* [PATCH v4] c++: Check for indirect change of active union member in constexpr [PR101631,PR102286]
[not found] ` <ZQ2sQNoVz0rblzDN@Thaum.localdomain>
@ 2023-09-23 0:38 ` Nathaniel Shead
2023-09-23 6:40 ` Jonathan Wakely
0 siblings, 1 reply; 11+ messages in thread
From: Nathaniel Shead @ 2023-09-23 0:38 UTC (permalink / raw)
To: Jason Merrill, libstdc++; +Cc: gcc-patches
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
>
^ permalink raw reply [flat|nested] 11+ messages in thread
* Re: [PATCH v4] c++: Check for indirect change of active union member in constexpr [PR101631,PR102286]
2023-09-23 0:38 ` [PATCH v4] c++: Check for indirect change of active union member in constexpr [PR101631,PR102286] Nathaniel Shead
@ 2023-09-23 6:40 ` Jonathan Wakely
2023-09-23 7:30 ` [PATCH] libstdc++: Ensure active union member is correctly set Nathaniel Shead
0 siblings, 1 reply; 11+ messages in thread
From: Jonathan Wakely @ 2023-09-23 6:40 UTC (permalink / raw)
To: Nathaniel Shead; +Cc: Jason Merrill, libstdc++, gcc-patches
[-- 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
> >
>
^ permalink raw reply [flat|nested] 11+ messages in thread
* [PATCH] libstdc++: Ensure active union member is correctly set
2023-09-23 6:40 ` Jonathan Wakely
@ 2023-09-23 7:30 ` Nathaniel Shead
2023-09-23 10:52 ` Jonathan Wakely
2023-09-27 14:13 ` Jonathan Wakely
0 siblings, 2 replies; 11+ messages in thread
From: Nathaniel Shead @ 2023-09-23 7:30 UTC (permalink / raw)
To: Jonathan Wakely; +Cc: Jason Merrill, libstdc++, gcc-patches
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
^ permalink raw reply [flat|nested] 11+ messages in thread
* Re: [PATCH] libstdc++: Ensure active union member is correctly set
2023-09-23 7:30 ` [PATCH] libstdc++: Ensure active union member is correctly set Nathaniel Shead
@ 2023-09-23 10:52 ` Jonathan Wakely
2023-09-27 14:13 ` Jonathan Wakely
1 sibling, 0 replies; 11+ messages in thread
From: Jonathan Wakely @ 2023-09-23 10:52 UTC (permalink / raw)
To: Nathaniel Shead; +Cc: Jason Merrill, libstdc++, gcc-patches
[-- 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
>
>
^ permalink raw reply [flat|nested] 11+ messages in thread
* Re: [PATCH] libstdc++: Ensure active union member is correctly set
2023-09-23 7:30 ` [PATCH] libstdc++: Ensure active union member is correctly set Nathaniel Shead
2023-09-23 10:52 ` Jonathan Wakely
@ 2023-09-27 14:13 ` Jonathan Wakely
2023-09-28 23:25 ` Nathaniel Shead
1 sibling, 1 reply; 11+ messages in thread
From: Jonathan Wakely @ 2023-09-27 14:13 UTC (permalink / raw)
To: Nathaniel Shead; +Cc: Jonathan Wakely, Jason Merrill, libstdc++, gcc-patches
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
>
^ permalink raw reply [flat|nested] 11+ messages in thread
* Re: [PATCH] libstdc++: Ensure active union member is correctly set
2023-09-27 14:13 ` Jonathan Wakely
@ 2023-09-28 23:25 ` Nathaniel Shead
2023-09-29 9:32 ` Jonathan Wakely
0 siblings, 1 reply; 11+ messages in thread
From: Nathaniel Shead @ 2023-09-28 23:25 UTC (permalink / raw)
To: Jonathan Wakely; +Cc: Jonathan Wakely, Jason Merrill, libstdc++, gcc-patches
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
^ permalink raw reply [flat|nested] 11+ messages in thread
* Re: [PATCH] libstdc++: Ensure active union member is correctly set
2023-09-28 23:25 ` Nathaniel Shead
