ia32: Fix alignment of _Atomic fields [PR65146]

Hi!

For _Atomic fields, lowering the alignment of long long or double etc.
fields on ia32 is undesirable, because then one really can't perform atomic
operations on those using cmpxchg8b.

The following patch stops lowering the alignment in fields for _Atomic
types (the x86_field_alignment change) and for -mpreferred-stack-boundary=2
also ensures we don't misalign _Atomic long long etc. automatic variables
(the ix86_{local,minimum}_alignment changes).
Not sure about iamcu_alignment change, I know next to nothing about IA MCU,
but unless it doesn't have cmpxchg8b instruction, it would surprise me if we
don't want to do it as well.
clang apparently doesn't lower the field alignment for _Atomic.

Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?

2020-08-27  Jakub Jelinek  <jakub@redhat.com>

	PR target/65146
	* config/i386/i386.c (iamcu_alignment): Don't decrease alignment
	for TYPE_ATOMIC types.
	(ix86_local_alignment): Likewise.
	(ix86_minimum_alignment): Likewise.
	(x86_field_alignment): Likewise, and emit a -Wpsabi diagnostic
	for it.

	* gcc.target/i386/pr65146.c: New test.


	Jakub

On Thu, Aug 27, 2020 at 10:35 AM Jakub Jelinek <jakub@redhat.com> wrote:
>
> Hi!
>
> For _Atomic fields, lowering the alignment of long long or double etc.
> fields on ia32 is undesirable, because then one really can't perform atomic
> operations on those using cmpxchg8b.
>
> The following patch stops lowering the alignment in fields for _Atomic
> types (the x86_field_alignment change) and for -mpreferred-stack-boundary=2
> also ensures we don't misalign _Atomic long long etc. automatic variables
> (the ix86_{local,minimum}_alignment changes).
> Not sure about iamcu_alignment change, I know next to nothing about IA MCU,
> but unless it doesn't have cmpxchg8b instruction, it would surprise me if we
> don't want to do it as well.
> clang apparently doesn't lower the field alignment for _Atomic.

Intel MCU implements pentium ISA, which includes cmpxchg8b.

> Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?
>
> 2020-08-27  Jakub Jelinek  <jakub@redhat.com>
>
>         PR target/65146
>         * config/i386/i386.c (iamcu_alignment): Don't decrease alignment
>         for TYPE_ATOMIC types.
>         (ix86_local_alignment): Likewise.
>         (ix86_minimum_alignment): Likewise.
>         (x86_field_alignment): Likewise, and emit a -Wpsabi diagnostic
>         for it.
>
>         * gcc.target/i386/pr65146.c: New test.

LGTM, but please allow some time for HJ to comment.

Thanks,
Uros.

