[PATCH] c++, v2: Alignment changes to layout compatibility/common initial sequence - DR2583

[Jakub Jelinek <jakub@redhat.com>]

On Tue, Nov 15, 2022 at 05:57:26PM -0500, Jason Merrill wrote:
> > 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.
> 
> Agreed.
> 
> > +#if __cpp_lib_is_layout_compatible >= 201907L
> > +static_assert (std::is_corresponding_member (&A::i, &B::i), "");
> > +static_assert (alignof (char) == 8 || !std::is_corresponding_member (&A::c, &B::c), "");
> 
> Maybe
> 
> std_is_corresponding_member (&A::c, &B::c) == (alignof (char) == 8)
> 
> ?
> 
> Could also use an alignas(alignof(type)) case to verify that it matches a
> member with no alignas.

So like this?
The previous patch successfully bootstrapped/regtested on x86_64-linux and
i686-linux, only the new testcase changed and that one passes make check, ok
for trunk?

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.jj	2022-11-13 04:53:46.010682269 -1200
+++ gcc/cp/typeck.cc	2022-11-13 23:14:41.355180354 -1200
@@ -1833,6 +1833,8 @@ next_common_initial_sequence (tree &memb1
   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, tr
   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);
--- gcc/testsuite/g++.dg/cpp2a/is-layout-compatible3.C.jj	2021-08-18 21:42:27.414421719 -1200
+++ gcc/testsuite/g++.dg/cpp2a/is-layout-compatible3.C	2022-11-13 23:20:05.008776825 -1200
@@ -55,10 +55,10 @@ static_assert (!std::is_layout_compatibl
 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>);
--- gcc/testsuite/g++.dg/DRs/dr2583.C.jj	2022-11-16 11:28:03.200831332 +0100
+++ gcc/testsuite/g++.dg/DRs/dr2583.C	2022-11-16 11:31:34.421978999 +0100
@@ -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


	Jakub