           (void)_M_use_local_data();
>             traits_type::copy(_M_local_buf, __str._M_local_buf,
>                               __str.length() + 1);
>             _M_length(__str.length());
> @@ -728,7 +730,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
>             _M_data(__str._M_data());
>             _M_length(__str.length());
>             _M_capacity(__str._M_allocated_capacity);
> -           __str._M_data(__str._M_local_buf);
> +           __str._M_data(__str._M_use_local_data());
>             __str._M_set_length(0);
>           }
>         else
> diff --git a/libstdc++-v3/include/bits/basic_string.tcc b/libstdc++-v3/include/bits/basic_string.tcc
> index 104a517f794..4bc98f2aea7 100644
> --- a/libstdc++-v3/include/bits/basic_string.tcc
> +++ b/libstdc++-v3/include/bits/basic_string.tcc
> @@ -79,6 +79,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>               }
>             else if (__s.length())
>               {
> +               (void)_M_use_local_data();
>                 traits_type::copy(_M_local_buf, __s._M_local_buf,
>                                   __s.length() + 1);
>                 _M_length(__s.length());
> @@ -87,6 +88,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>               }
>             else if (length())
>               {
> +               (void)__s._M_use_local_data();
>                 traits_type::copy(__s._M_local_buf, _M_local_buf,
>                                   length() + 1);
>                 __s._M_length(length());
> @@ -97,6 +99,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>         else
>           {
>             const size_type __tmp_capacity = __s._M_allocated_capacity;
> +           (void)__s._M_use_local_data();
>             traits_type::copy(__s._M_local_buf, _M_local_buf,
>                               length() + 1);
>             _M_data(__s._M_data());
> @@ -108,6 +111,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>           const size_type __tmp_capacity = _M_allocated_capacity;
>           if (__s._M_is_local())
>             {
> +             (void)_M_use_local_data();
>               traits_type::copy(_M_local_buf, __s._M_local_buf,
>                                 __s.length() + 1);
>               __s._M_data(_M_data());
> diff --git a/libstdc++-v3/include/std/variant b/libstdc++-v3/include/std/variant
> index c0e41740dcf..7f24e760bb1 100644
> --- a/libstdc++-v3/include/std/variant
> +++ b/libstdc++-v3/include/std/variant
> @@ -320,6 +320,33 @@ namespace __variant
>      __get(_Variant&& __v) noexcept
>      { return __variant::__get_n<_Np>(std::forward<_Variant>(__v)._M_u); }
>
> +  // Gets the _Uninitialized to construct into for __u.
> +  template<size_t _Np, typename _Union>
> +    constexpr decltype(auto)
> +    __construct_n(_Union& __u) noexcept
> +    {
> +      if constexpr (_Np == 0)
> +       return &__u._M_first;
> +      else if constexpr (_Np == 1)
> +       {
> +         std::_Construct(&__u._M_rest);
> +         return &__u._M_rest._M_first;
> +       }
> +      else if constexpr (_Np == 2)
> +       {
> +         std::_Construct(&__u._M_rest);
> +         std::_Construct(&__u._M_rest._M_rest);
> +         return &__u._M_rest._M_rest._M_first;
> +       }
> +      else
> +       {
> +         std::_Construct(&__u._M_rest);
> +         std::_Construct(&__u._M_rest._M_rest);
> +         std::_Construct(&__u._M_rest._M_rest._M_rest);
> +         return __variant::__construct_n<_Np - 3>(__u._M_rest._M_rest._M_rest);
> +       }
> +    }
> +
>    template<typename... _Types>
>      struct _Traits
>      {
> @@ -536,8 +563,9 @@ namespace __variant
>      __emplace(_Variant_storage<_Triv, _Types...>& __v, _Args&&... __args)
>      {
>        __v._M_reset();
> -      auto* __addr = std::__addressof(__variant::__get_n<_Np>(__v._M_u));
> -      std::_Construct(__addr, std::forward<_Args>(__args)...);
> +      auto* __addr = __variant::__construct_n<_Np>(__v._M_u);
> +      std::_Construct(__addr, in_place_index<0>,
> +                     std::forward<_Args>(__args)...);
>        // Construction didn't throw, so can set the new index now:
>        __v._M_index = _Np;
>      }
> --
> 2.41.0
>

Re: [PATCH] libstdc++: Ensure active union member is correctly set

[Jonathan Wakely]

[-- Attachment #1: Type: text/plain, Size: 1688 bytes --]

On Fri, 29 Sept 2023 at 10:32, Jonathan Wakely <jwakely@redhat.com> wrote:
> > Thanks for the comments, here's an updated version of the patch.
> > Bootstrapped and regtested on x86_64-pc-linux-gnu.
>
> Great, I'll get this committed today - thanks!

That's done now.

> >
> > I'll note that there are some existing calls to `_M_use_local_data()`
> > already used only for their side effects without a cast to void, e.g.
> >
> >       /**
> >        *  @brief  Default constructor creates an empty string.
> >        */
> >       _GLIBCXX20_CONSTEXPR
> >       basic_string()
> >       _GLIBCXX_NOEXCEPT_IF(is_nothrow_default_constructible<_Alloc>::value)
> >       : _M_dataplus(_M_local_data())
> >       {
> >         _M_use_local_data();
> >         _M_set_length(0);
> >       }
> >
> > I haven't updated these, but should this be changed for consistency?
>
> Yes, good idea. I can do that.

I started to do that, and decided it made more sense to split out the
constexpr loop from _M_use_local_data() into a separate function,
_M_init_local_buf(). Then we can use that for all the places where we
don't care about the return value. That avoids the overhead of using
pointer_traits::pointer_to when we don't need the return value (which
is admittedly only going to be an issue for -O0 code, but I think it's
cleaner this way anyway).

Please see the attached patch and let me know what you think.


> Thanks again for fixing these. I think this might fix some bug reports
> about clang rejecting our std::string in constant expressions, so I'll
> check those.

Your patch fixes https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110900
(so we should backport it to gcc-13 and gcc-12 too).

[-- Attachment #2: patch.txt --]
[-- Type: text/plain, Size: 6292 bytes --]

commit 2668979d3206ff6c039ac0165aae29377a15666c
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Fri Sep 29 12:12:22 2023

    libstdc++: Split std::basic_string::_M_use_local_data into two functions
    
    This splits out the activate-the-union-member-for-constexpr logic from
    _M_use_local_data, so that it can be used separately in cases that don't
    need to use std::pointer_traits<pointer>::pointer_to to obtain the
    return value.
    