> --- gcc/config/i386/i386.c.jj   2020-08-24 10:00:01.321258451 +0200
> +++ gcc/config/i386/i386.c      2020-08-26 19:19:11.834297395 +0200
> @@ -16487,7 +16487,11 @@ iamcu_alignment (tree type, int align)
>
>    /* Intel MCU psABI specifies scalar types > 4 bytes aligned to 4
>       bytes.  */
> -  mode = TYPE_MODE (strip_array_types (type));
> +  type = strip_array_types (type);
> +  if (TYPE_ATOMIC (type))
> +    return align;
> +
> +  mode = TYPE_MODE (type);
>    switch (GET_MODE_CLASS (mode))
>      {
>      case MODE_INT:
> @@ -16644,7 +16648,8 @@ ix86_local_alignment (tree exp, machine_
>        && align == 64
>        && ix86_preferred_stack_boundary < 64
>        && (mode == DImode || (type && TYPE_MODE (type) == DImode))
> -      && (!type || !TYPE_USER_ALIGN (type))
> +      && (!type || (!TYPE_USER_ALIGN (type)
> +                   && !TYPE_ATOMIC (strip_array_types (type))))
>        && (!decl || !DECL_USER_ALIGN (decl)))
>      align = 32;
>
> @@ -16757,7 +16762,8 @@ ix86_minimum_alignment (tree exp, machin
>    /* Don't do dynamic stack realignment for long long objects with
>       -mpreferred-stack-boundary=2.  */
>    if ((mode == DImode || (type && TYPE_MODE (type) == DImode))
> -      && (!type || !TYPE_USER_ALIGN (type))
> +      && (!type || (!TYPE_USER_ALIGN (type)
> +                   && !TYPE_ATOMIC (strip_array_types (type))))
>        && (!decl || !DECL_USER_ALIGN (decl)))
>      {
>        gcc_checking_assert (!TARGET_STV);
> @@ -20293,11 +20299,30 @@ x86_field_alignment (tree type, int comp
>      return computed;
>    if (TARGET_IAMCU)
>      return iamcu_alignment (type, computed);
> -  mode = TYPE_MODE (strip_array_types (type));
> +  type = strip_array_types (type);
> +  mode = TYPE_MODE (type);
>    if (mode == DFmode || mode == DCmode
>        || GET_MODE_CLASS (mode) == MODE_INT
>        || GET_MODE_CLASS (mode) == MODE_COMPLEX_INT)
> -    return MIN (32, computed);
> +    {
> +      if (TYPE_ATOMIC (type) && computed > 32)
> +       {
> +         static bool warned;
> +
> +         if (!warned && warn_psabi)
> +           {
> +             const char *url
> +               = CHANGES_ROOT_URL "gcc-11/changes.html#ia32_atomic";
> +
> +             warned = true;
> +             inform (input_location, "the alignment of %<_Atomic %T%> "
> +                                     "fields changed in %{GCC 11.1%}",
> +                     TYPE_MAIN_VARIANT (type), url);
> +           }
> +       }
> +      else
> +      return MIN (32, computed);
> +    }
>    return computed;
>  }
>
> --- gcc/testsuite/gcc.target/i386/pr65146.c.jj  2020-08-26 19:25:27.720023020 +0200
> +++ gcc/testsuite/gcc.target/i386/pr65146.c     2020-08-26 19:28:24.982535651 +0200
> @@ -0,0 +1,12 @@
> +/* PR target/65146 */
> +/* { dg-do compile } */
> +/* { dg-options "-Wno-psabi" } */
> +
> +struct A { char a; _Atomic long long b; };
> +struct B { char a; _Atomic double b; };
> +struct C { char a; _Atomic long long b[2]; };
> +struct D { char a; _Atomic double b[2]; };
> +extern int a[__builtin_offsetof (struct A, b) == 8 ? 1 : -1];
> +extern int b[__builtin_offsetof (struct B, b) == 8 ? 1 : -1];
> +extern int c[__builtin_offsetof (struct C, b) == 8 ? 1 : -1];
> +extern int d[__builtin_offsetof (struct D, b) == 8 ? 1 : -1];
>
>         Jakub
>

On Thu, Aug 27, 2020 at 5:20 AM Uros Bizjak <ubizjak@gmail.com> wrote:
>
> On Thu, Aug 27, 2020 at 10:35 AM Jakub Jelinek <jakub@redhat.com> wrote:
> >
> > Hi!
> >
> > For _Atomic fields, lowering the alignment of long long or double etc.
> > fields on ia32 is undesirable, because then one really can't perform atomic
> > operations on those using cmpxchg8b.
> >
> > The following patch stops lowering the alignment in fields for _Atomic
> > types (the x86_field_alignment change) and for -mpreferred-stack-boundary=2
> > also ensures we don't misalign _Atomic long long etc. automatic variables
> > (the ix86_{local,minimum}_alignment changes).
> > Not sure about iamcu_alignment change, I know next to nothing about IA MCU,
> > but unless it doesn't have cmpxchg8b instruction, it would surprise me if we
> > don't want to do it as well.
> > clang apparently doesn't lower the field alignment for _Atomic.
>
> Intel MCU implements pentium ISA, which includes cmpxchg8b.

That is correct.

> > Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?
> >
> > 2020-08-27  Jakub Jelinek  <jakub@redhat.com>
> >
> >         PR target/65146
> >         * config/i386/i386.c (iamcu_alignment): Don't decrease alignment
> >         for TYPE_ATOMIC types.
> >         (ix86_local_alignment): Likewise.
> >         (ix86_minimum_alignment): Likewise.
> >         (x86_field_alignment): Likewise, and emit a -Wpsabi diagnostic
> >         for it.
> >
> >         * gcc.target/i386/pr65146.c: New test.
>
> LGTM, but please allow some time for HJ to comment.

LGTM too.  Thanks.

