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

[Jason Merrill <jason@redhat.com>]

On 11/14/22 01:28, Jakub Jelinek wrote:
> Hi!
> 
> Working virtually out of Baker Island.
> 
> 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.

Agreed.

> Lightly tested (on is-layout*/is-corresponding*/dr2583.C only) so far,
> ok if it passes full bootstrap/regtest?
> Or do we need different rules?
> 
> 2022-11-14  Jakub Jelinek  <jakub@redhat.com>
> 
> 	* typeck.cc (next_common_initial_seqence): 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_seqence (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	2022-11-13 22:58:11.977640606 -1200
> +++ gcc/testsuite/g++.dg/DRs/dr2583.C	2022-11-13 23:18:04.630414835 -1200
> @@ -0,0 +1,31 @@
> +// 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 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 (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.

> +static_assert (alignof (char) == 16 || !std::is_corresponding_member (&S0::x, &S1::x), "");
> +static_assert (alignof (char) == 16 || !std::is_corresponding_member (&S0::i, &S1::i), "");
> +#endif
> 
> 	Jakub
>