    This leaves only three uses of _M_use_local_data() which are all the
    same form:
    
      __s._M_data(_M_use_local_data());
      __s._M_set_length(0);
    
    We could remove _M_use_local_data() and change those three places to use
    a new _M_reset() function that does:
    
      _M_init_local_buf();
      _M_data(_M_local_data());
      _M_set_length(0);
    
    This is left for a future change.
    
    libstdc++-v3/ChangeLog:
    
            * include/bits/basic_string.h (_M_init_local_buf()): New
            function.
            (_M_use_local_data()): Use _M_init_local_buf.
            (basic_string(), basic_string(const Alloc&))
            (basic_string(basic_string&&))
            (basic_string(basic_string&&, const Alloc&)): Use
            _M_init_local_buf instead of _M_use_local_data().
            * include/bits/basic_string.tcc (swap(basic_string&))
            (_M_construct(InIter, InIter, forward_iterator_tag))
            (_M_construct(size_type, CharT), reserve()): Likewise.
            (_M_construct(InIter, InIter, input_iterator_tag)): Remove call
            to _M_use_local_data() and initialize the local buffer directly.

diff --git a/libstdc++-v3/include/bits/basic_string.h b/libstdc++-v3/include/bits/basic_string.h
index 4f94cd967cf..18a19b8dcbc 100644
--- a/libstdc++-v3/include/bits/basic_string.h
+++ b/libstdc++-v3/include/bits/basic_string.h
@@ -353,13 +353,23 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
       // Ensure that _M_local_buf is the active member of the union.
       __attribute__((__always_inline__))
       _GLIBCXX14_CONSTEXPR
-      pointer
-      _M_use_local_data() _GLIBCXX_NOEXCEPT
+      void
+      _M_init_local_buf() _GLIBCXX_NOEXCEPT
       {
 #if __cpp_lib_is_constant_evaluated
 	if (std::is_constant_evaluated())
 	  for (size_type __i = 0; __i <= _S_local_capacity; ++__i)
 	    _M_local_buf[__i] = _CharT();
+#endif
+      }
+
+      __attribute__((__always_inline__))
+      _GLIBCXX14_CONSTEXPR
+      pointer
+      _M_use_local_data() _GLIBCXX_NOEXCEPT
+      {
+#if __glibcxx_is_constant_evaluated
+	_M_init_local_buf();
 #endif
 	return _M_local_data();
       }
@@ -522,7 +532,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
       _GLIBCXX_NOEXCEPT_IF(is_nothrow_default_constructible<_Alloc>::value)
       : _M_dataplus(_M_local_data())
       {
-	_M_use_local_data();
+	_M_init_local_buf();
 	_M_set_length(0);
       }
 
@@ -534,7 +544,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
       basic_string(const _Alloc& __a) _GLIBCXX_NOEXCEPT
       : _M_dataplus(_M_local_data(), __a)
       {
-	_M_use_local_data();
+	_M_init_local_buf();
 	_M_set_length(0);
       }
 
@@ -678,7 +688,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
       {
 	if (__str._M_is_local())
 	  {
-	    (void)_M_use_local_data();
+	    _M_init_local_buf();
 	    traits_type::copy(_M_local_buf, __str._M_local_buf,
 			      __str.length() + 1);
 	  }
@@ -718,7 +728,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
       {
 	if (__str._M_is_local())
 	  {
-	    (void)_M_use_local_data();
+	    _M_init_local_buf();
 	    traits_type::copy(_M_local_buf, __str._M_local_buf,
 			      __str.length() + 1);
 	    _M_length(__str.length());
diff --git a/libstdc++-v3/include/bits/basic_string.tcc b/libstdc++-v3/include/bits/basic_string.tcc
index 4bc98f2aea7..052eeb9e846 100644
--- a/libstdc++-v3/include/bits/basic_string.tcc
+++ b/libstdc++-v3/include/bits/basic_string.tcc
@@ -79,7 +79,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	      }
 	    else if (__s.length())
 	      {
-		(void)_M_use_local_data();
+		_M_init_local_buf();
 		traits_type::copy(_M_local_buf, __s._M_local_buf,
 				  __s.length() + 1);
 		_M_length(__s.length());
@@ -88,7 +88,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	      }
 	    else if (length())
 	      {
-		(void)__s._M_use_local_data();
+		__s._M_init_local_buf();
 		traits_type::copy(__s._M_local_buf, _M_local_buf,
 				  length() + 1);
 		__s._M_length(length());
@@ -99,7 +99,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	else
 	  {
 	    const size_type __tmp_capacity = __s._M_allocated_capacity;
-	    (void)__s._M_use_local_data();
+	    __s._M_init_local_buf();
 	    traits_type::copy(__s._M_local_buf, _M_local_buf,
 			      length() + 1);
 	    _M_data(__s._M_data());
@@ -111,7 +111,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	  const size_type __tmp_capacity = _M_allocated_capacity;
 	  if (__s._M_is_local())
 	    {
-	      (void)_M_use_local_data();
+	      _M_init_local_buf();
 	      traits_type::copy(_M_local_buf, __s._M_local_buf,
 				__s.length() + 1);
 	      __s._M_data(_M_data());
@@ -174,14 +174,18 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	size_type __len = 0;
 	size_type __capacity = size_type(_S_local_capacity);
 
-	pointer __p = _M_use_local_data();
-
 	while (__beg != __end && __len < __capacity)
 	  {
-	    __p[__len++] = *__beg;
+	    _M_local_buf[__len++] = *__beg;
 	    ++__beg;
 	  }
 
+#if __glibcxx_is_constant_evaluated
+	if (std::is_constant_evaluated())
+	  for (size_type __i = __len; __i <= __capacity; ++__i)
+	    _M_local_buf[__i] = _CharT();
+#endif
+
 	struct _Guard
 	{
 	  _GLIBCXX20_CONSTEXPR
@@ -230,7 +234,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	    _M_capacity(__dnew);
 	  }
 	else
-	  _M_use_local_data();
+	  _M_init_local_buf();
 
 	// Check for out_of_range and length_error exceptions.
 	struct _Guard
@@ -263,7 +267,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	  _M_capacity(__n);
 	}
       else
-	_M_use_local_data();
+	_M_init_local_buf();
 
       if (__n)
 	this->_S_assign(_M_data(), __n, __c);
@@ -372,7 +376,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 
       if (__length <= size_type(_S_local_capacity))
 	{
-	  this->_S_copy(_M_use_local_data(), _M_data(), __length + 1);
+	  _M_init_local_buf();
+	  this->_S_copy(_M_local_buf, _M_data(), __length + 1);
 	  _M_destroy(__capacity);
 	  _M_data(_M_local_data());
 	}

Re: [PATCH] libstdc++: Ensure active union member is correctly set

[Nathaniel Shead]

On Fri, Sep 29, 2023 at 04:06:33PM +0100, Jonathan Wakely wrote:
> On Fri, 29 Sept 2023 at 10:32, Jonathan Wakely <jwakely@redhat.com> wrote:
> > > Thanks for the comments, here's an updated version of the patch.
> > > Bootstrapped and regtested on x86_64-pc-linux-gnu.
> >
> > Great, I'll get this committed today - thanks!
> 
> That's done now.
> 

Thanks!

