From c5e8c6c193d9a1ab80ef1a05e5158ddf35d54009 Mon Sep 17 00:00:00 2001
From: Jakub Jelinek <jakub@redhat.com>
Date: Wed, 16 Nov 2022 14:49:47 +0100
Subject: [PATCH] c++: Alignment changes to layout compatibility/common initial
sequence - DR2583
When trying to figure out what to do about alignment,
layout_compatible_type_p returns false if TYPE_ALIGN on
ENUMERAL_TYPE/CLASS_TYPE_P (but not scalar types?) differ, or if members
don't have the same positions.
What is in DR2583 doesn't say anything like that though, on the other side
it says that if the corresponding entities don't have the same alignment
requirements, they aren't part of the common initial sequence.
So, my understanding of this is we shouldn't check TYPE_ALIGN in
layout_compatible_type_p, but instead DECL_ALIGN in
next_common_initial_seqence.
2022-11-16 Jakub Jelinek <jakub@redhat.com>
* typeck.cc (next_common_initial_sequence): Return false members have
different DECL_ALIGN.
(layout_compatible_type_p): Don't test TYPE_ALIGN of ENUMERAL_TYPE
or CLASS_TYPE_P.
* g++.dg/cpp2a/is-layout-compatible3.C: Expect enums with different
alignas to be layout compatible, while classes with different
alignas on members layout incompatible.
* g++.dg/DRs/dr2583.C: New test.
---
gcc/cp/typeck.cc | 6 +--
gcc/testsuite/g++.dg/DRs/dr2583.C | 45 +++++++++++++++++++
.../g++.dg/cpp2a/is-layout-compatible3.C | 4 +-
3 files changed, 50 insertions(+), 5 deletions(-)
create mode 100644 gcc/testsuite/g++.dg/DRs/dr2583.C
diff --git a/gcc/cp/typeck.cc b/gcc/cp/typeck.cc
index 6c911827778c..281d281629b3 100644
--- a/gcc/cp/typeck.cc
+++ b/gcc/cp/typeck.cc
@@ -1833,6 +1833,8 @@ next_common_initial_sequence (tree &memb1, tree &memb2)
if ((!lookup_attribute ("no_unique_address", DECL_ATTRIBUTES (memb1)))
!= !lookup_attribute ("no_unique_address", DECL_ATTRIBUTES (memb2)))
return false;
+ if (DECL_ALIGN (memb1) != DECL_ALIGN (memb2))
+ return false;
if (!tree_int_cst_equal (bit_position (memb1), bit_position (memb2)))
return false;
return true;
@@ -1854,15 +1856,13 @@ layout_compatible_type_p (tree type1, tree type2)
type2 = cp_build_qualified_type (type2, TYPE_UNQUALIFIED);
if (TREE_CODE (type1) == ENUMERAL_TYPE)
- return (TYPE_ALIGN (type1) == TYPE_ALIGN (type2)
- && tree_int_cst_equal (TYPE_SIZE (type1), TYPE_SIZE (type2))
+ return (tree_int_cst_equal (TYPE_SIZE (type1), TYPE_SIZE (type2))
&& same_type_p (finish_underlying_type (type1),
finish_underlying_type (type2)));
if (CLASS_TYPE_P (type1)
&& std_layout_type_p (type1)
&& std_layout_type_p (type2)
- && TYPE_ALIGN (type1) == TYPE_ALIGN (type2)
&& tree_int_cst_equal (TYPE_SIZE (type1), TYPE_SIZE (type2)))
{
tree field1 = TYPE_FIELDS (type1);
diff --git a/gcc/testsuite/g++.dg/DRs/dr2583.C b/gcc/testsuite/g++.dg/DRs/dr2583.C
new file mode 100644
index 000000000000..e80834c62159
--- /dev/null
+++ b/gcc/testsuite/g++.dg/DRs/dr2583.C
@@ -0,0 +1,45 @@
+// DR 2583 - Common initial sequence should consider over-alignment.
+// { dg-do compile { target c++11 } }
+
+#include <type_traits>
+
+struct A {
+ int i;
+ char c;
+};
+
+struct B {
+ int i;
+ alignas(8) char c;
+};
+
+struct C {
+ int i;
+ alignas(alignof(char)) char c;
+};
+
+struct D {
+ alignas(alignof(int)) int i;
+ char c;
+};
+
+struct S0 {
+ alignas(16) char x[128];
+ int i;
+};
+
+struct alignas(16) S1 {
+ char x[128];
+ int i;
+};
+
+#if __cpp_lib_is_layout_compatible >= 201907L
+static_assert (std::is_corresponding_member (&A::i, &B::i), "");
+static_assert (std::is_corresponding_member (&A::c, &B::c) == (alignof (char) == 8), "");
+static_assert (std::is_corresponding_member (&A::i, &C::i), "");
+static_assert (std::is_corresponding_member (&A::c, &C::c), "");
+static_assert (std::is_corresponding_member (&A::i, &D::i), "");
+static_assert (std::is_corresponding_member (&A::c, &D::c), "");
+static_assert (std::is_corresponding_member (&S0::x, &S1::x) == (alignof (char) == 16), "");
+static_assert (std::is_corresponding_member (&S0::i, &S1::i) == (alignof (char) == 16), "");
+#endif
diff --git a/gcc/testsuite/g++.dg/cpp2a/is-layout-compatible3.C b/gcc/testsuite/g++.dg/cpp2a/is-layout-compatible3.C
index c54858746606..8f48ba8679a1 100644
--- a/gcc/testsuite/g++.dg/cpp2a/is-layout-compatible3.C
+++ b/gcc/testsuite/g++.dg/cpp2a/is-layout-compatible3.C
@@ -55,10 +55,10 @@ static_assert (!std::is_layout_compatible_v<K, L>);
static_assert (!std::is_layout_compatible_v<M, N>);
static_assert (!std::is_layout_compatible_v<O, P>);
static_assert (!std::is_layout_compatible_v<P, D>);
-static_assert (!std::is_layout_compatible_v<Q, R>);
+static_assert (std::is_layout_compatible_v<Q, R>);
static_assert (!std::is_layout_compatible_v<U, V>);
static_assert (!std::is_layout_compatible_v<A, I>);
static_assert (!std::is_layout_compatible_v<C, I>);
-static_assert (std::is_layout_compatible_v<E, F>);
+static_assert (!std::is_layout_compatible_v<E, F>);
static_assert (std::is_layout_compatible_v<G, H>);
static_assert (std::is_layout_compatible_v<C1, D1>);
--
GitLab