From: Jonathan Wakely <jwakely.gcc@gmail.com>
To: Nathaniel Shead <nathanieloshead@gmail.com>
Cc: Jason Merrill <jason@redhat.com>,
"libstdc++" <libstdc++@gcc.gnu.org>,
gcc-patches <gcc-patches@gcc.gnu.org>
Subject: Re: [PATCH v4] c++: Check for indirect change of active union member in constexpr [PR101631,PR102286]
Date: Sat, 23 Sep 2023 07:40:48 +0100 [thread overview]
Message-ID: <CAH6eHdTjqNanT164HDX7=6FhZ5BYPP_698_jt3SXfzTfvaGxiw@mail.gmail.com> (raw)
In-Reply-To: <ZQ4zeXtkl2hnVNRc@Thaum.localdomain>
[-- 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
> >
>
next prev parent reply other threads:[~2023-09-23 6:41 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <ZO30PQql2TablzpJ@Thaum.localdomain>
[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 ` Nathaniel Shead
2023-09-23 6:40 ` Jonathan Wakely [this message]
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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