> > >
> > > I'll note that there are some existing calls to `_M_use_local_data()`
> > > already used only for their side effects without a cast to void, e.g.
> > >
> > >       /**
> > >        *  @brief  Default constructor creates an empty string.
> > >        */
> > >       _GLIBCXX20_CONSTEXPR
> > >       basic_string()
> > >       _GLIBCXX_NOEXCEPT_IF(is_nothrow_default_constructible<_Alloc>::value)
> > >       : _M_dataplus(_M_local_data())
> > >       {
> > >         _M_use_local_data();
> > >         _M_set_length(0);
> > >       }
> > >
> > > I haven't updated these, but should this be changed for consistency?
> >
> > Yes, good idea. I can do that.
> 
> I started to do that, and decided it made more sense to split out the
> constexpr loop from _M_use_local_data() into a separate function,
> _M_init_local_buf(). Then we can use that for all the places where we
> don't care about the return value. That avoids the overhead of using
> pointer_traits::pointer_to when we don't need the return value (which
> is admittedly only going to be an issue for -O0 code, but I think it's
> cleaner this way anyway).
> 
> Please see the attached patch and let me know what you think.

I agree, and it also looks clearer to me what is happening.

> 
> > Thanks again for fixing these. I think this might fix some bug reports
> > about clang rejecting our std::string in constant expressions, so I'll
> > check those.
> 
> Your patch fixes https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110900
> (so we should backport it to gcc-13 and gcc-12 too).

> commit 2668979d3206ff6c039ac0165aae29377a15666c
> Author: Jonathan Wakely <jwakely@redhat.com>
> Date:   Fri Sep 29 12:12:22 2023
> 
>     libstdc++: Split std::basic_string::_M_use_local_data into two functions
>     
>     This splits out the activate-the-union-member-for-constexpr logic from
>     _M_use_local_data, so that it can be used separately in cases that don't
>     need to use std::pointer_traits<pointer>::pointer_to to obtain the
>     return value.
>     
>     This leaves only three uses of _M_use_local_data() which are all the
>     same form:
>     
>       __s._M_data(_M_use_local_data());
>       __s._M_set_length(0);
>     
>     We could remove _M_use_local_data() and change those three places to use
>     a new _M_reset() function that does:
>     
>       _M_init_local_buf();
>       _M_data(_M_local_data());
>       _M_set_length(0);
>     
>     This is left for a future change.
>     
>     libstdc++-v3/ChangeLog:
>     
>             * include/bits/basic_string.h (_M_init_local_buf()): New
>             function.
>             (_M_use_local_data()): Use _M_init_local_buf.
>             (basic_string(), basic_string(const Alloc&))
>             (basic_string(basic_string&&))
>             (basic_string(basic_string&&, const Alloc&)): Use
>             _M_init_local_buf instead of _M_use_local_data().
>             * include/bits/basic_string.tcc (swap(basic_string&))
>             (_M_construct(InIter, InIter, forward_iterator_tag))
>             (_M_construct(size_type, CharT), reserve()): Likewise.
>             (_M_construct(InIter, InIter, input_iterator_tag)): Remove call
>             to _M_use_local_data() and initialize the local buffer directly.
> 
> diff --git a/libstdc++-v3/include/bits/basic_string.h b/libstdc++-v3/include/bits/basic_string.h
> index 4f94cd967cf..18a19b8dcbc 100644
> --- a/libstdc++-v3/include/bits/basic_string.h
> +++ b/libstdc++-v3/include/bits/basic_string.h
> @@ -353,13 +353,23 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
>        // Ensure that _M_local_buf is the active member of the union.
>        __attribute__((__always_inline__))
>        _GLIBCXX14_CONSTEXPR
> -      pointer
> -      _M_use_local_data() _GLIBCXX_NOEXCEPT
> +      void
> +      _M_init_local_buf() _GLIBCXX_NOEXCEPT
>        {
>  #if __cpp_lib_is_constant_evaluated
>  	if (std::is_constant_evaluated())
>  	  for (size_type __i = 0; __i <= _S_local_capacity; ++__i)
>  	    _M_local_buf[__i] = _CharT();
> +#endif
> +      }
> +
> +      __attribute__((__always_inline__))
> +      _GLIBCXX14_CONSTEXPR
> +      pointer
> +      _M_use_local_data() _GLIBCXX_NOEXCEPT
> +      {
> +#if __glibcxx_is_constant_evaluated
> +	_M_init_local_buf();
>  #endif
>  	return _M_local_data();
>        }

What's the difference between __cpp_lib_is_constant_evaluated and
__glibcxx_is_constant_evaluated? Should these lines be using the same
macro here?