@ 2023-09-29 9:32 ` Jonathan Wakely
2023-09-29 15:06 ` Jonathan Wakely
0 siblings, 1 reply; 11+ messages in thread
From: Jonathan Wakely @ 2023-09-29 9:32 UTC (permalink / raw)
To: Nathaniel Shead; +Cc: Jonathan Wakely, Jason Merrill, libstdc++, gcc-patches
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
>
^ permalink raw reply [flat|nested] 11+ messages in thread
* Re: [PATCH] libstdc++: Ensure active union member is correctly set
2023-09-29 9:32 ` Jonathan Wakely
@ 2023-09-29 15:06 ` Jonathan Wakely
2023-09-29 16:29 ` Nathaniel Shead
0 siblings, 1 reply; 11+ messages in thread
From: Jonathan Wakely @ 2023-09-29 15:06 UTC (permalink / raw)
To: Nathaniel Shead; +Cc: Jonathan Wakely, Jason Merrill, libstdc++, gcc-patches
[-- 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());
}
^ permalink raw reply [flat|nested] 11+ messages in thread
* Re: [PATCH] libstdc++: Ensure active union member is correctly set
2023-09-29 15:06 ` Jonathan Wakely
@ 2023-09-29 16:29 ` Nathaniel Shead
2023-09-29 16:46 ` Jonathan Wakely
0 siblings, 1 reply; 11+ messages in thread
From: Nathaniel Shead @ 2023-09-29 16:29 UTC (permalink / raw)
To: Jonathan Wakely; +Cc: Jonathan Wakely, Jason Merrill, libstdc++, gcc-patches
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.
^ permalink raw reply [flat|nested] 11+ messages in thread
* Re: [PATCH] libstdc++: Ensure active union member is correctly set
2023-09-29 16:29 ` Nathaniel Shead
@ 2023-09-29 16:46 ` Jonathan Wakely
2023-10-21 14:45 ` Jonathan Wakely
0 siblings, 1 reply; 11+ messages in thread
From: Jonathan Wakely @ 2023-09-29 16:46 UTC (permalink / raw)
To: Nathaniel Shead; +Cc: Jonathan Wakely, Jason Merrill, libstdc++, gcc-patches
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.
^ permalink raw reply [flat|nested] 11+ messages in thread
* Re: [PATCH] libstdc++: Ensure active union member is correctly set
2023-09-29 16:46 ` Jonathan Wakely
@ 2023-10-21 14:45 ` Jonathan Wakely
0 siblings, 0 replies; 11+ messages in thread
From: Jonathan Wakely @ 2023-10-21 14:45 UTC (permalink / raw)
To: Jonathan Wakely; +Cc: Nathaniel Shead, Jason Merrill, libstdc++, gcc-patches
[-- 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());
}
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[not found] ` <053faf76-f918-7527-4a41-755a18d0018a@redhat.com>
[not found] ` <ZPHXiqquRCNCREoX@Thaum.localdomain>
[not found] ` <e5fb1597-28a9-1be6-f914-5ed475732da2@redhat.com>
[not found] ` <ZQpDAkaSSdkc0Q+R@Thaum.localdomain>
[not found] ` <e11829bb-68c2-98b6-c7d0-9f9dbd58cb25@redhat.com>
[not found] ` <ZQxIGK4oIxNMun2i@Thaum.localdomain>
[not found] ` <bfe816fe-5ce5-8dc8-3d00-455ef37d67df@redhat.com>
[not found] ` <ZQ2sQNoVz0rblzDN@Thaum.localdomain>
2023-09-23 0:38 ` [PATCH v4] c++: Check for indirect change of active union member in constexpr [PR101631,PR102286] Nathaniel Shead
2023-09-23 6:40 ` Jonathan Wakely
2023-09-23 7:30 ` [PATCH] libstdc++: Ensure active union member is correctly set Nathaniel Shead
2023-09-23 10:52 ` Jonathan Wakely
2023-09-27 14:13 ` Jonathan Wakely
2023-09-28 23:25 ` Nathaniel Shead
2023-09-29 9:32 ` Jonathan Wakely
2023-09-29 15:06 ` Jonathan Wakely
2023-09-29 16:29 ` Nathaniel Shead
2023-09-29 16:46 ` Jonathan Wakely
2023-10-21 14:45 ` Jonathan Wakely
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