> Thanks,
> Uros.
>
> > --- gcc/config/i386/i386.c.jj   2020-08-24 10:00:01.321258451 +0200
> > +++ gcc/config/i386/i386.c      2020-08-26 19:19:11.834297395 +0200
> > @@ -16487,7 +16487,11 @@ iamcu_alignment (tree type, int align)
> >
> >    /* Intel MCU psABI specifies scalar types > 4 bytes aligned to 4
> >       bytes.  */
> > -  mode = TYPE_MODE (strip_array_types (type));
> > +  type = strip_array_types (type);
> > +  if (TYPE_ATOMIC (type))
> > +    return align;
> > +
> > +  mode = TYPE_MODE (type);
> >    switch (GET_MODE_CLASS (mode))
> >      {
> >      case MODE_INT:
> > @@ -16644,7 +16648,8 @@ ix86_local_alignment (tree exp, machine_
> >        && align == 64
> >        && ix86_preferred_stack_boundary < 64
> >        && (mode == DImode || (type && TYPE_MODE (type) == DImode))
> > -      && (!type || !TYPE_USER_ALIGN (type))
> > +      && (!type || (!TYPE_USER_ALIGN (type)
> > +                   && !TYPE_ATOMIC (strip_array_types (type))))
> >        && (!decl || !DECL_USER_ALIGN (decl)))
> >      align = 32;
> >
> > @@ -16757,7 +16762,8 @@ ix86_minimum_alignment (tree exp, machin
> >    /* Don't do dynamic stack realignment for long long objects with
> >       -mpreferred-stack-boundary=2.  */
> >    if ((mode == DImode || (type && TYPE_MODE (type) == DImode))
> > -      && (!type || !TYPE_USER_ALIGN (type))
> > +      && (!type || (!TYPE_USER_ALIGN (type)
> > +                   && !TYPE_ATOMIC (strip_array_types (type))))
> >        && (!decl || !DECL_USER_ALIGN (decl)))
> >      {
> >        gcc_checking_assert (!TARGET_STV);
> > @@ -20293,11 +20299,30 @@ x86_field_alignment (tree type, int comp
> >      return computed;
> >    if (TARGET_IAMCU)
> >      return iamcu_alignment (type, computed);
> > -  mode = TYPE_MODE (strip_array_types (type));
> > +  type = strip_array_types (type);
> > +  mode = TYPE_MODE (type);
> >    if (mode == DFmode || mode == DCmode
> >        || GET_MODE_CLASS (mode) == MODE_INT
> >        || GET_MODE_CLASS (mode) == MODE_COMPLEX_INT)
> > -    return MIN (32, computed);
> > +    {
> > +      if (TYPE_ATOMIC (type) && computed > 32)
> > +       {
> > +         static bool warned;
> > +
> > +         if (!warned && warn_psabi)
> > +           {
> > +             const char *url
> > +               = CHANGES_ROOT_URL "gcc-11/changes.html#ia32_atomic";
> > +
> > +             warned = true;
> > +             inform (input_location, "the alignment of %<_Atomic %T%> "
> > +                                     "fields changed in %{GCC 11.1%}",
> > +                     TYPE_MAIN_VARIANT (type), url);
> > +           }
> > +       }
> > +      else
> > +      return MIN (32, computed);
> > +    }
> >    return computed;
> >  }
> >
> > --- gcc/testsuite/gcc.target/i386/pr65146.c.jj  2020-08-26 19:25:27.720023020 +0200
> > +++ gcc/testsuite/gcc.target/i386/pr65146.c     2020-08-26 19:28:24.982535651 +0200
> > @@ -0,0 +1,12 @@
> > +/* PR target/65146 */
> > +/* { dg-do compile } */
> > +/* { dg-options "-Wno-psabi" } */
> > +
> > +struct A { char a; _Atomic long long b; };
> > +struct B { char a; _Atomic double b; };
> > +struct C { char a; _Atomic long long b[2]; };
> > +struct D { char a; _Atomic double b[2]; };
> > +extern int a[__builtin_offsetof (struct A, b) == 8 ? 1 : -1];
> > +extern int b[__builtin_offsetof (struct B, b) == 8 ? 1 : -1];
> > +extern int c[__builtin_offsetof (struct C, b) == 8 ? 1 : -1];
> > +extern int d[__builtin_offsetof (struct D, b) == 8 ? 1 : -1];
> >
> >         Jakub
> >