> @@ -522,7 +532,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
>        _GLIBCXX_NOEXCEPT_IF(is_nothrow_default_constructible<_Alloc>::value)
>        : _M_dataplus(_M_local_data())
>        {
> -	_M_use_local_data();
> +	_M_init_local_buf();
>  	_M_set_length(0);
>        }
>  
> @@ -534,7 +544,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
>        basic_string(const _Alloc& __a) _GLIBCXX_NOEXCEPT
>        : _M_dataplus(_M_local_data(), __a)
>        {
> -	_M_use_local_data();
> +	_M_init_local_buf();
>  	_M_set_length(0);
>        }
>  
> @@ -678,7 +688,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
>        {
>  	if (__str._M_is_local())
>  	  {
> -	    (void)_M_use_local_data();
> +	    _M_init_local_buf();
>  	    traits_type::copy(_M_local_buf, __str._M_local_buf,
>  			      __str.length() + 1);
>  	  }
> @@ -718,7 +728,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
>        {
>  	if (__str._M_is_local())
>  	  {
> -	    (void)_M_use_local_data();
> +	    _M_init_local_buf();
>  	    traits_type::copy(_M_local_buf, __str._M_local_buf,
>  			      __str.length() + 1);
>  	    _M_length(__str.length());
> diff --git a/libstdc++-v3/include/bits/basic_string.tcc b/libstdc++-v3/include/bits/basic_string.tcc
> index 4bc98f2aea7..052eeb9e846 100644
> --- a/libstdc++-v3/include/bits/basic_string.tcc
> +++ b/libstdc++-v3/include/bits/basic_string.tcc
> @@ -79,7 +79,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	      }
>  	    else if (__s.length())
>  	      {
> -		(void)_M_use_local_data();
> +		_M_init_local_buf();
>  		traits_type::copy(_M_local_buf, __s._M_local_buf,
>  				  __s.length() + 1);
>  		_M_length(__s.length());
> @@ -88,7 +88,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	      }
>  	    else if (length())
>  	      {
> -		(void)__s._M_use_local_data();
> +		__s._M_init_local_buf();
>  		traits_type::copy(__s._M_local_buf, _M_local_buf,
>  				  length() + 1);
>  		__s._M_length(length());
> @@ -99,7 +99,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	else
>  	  {
>  	    const size_type __tmp_capacity = __s._M_allocated_capacity;
> -	    (void)__s._M_use_local_data();
> +	    __s._M_init_local_buf();
>  	    traits_type::copy(__s._M_local_buf, _M_local_buf,
>  			      length() + 1);
>  	    _M_data(__s._M_data());
> @@ -111,7 +111,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	  const size_type __tmp_capacity = _M_allocated_capacity;
>  	  if (__s._M_is_local())
>  	    {
> -	      (void)_M_use_local_data();
> +	      _M_init_local_buf();
>  	      traits_type::copy(_M_local_buf, __s._M_local_buf,
>  				__s.length() + 1);
>  	      __s._M_data(_M_data());
> @@ -174,14 +174,18 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	size_type __len = 0;
>  	size_type __capacity = size_type(_S_local_capacity);
>  
> -	pointer __p = _M_use_local_data();
> -
>  	while (__beg != __end && __len < __capacity)
>  	  {
> -	    __p[__len++] = *__beg;
> +	    _M_local_buf[__len++] = *__beg;
>  	    ++__beg;
>  	  }
>  
> +#if __glibcxx_is_constant_evaluated
> +	if (std::is_constant_evaluated())
> +	  for (size_type __i = __len; __i <= __capacity; ++__i)
> +	    _M_local_buf[__i] = _CharT();
> +#endif
> +

I wonder if maybe this should still be a call to `_M_init_local_buf()`
above, where the `_M_use_local_data()` used to be? That way the logic
stays in one place, and I don't imagine the compile time savings of not
immediately overwriting the first __len characters would be significant.

>  	struct _Guard
>  	{
>  	  _GLIBCXX20_CONSTEXPR
> @@ -230,7 +234,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	    _M_capacity(__dnew);
>  	  }
>  	else
> -	  _M_use_local_data();
> +	  _M_init_local_buf();
>  
>  	// Check for out_of_range and length_error exceptions.
>  	struct _Guard
> @@ -263,7 +267,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  	  _M_capacity(__n);
>  	}
>        else
> -	_M_use_local_data();
> +	_M_init_local_buf();
>  
>        if (__n)
>  	this->_S_assign(_M_data(), __n, __c);
> @@ -372,7 +376,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  
>        if (__length <= size_type(_S_local_capacity))
>  	{
> -	  this->_S_copy(_M_use_local_data(), _M_data(), __length + 1);
> +	  _M_init_local_buf();
> +	  this->_S_copy(_M_local_buf, _M_data(), __length + 1);
>  	  _M_destroy(__capacity);
>  	  _M_data(_M_local_data());
>  	}

But looks good to me; I tried applying this on top of my frontend patch
and it looks like everything still works correctly.

Re: [PATCH] libstdc++: Ensure active union member is correctly set

[Jonathan Wakely]

On Fri, 29 Sept 2023 at 17:29, Nathaniel Shead
<nathanieloshead@gmail.com> wrote:
>
> On Fri, Sep 29, 2023 at 04:06:33PM +0100, Jonathan Wakely wrote:
> > On Fri, 29 Sept 2023 at 10:32, Jonathan Wakely <jwakely@redhat.com> wrote:
> > > > Thanks for the comments, here's an updated version of the patch.
> > > > Bootstrapped and regtested on x86_64-pc-linux-gnu.
> > >
> > > Great, I'll get this committed today - thanks!
> >
> > That's done now.
> >
>
> Thanks!
>
> > > >
> > > > I'll note that there are some existing calls to `_M_use_local_data()`
> > > > already used only for their side effects without a cast to void, e.g.
> > > >
> > > >       /**
> > > >        *  @brief  Default constructor creates an empty string.
> > > >        */
> > > >       _GLIBCXX20_CONSTEXPR
> > > >       basic_string()
> > > >       _GLIBCXX_NOEXCEPT_IF(is_nothrow_default_constructible<_Alloc>::value)
> > > >       : _M_dataplus(_M_local_data())
> > > >       {
> > > >         _M_use_local_data();
> > > >         _M_set_length(0);
> > > >       }
> > > >
> > > > I haven't updated these, but should this be changed for consistency?
> > >
> > > Yes, good idea. I can do that.
> >
> > I started to do that, and decided it made more sense to split out the
> > constexpr loop from _M_use_local_data() into a separate function,
> > _M_init_local_buf(). Then we can use that for all the places where we
> > don't care about the return value. That avoids the overhead of using
> > pointer_traits::pointer_to when we don't need the return value (which
> > is admittedly only going to be an issue for -O0 code, but I think it's
> > cleaner this way anyway).
> >
> > Please see the attached patch and let me know what you think.
>
> I agree, and it also looks clearer to me what is happening.

Good, I'll make this change next week then.


>
> >
> > > Thanks again for fixing these. I think this might fix some bug reports
> > > about clang rejecting our std::string in constant expressions, so I'll
> > > check those.
> >
> > Your patch fixes https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110900
> > (so we should backport it to gcc-13 and gcc-12 too).
>
> > commit 2668979d3206ff6c039ac0165aae29377a15666c
> > Author: Jonathan Wakely <jwakely@redhat.com>
> > Date:   Fri Sep 29 12:12:22 2023
> >
> >     libstdc++: Split std::basic_string::_M_use_local_data into two functions
> >
> >     This splits out the activate-the-union-member-for-constexpr logic from
> >     _M_use_local_data, so that it can be used separately in cases that don't
> >     need to use std::pointer_traits<pointer>::pointer_to to obtain the
> >     return value.
> >
> >     This leaves only three uses of _M_use_local_data() which are all the
> >     same form:
> >
> >       __s._M_data(_M_use_local_data());
> >       __s._M_set_length(0);
> >
> >     We could remove _M_use_local_data() and change those three places to use
> >     a new _M_reset() function that does:
> >
> >       _M_init_local_buf();
> >       _M_data(_M_local_data());
> >       _M_set_length(0);
> >
> >     This is left for a future change.
> >
> >     libstdc++-v3/ChangeLog:
> >
> >             * include/bits/basic_string.h (_M_init_local_buf()): New
> >             function.
> >             (_M_use_local_data()): Use _M_init_local_buf.
> >             (basic_string(), basic_string(const Alloc&))
> >             (basic_string(basic_string&&))
> >             (basic_string(basic_string&&, const Alloc&)): Use
> >             _M_init_local_buf instead of _M_use_local_data().
> >             * include/bits/basic_string.tcc (swap(basic_string&))
> >             (_M_construct(InIter, InIter, forward_iterator_tag))
> >             (_M_construct(size_type, CharT), reserve()): Likewise.
> >             (_M_construct(InIter, InIter, input_iterator_tag)): Remove call
> >             to _M_use_local_data() and initialize the local buffer directly.
> >
> > diff --git a/libstdc++-v3/include/bits/basic_string.h b/libstdc++-v3/include/bits/basic_string.h
> > index 4f94cd967cf..18a19b8dcbc 100644
> > --- a/libstdc++-v3/include/bits/basic_string.h
> > +++ b/libstdc++-v3/include/bits/basic_string.h
> > @@ -353,13 +353,23 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
> >        // Ensure that _M_local_buf is the active member of the union.
> >        __attribute__((__always_inline__))
> >        _GLIBCXX14_CONSTEXPR
> > -      pointer
> > -      _M_use_local_data() _GLIBCXX_NOEXCEPT
> > +      void
> > +      _M_init_local_buf() _GLIBCXX_NOEXCEPT
> >        {
> >  #if __cpp_lib_is_constant_evaluated
> >       if (std::is_constant_evaluated())
> >         for (size_type __i = 0; __i <= _S_local_capacity; ++__i)
> >           _M_local_buf[__i] = _CharT();
> > +#endif
> > +      }
> > +
> > +      __attribute__((__always_inline__))
> > +      _GLIBCXX14_CONSTEXPR
> > +      pointer
> > +      _M_use_local_data() _GLIBCXX_NOEXCEPT
> > +      {
> > +#if __glibcxx_is_constant_evaluated
> > +     _M_init_local_buf();
> >  #endif
> >       return _M_local_data();
> >        }
>
> What's the difference between __cpp_lib_is_constant_evaluated and
> __glibcxx_is_constant_evaluated? Should these lines be using the same
> macro here?

Ah, yeah, they could be the same.

The difference is that (for most feature test macros) the
__glibcxx_ftm form is defined after including <bits/version.h>, but
the __cpp_lib_ftm form is only defined only by headers that do:

#define __glibcxx_want_ftm
#include <bits/version.h>

This means we can ensure that the __cpp_lib_ftm form is only defined
by headers where we actually want to define it, e.g. in the headers
that [version.syn] in the standard says should define the macro. So
for our own internal uses, we should generally rely on the
__glibcxx_ftm one. Users should rely on __cpp_lib_ftm after including
the correct header. For example, __glibcxx_atomic_wait is defined
after including <bits/atomic_wait.h> and so is available for our own
uses, but __cpp_lib_atomic_wait is only defined after including
<atomic>, not just <bits/atomic_wait.h>. Or at least, that's the plan
- I have a pending patch to make everything I just said true :-)
Currently we "leak" the __cpp_lib_ftm forms into lots of <bits/xxx.h>
internal headers. That will change next week.

In this specific case, it doesn't matter, because
__cpp_lib_is_constant_evaluated is defined by <type_traits>, and every
header includes that. So it doesn't really matter whether our internal
uses are __cpp_lib_is_constant_evaluated or
__glibcxx_is_constant_evaluated. But it would be good to be
consistent.

>
> > @@ -522,7 +532,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
> >        _GLIBCXX_NOEXCEPT_IF(is_nothrow_default_constructible<_Alloc>::value)
> >        : _M_dataplus(_M_local_data())
> >        {
> > -     _M_use_local_data();
> > +     _M_init_local_buf();
> >       _M_set_length(0);
> >        }
> >
> > @@ -534,7 +544,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
> >        basic_string(const _Alloc& __a) _GLIBCXX_NOEXCEPT
> >        : _M_dataplus(_M_local_data(), __a)
> >        {
> > -     _M_use_local_data();
> > +     _M_init_local_buf();
> >       _M_set_length(0);
> >        }
> >
> > @@ -678,7 +688,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
> >        {
> >       if (__str._M_is_local())
> >         {
> > -         (void)_M_use_local_data();
> > +         _M_init_local_buf();
> >           traits_type::copy(_M_local_buf, __str._M_local_buf,
> >                             __str.length() + 1);
> >         }
> > @@ -718,7 +728,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
> >        {
> >       if (__str._M_is_local())
> >         {
> > -         (void)_M_use_local_data();
> > +         _M_init_local_buf();
> >           traits_type::copy(_M_local_buf, __str._M_local_buf,
> >                             __str.length() + 1);
> >           _M_length(__str.length());
> > diff --git a/libstdc++-v3/include/bits/basic_string.tcc b/libstdc++-v3/include/bits/basic_string.tcc
> > index 4bc98f2aea7..052eeb9e846 100644
> > --- a/libstdc++-v3/include/bits/basic_string.tcc
> > +++ b/libstdc++-v3/include/bits/basic_string.tcc
> > @@ -79,7 +79,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> >             }
> >           else if (__s.length())
> >             {
> > -             (void)_M_use_local_data();
> > +             _M_init_local_buf();
> >               traits_type::copy(_M_local_buf, __s._M_local_buf,
> >                                 __s.length() + 1);
> >               _M_length(__s.length());
> > @@ -88,7 +88,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> >             }
> >           else if (length())
> >             {
> > -             (void)__s._M_use_local_data();
> > +             __s._M_init_local_buf();
> >               traits_type::copy(__s._M_local_buf, _M_local_buf,
> >                                 length() + 1);
> >               __s._M_length(length());
> > @@ -99,7 +99,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> >       else
> >         {
> >           const size_type __tmp_capacity = __s._M_allocated_capacity;
> > -         (void)__s._M_use_local_data();
> > +         __s._M_init_local_buf();
> >           traits_type::copy(__s._M_local_buf, _M_local_buf,
> >                             length() + 1);
> >           _M_data(__s._M_data());
> > @@ -111,7 +111,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> >         const size_type __tmp_capacity = _M_allocated_capacity;
> >         if (__s._M_is_local())
> >           {
> > -           (void)_M_use_local_data();
> > +           _M_init_local_buf();
> >             traits_type::copy(_M_local_buf, __s._M_local_buf,
> >                               __s.length() + 1);
> >             __s._M_data(_M_data());
> > @@ -174,14 +174,18 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> >       size_type __len = 0;
> >       size_type __capacity = size_type(_S_local_capacity);
> >
> > -     pointer __p = _M_use_local_data();
> > -
> >       while (__beg != __end && __len < __capacity)
> >         {
> > -         __p[__len++] = *__beg;
> > +         _M_local_buf[__len++] = *__beg;
> >           ++__beg;
> >         }
> >
> > +#if __glibcxx_is_constant_evaluated
> > +     if (std::is_constant_evaluated())
> > +       for (size_type __i = __len; __i <= __capacity; ++__i)
> > +         _M_local_buf[__i] = _CharT();
> > +#endif
> > +
>
> I wonder if maybe this should still be a call to `_M_init_local_buf()`
> above, where the `_M_use_local_data()` used to be? That way the logic
> stays in one place, and I don't imagine the compile time savings of not
> immediately overwriting the first __len characters would be significant.


Yeah, I went back and forth on that, but you're probably right. I'll
do as you suggested.

Re: [PATCH] libstdc++: Ensure active union member is correctly set

[Jonathan Wakely]

[-- Attachment #1: Type: text/plain, Size: 11322 bytes --]

On Fri, 29 Sept 2023 at 17:46, Jonathan Wakely <jwakely.gcc@gmail.com> wrote:
>
> On Fri, 29 Sept 2023 at 17:29, Nathaniel Shead
> <nathanieloshead@gmail.com> wrote:
> >
> > On Fri, Sep 29, 2023 at 04:06:33PM +0100, Jonathan Wakely wrote:
> > > On Fri, 29 Sept 2023 at 10:32, Jonathan Wakely <jwakely@redhat.com> wrote:
> > > > > Thanks for the comments, here's an updated version of the patch.
> > > > > Bootstrapped and regtested on x86_64-pc-linux-gnu.
> > > >
> > > > Great, I'll get this committed today - thanks!
> > >
> > > That's done now.
> > >
> >
> > Thanks!
> >
> > > > >
> > > > > I'll note that there are some existing calls to `_M_use_local_data()`
> > > > > already used only for their side effects without a cast to void, e.g.
> > > > >
> > > > >       /**
> > > > >        *  @brief  Default constructor creates an empty string.
> > > > >        */
> > > > >       _GLIBCXX20_CONSTEXPR
> > > > >       basic_string()
> > > > >       _GLIBCXX_NOEXCEPT_IF(is_nothrow_default_constructible<_Alloc>::value)
> > > > >       : _M_dataplus(_M_local_data())
> > > > >       {
> > > > >         _M_use_local_data();
> > > > >         _M_set_length(0);
> > > > >       }
> > > > >
> > > > > I haven't updated these, but should this be changed for consistency?
> > > >
> > > > Yes, good idea. I can do that.
> > >
> > > I started to do that, and decided it made more sense to split out the
> > > constexpr loop from _M_use_local_data() into a separate function,
> > > _M_init_local_buf(). Then we can use that for all the places where we
> > > don't care about the return value. That avoids the overhead of using
> > > pointer_traits::pointer_to when we don't need the return value (which
> > > is admittedly only going to be an issue for -O0 code, but I think it's
> > > cleaner this way anyway).
> > >
> > > Please see the attached patch and let me know what you think.
> >
> > I agree, and it also looks clearer to me what is happening.
>
> Good, I'll make this change next week then.
>
>
> >
> > >
> > > > Thanks again for fixing these. I think this might fix some bug reports
> > > > about clang rejecting our std::string in constant expressions, so I'll
> > > > check those.
> > >
> > > Your patch fixes https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110900
> > > (so we should backport it to gcc-13 and gcc-12 too).
> >
> > > commit 2668979d3206ff6c039ac0165aae29377a15666c
> > > Author: Jonathan Wakely <jwakely@redhat.com>
> > > Date:   Fri Sep 29 12:12:22 2023
> > >
> > >     libstdc++: Split std::basic_string::_M_use_local_data into two functions
> > >
> > >     This splits out the activate-the-union-member-for-constexpr logic from
> > >     _M_use_local_data, so that it can be used separately in cases that don't
> > >     need to use std::pointer_traits<pointer>::pointer_to to obtain the
> > >     return value.
> > >
> > >     This leaves only three uses of _M_use_local_data() which are all the
> > >     same form:
> > >
> > >       __s._M_data(_M_use_local_data());
> > >       __s._M_set_length(0);
> > >
> > >     We could remove _M_use_local_data() and change those three places to use
> > >     a new _M_reset() function that does:
> > >
> > >       _M_init_local_buf();
> > >       _M_data(_M_local_data());
> > >       _M_set_length(0);
> > >
> > >     This is left for a future change.
> > >
> > >     libstdc++-v3/ChangeLog:
> > >
> > >             * include/bits/basic_string.h (_M_init_local_buf()): New
> > >             function.
> > >             (_M_use_local_data()): Use _M_init_local_buf.
> > >             (basic_string(), basic_string(const Alloc&))
> > >             (basic_string(basic_string&&))
> > >             (basic_string(basic_string&&, const Alloc&)): Use
> > >             _M_init_local_buf instead of _M_use_local_data().
> > >             * include/bits/basic_string.tcc (swap(basic_string&))
> > >             (_M_construct(InIter, InIter, forward_iterator_tag))
> > >             (_M_construct(size_type, CharT), reserve()): Likewise.
> > >             (_M_construct(InIter, InIter, input_iterator_tag)): Remove call
> > >             to _M_use_local_data() and initialize the local buffer directly.
> > >
> > > diff --git a/libstdc++-v3/include/bits/basic_string.h b/libstdc++-v3/include/bits/basic_string.h
> > > index 4f94cd967cf..18a19b8dcbc 100644
> > > --- a/libstdc++-v3/include/bits/basic_string.h
> > > +++ b/libstdc++-v3/include/bits/basic_string.h
> > > @@ -353,13 +353,23 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
> > >        // Ensure that _M_local_buf is the active member of the union.
> > >        __attribute__((__always_inline__))
> > >        _GLIBCXX14_CONSTEXPR
> > > -      pointer
> > > -      _M_use_local_data() _GLIBCXX_NOEXCEPT
> > > +      void
> > > +      _M_init_local_buf() _GLIBCXX_NOEXCEPT
> > >        {
> > >  #if __cpp_lib_is_constant_evaluated
> > >       if (std::is_constant_evaluated())
> > >         for (size_type __i = 0; __i <= _S_local_capacity; ++__i)
> > >           _M_local_buf[__i] = _CharT();
> > > +#endif
> > > +      }
> > > +
> > > +      __attribute__((__always_inline__))
> > > +      _GLIBCXX14_CONSTEXPR
> > > +      pointer
> > > +      _M_use_local_data() _GLIBCXX_NOEXCEPT
> > > +      {
> > > +#if __glibcxx_is_constant_evaluated
> > > +     _M_init_local_buf();
> > >  #endif
> > >       return _M_local_data();
> > >        }
> >
> > What's the difference between __cpp_lib_is_constant_evaluated and
> > __glibcxx_is_constant_evaluated? Should these lines be using the same
> > macro here?
>
> Ah, yeah, they could be the same.
>
> The difference is that (for most feature test macros) the
> __glibcxx_ftm form is defined after including <bits/version.h>, but
> the __cpp_lib_ftm form is only defined only by headers that do:
>
> #define __glibcxx_want_ftm
> #include <bits/version.h>
>
> This means we can ensure that the __cpp_lib_ftm form is only defined
> by headers where we actually want to define it, e.g. in the headers
> that [version.syn] in the standard says should define the macro. So
> for our own internal uses, we should generally rely on the
> __glibcxx_ftm one. Users should rely on __cpp_lib_ftm after including
> the correct header. For example, __glibcxx_atomic_wait is defined
> after including <bits/atomic_wait.h> and so is available for our own
> uses, but __cpp_lib_atomic_wait is only defined after including
> <atomic>, not just <bits/atomic_wait.h>. Or at least, that's the plan
> - I have a pending patch to make everything I just said true :-)
> Currently we "leak" the __cpp_lib_ftm forms into lots of <bits/xxx.h>
> internal headers. That will change next week.
>
> In this specific case, it doesn't matter, because
> __cpp_lib_is_constant_evaluated is defined by <type_traits>, and every
> header includes that. So it doesn't really matter whether our internal
> uses are __cpp_lib_is_constant_evaluated or
> __glibcxx_is_constant_evaluated. But it would be good to be
> consistent.
>
> >
> > > @@ -522,7 +532,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
> > >        _GLIBCXX_NOEXCEPT_IF(is_nothrow_default_constructible<_Alloc>::value)
> > >        : _M_dataplus(_M_local_data())
> > >        {
> > > -     _M_use_local_data();
> > > +     _M_init_local_buf();
> > >       _M_set_length(0);
> > >        }
> > >
> > > @@ -534,7 +544,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
> > >        basic_string(const _Alloc& __a) _GLIBCXX_NOEXCEPT
> > >        : _M_dataplus(_M_local_data(), __a)
> > >        {
> > > -     _M_use_local_data();
> > > +     _M_init_local_buf();
> > >       _M_set_length(0);
> > >        }
> > >
> > > @@ -678,7 +688,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
> > >        {
> > >       if (__str._M_is_local())
> > >         {
> > > -         (void)_M_use_local_data();
> > > +         _M_init_local_buf();
> > >           traits_type::copy(_M_local_buf, __str._M_local_buf,
> > >                             __str.length() + 1);
> > >         }
> > > @@ -718,7 +728,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
> > >        {
> > >       if (__str._M_is_local())
> > >         {
> > > -         (void)_M_use_local_data();
> > > +         _M_init_local_buf();
> > >           traits_type::copy(_M_local_buf, __str._M_local_buf,
> > >                             __str.length() + 1);
> > >           _M_length(__str.length());
> > > diff --git a/libstdc++-v3/include/bits/basic_string.tcc b/libstdc++-v3/include/bits/basic_string.tcc
> > > index 4bc98f2aea7..052eeb9e846 100644
> > > --- a/libstdc++-v3/include/bits/basic_string.tcc
> > > +++ b/libstdc++-v3/include/bits/basic_string.tcc
> > > @@ -79,7 +79,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> > >             }
> > >           else if (__s.length())
> > >             {
> > > -             (void)_M_use_local_data();
> > > +             _M_init_local_buf();
> > >               traits_type::copy(_M_local_buf, __s._M_local_buf,
> > >                                 __s.length() + 1);
> > >               _M_length(__s.length());
> > > @@ -88,7 +88,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> > >             }
> > >           else if (length())
> > >             {
> > > -             (void)__s._M_use_local_data();
> > > +             __s._M_init_local_buf();
> > >               traits_type::copy(__s._M_local_buf, _M_local_buf,
> > >                                 length() + 1);
> > >               __s._M_length(length());
> > > @@ -99,7 +99,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> > >       else
> > >         {
> > >           const size_type __tmp_capacity = __s._M_allocated_capacity;
> > > -         (void)__s._M_use_local_data();
> > > +         __s._M_init_local_buf();
> > >           traits_type::copy(__s._M_local_buf, _M_local_buf,
> > >                             length() + 1);
> > >           _M_data(__s._M_data());
> > > @@ -111,7 +111,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> > >         const size_type __tmp_capacity = _M_allocated_capacity;
> > >         if (__s._M_is_local())
> > >           {
> > > -           (void)_M_use_local_data();
> > > +           _M_init_local_buf();
> > >             traits_type::copy(_M_local_buf, __s._M_local_buf,
> > >                               __s.length() + 1);
> > >             __s._M_data(_M_data());
> > > @@ -174,14 +174,18 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> > >       size_type __len = 0;
> > >       size_type __capacity = size_type(_S_local_capacity);
> > >
> > > -     pointer __p = _M_use_local_data();
> > > -
> > >       while (__beg != __end && __len < __capacity)
> > >         {
> > > -         __p[__len++] = *__beg;
> > > +         _M_local_buf[__len++] = *__beg;
> > >           ++__beg;
> > >         }
> > >
> > > +#if __glibcxx_is_constant_evaluated
> > > +     if (std::is_constant_evaluated())
> > > +       for (size_type __i = __len; __i <= __capacity; ++__i)
> > > +         _M_local_buf[__i] = _CharT();
> > > +#endif
> > > +
> >
> > I wonder if maybe this should still be a call to `_M_init_local_buf()`
> > above, where the `_M_use_local_data()` used to be? That way the logic
> > stays in one place, and I don't imagine the compile time savings of not
> > immediately overwriting the first __len characters would be significant.
>
>
> Yeah, I went back and forth on that, but you're probably right. I'll
> do as you suggested.

Here's what I've pushed to trunk after testing on x86_64-linux.

Thanks again for the fixes.

[-- Attachment #2: patch.txt --]
[-- Type: text/plain, Size: 6006 bytes --]

commit 405a4140fc30bce86b1ec885a98bb17704e0c8c6
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Fri Sep 29 12:12:22 2023

    libstdc++: Split std::basic_string::_M_use_local_data into two functions
    
    This splits out the activate-the-union-member-for-constexpr logic from
    _M_use_local_data, so that it can be used separately in cases that don't
    need to use std::pointer_traits<pointer>::pointer_to to obtain the
    return value.
    
    This leaves only three uses of _M_use_local_data() which are all of the
    same form:
    
      __s._M_data(_M_use_local_data());
      __s._M_set_length(0);
    
    We could remove _M_use_local_data() and change those three places to use
    a new _M_reset() function that does:
    
      _M_init_local_buf();
      _M_data(_M_local_data());
      _M_set_length(0);
    
    This is left for a future change.
    
    libstdc++-v3/ChangeLog:
    
            * include/bits/basic_string.h (_M_init_local_buf()): New
            function.
            (_M_use_local_data()): Use _M_init_local_buf.
            (basic_string(), basic_string(const Alloc&))
            (basic_string(basic_string&&))
            (basic_string(basic_string&&, const Alloc&)): Use
            _M_init_local_buf instead of _M_use_local_data().
            * include/bits/basic_string.tcc (swap(basic_string&))
            (_M_construct(InIter, InIter, input_iterator_tag))
            (_M_construct(InIter, InIter, forward_iterator_tag))
            (_M_construct(size_type, CharT), reserve()): Likewise.

diff --git a/libstdc++-v3/include/bits/basic_string.h b/libstdc++-v3/include/bits/basic_string.h
index 4f94cd967cf..0fa32afeb84 100644
--- a/libstdc++-v3/include/bits/basic_string.h
+++ b/libstdc++-v3/include/bits/basic_string.h
@@ -353,13 +353,23 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
       // Ensure that _M_local_buf is the active member of the union.
       __attribute__((__always_inline__))
       _GLIBCXX14_CONSTEXPR
-      pointer
-      _M_use_local_data() _GLIBCXX_NOEXCEPT
+      void
+      _M_init_local_buf() _GLIBCXX_NOEXCEPT
       {
 #if __cpp_lib_is_constant_evaluated
 	if (std::is_constant_evaluated())
 	  for (size_type __i = 0; __i <= _S_local_capacity; ++__i)
 	    _M_local_buf[__i] = _CharT();
+#endif
+      }
+
+      __attribute__((__always_inline__))
+      _GLIBCXX14_CONSTEXPR
+      pointer
+      _M_use_local_data() _GLIBCXX_NOEXCEPT
+      {
+#if __cpp_lib_is_constant_evaluated
+	_M_init_local_buf();
 #endif
 	return _M_local_data();
       }
@@ -522,7 +532,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
       _GLIBCXX_NOEXCEPT_IF(is_nothrow_default_constructible<_Alloc>::value)
       : _M_dataplus(_M_local_data())
       {
-	_M_use_local_data();
+	_M_init_local_buf();
 	_M_set_length(0);
       }
 
@@ -534,7 +544,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
       basic_string(const _Alloc& __a) _GLIBCXX_NOEXCEPT
       : _M_dataplus(_M_local_data(), __a)
       {
-	_M_use_local_data();
+	_M_init_local_buf();
 	_M_set_length(0);
       }
 
@@ -678,7 +688,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
       {
 	if (__str._M_is_local())
 	  {
-	    (void)_M_use_local_data();
+	    _M_init_local_buf();
 	    traits_type::copy(_M_local_buf, __str._M_local_buf,
 			      __str.length() + 1);
 	  }
@@ -718,7 +728,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
       {
 	if (__str._M_is_local())
 	  {
-	    (void)_M_use_local_data();
+	    _M_init_local_buf();
 	    traits_type::copy(_M_local_buf, __str._M_local_buf,
 			      __str.length() + 1);
 	    _M_length(__str.length());
diff --git a/libstdc++-v3/include/bits/basic_string.tcc b/libstdc++-v3/include/bits/basic_string.tcc
index 4bc98f2aea7..f0a44e5e881 100644
--- a/libstdc++-v3/include/bits/basic_string.tcc
+++ b/libstdc++-v3/include/bits/basic_string.tcc
@@ -79,7 +79,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	      }
 	    else if (__s.length())
 	      {
-		(void)_M_use_local_data();
+		_M_init_local_buf();
 		traits_type::copy(_M_local_buf, __s._M_local_buf,
 				  __s.length() + 1);
 		_M_length(__s.length());
@@ -88,7 +88,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	      }
 	    else if (length())
 	      {
-		(void)__s._M_use_local_data();
+		__s._M_init_local_buf();
 		traits_type::copy(__s._M_local_buf, _M_local_buf,
 				  length() + 1);
 		__s._M_length(length());
@@ -99,7 +99,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	else
 	  {
 	    const size_type __tmp_capacity = __s._M_allocated_capacity;
-	    (void)__s._M_use_local_data();
+	    __s._M_init_local_buf();
 	    traits_type::copy(__s._M_local_buf, _M_local_buf,
 			      length() + 1);
 	    _M_data(__s._M_data());
@@ -111,7 +111,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	  const size_type __tmp_capacity = _M_allocated_capacity;
 	  if (__s._M_is_local())
 	    {
-	      (void)_M_use_local_data();
+	      _M_init_local_buf();
 	      traits_type::copy(_M_local_buf, __s._M_local_buf,
 				__s.length() + 1);
 	      __s._M_data(_M_data());
@@ -174,11 +174,11 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	size_type __len = 0;
 	size_type __capacity = size_type(_S_local_capacity);
 
-	pointer __p = _M_use_local_data();
+	_M_init_local_buf();
 
 	while (__beg != __end && __len < __capacity)
 	  {
-	    __p[__len++] = *__beg;
+	    _M_local_buf[__len++] = *__beg;
 	    ++__beg;
 	  }
 
@@ -230,7 +230,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	    _M_capacity(__dnew);
 	  }
 	else
-	  _M_use_local_data();
+	  _M_init_local_buf();
 
 	// Check for out_of_range and length_error exceptions.
 	struct _Guard
@@ -263,7 +263,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	  _M_capacity(__n);
 	}
       else
-	_M_use_local_data();
+	_M_init_local_buf();
 
       if (__n)
 	this->_S_assign(_M_data(), __n, __c);
@@ -372,7 +372,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 
       if (__length <= size_type(_S_local_capacity))
 	{
-	  this->_S_copy(_M_use_local_data(), _M_data(), __length + 1);
+	  _M_init_local_buf();
+	  this->_S_copy(_M_local_buf, _M_data(), __length + 1);
 	  _M_destroy(__capacity);
 	  _M_data(_M_local_data());
 	}