[Bug middle-end/115954] New: Alignment of _Atomic structs incompatible between GCC and LLVM

[Bug middle-end/115954] New: Alignment of _Atomic structs incompatible between GCC and LLVM

[wilco at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115954

            Bug ID: 115954
           Summary: Alignment of _Atomic structs incompatible between GCC
                    and LLVM
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: middle-end
          Assignee: unassigned at gcc dot gnu.org
          Reporter: wilco at gcc dot gnu.org
  Target Milestone: ---

The following code shows ABI inconsistencies between GCC and LLVM:

#include <stdio.h>
#include <stdatomic.h>
#include <stdalign.h>

_Atomic struct A3 { char a[3]; } a3;
_Atomic struct A7 { char a[7]; } a7;
_Atomic struct A8 { char a[8]; } a8;
_Atomic struct A9 { char a[9]; } a9;
_Atomic struct A16 { char a[16]; } a16;

int main (void)
{
   printf("size %ld align %ld lockfree %d\n", sizeof (a3), alignof (a3),
atomic_is_lock_free (&a3));
   printf("size %ld align %ld lockfree %d\n", sizeof (a7), alignof (a7),
atomic_is_lock_free (&a7));
   printf("size %ld align %ld lockfree %d\n", sizeof (a8), alignof (a8),
atomic_is_lock_free (&a8));
   printf("size %ld align %ld lockfree %d\n", sizeof (a9), alignof (a9),
atomic_is_lock_free (&a9));
   printf("size %ld align %ld lockfree %d\n", sizeof (a16), alignof (a16),
atomic_is_lock_free (&a16));
   return 0;
}

Compiled with GCC -O2 -latomic I get this on AArch64:

size 3 align 1 lockfree 1
size 7 align 1 lockfree 1
size 8 align 8 lockfree 1
size 9 align 1 lockfree 0
size 16 align 16 lockfree 0

However LLVM reports:

size 4 align 4 lockfree 1
size 8 align 8 lockfree 1
size 8 align 8 lockfree 1
size 16 align 16 lockfree 1
size 16 align 16 lockfree 1

The same is true for x86_64 GCC:

size 3 align 1 lockfree 0
size 7 align 1 lockfree 1  (due to alignment in libatomic)
size 8 align 8 lockfree 1
size 9 align 1 lockfree 0
size 16 align 16 lockfree 0

and LLVM:

size 4 align 4 lockfree 1
size 8 align 8 lockfree 1
size 8 align 8 lockfree 1
size 16 align 16 lockfree 0
size 16 align 16 lockfree 0

Increasing the alignment of small _Atomic structs to a power of 2 means these
will always be lock free rather than sometimes depending on alignment.

This also has the nice property that all types smaller than the maximum
supported atomic size are always lock free so there is no need to make
libatomic calls.

[Bug middle-end/115954] Alignment of _Atomic structs incompatible between GCC and LLVM

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115954

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Target|                            |aarch64 x86_64-*-*
                 CC|                            |matz at gcc dot gnu.org

--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
Not sure what the x86 psABI says here (possibly nothing for aggregate _Atomic).

[Bug middle-end/115954] Alignment of _Atomic structs incompatible between GCC and LLVM

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115954

--- Comment #2 from Richard Biener <rguenth at gcc dot gnu.org> ---
(In reply to Richard Biener from comment #1)
> Not sure what the x86 psABI says here (possibly nothing for aggregate
> _Atomic).

It doesn't consider _Atomic [influencing the ABI] at all.

Note I think your test queries actual object alignment which a compiler
can of course increase vs. what the ABI requires as minimum alignment,
you should possibly cross-check with alignof/sizeof of the type.

On x86 clang returns size 8 and align 8 for the atomic A7 type (GCC does not).

[Bug middle-end/115954] Alignment of _Atomic structs incompatible between GCC and LLVM

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115954

--- Comment #3 from Richard Biener <rguenth at gcc dot gnu.org> ---
And I'll note the original JTC1/SC22/WG14 - N2771 Title: C23 Atomics paper
mentions "ABI would have been fully determined to be compatible with non-atomic
type, leaving no room to implementations for introducing inconsistencies." but
I can't find where/if this went into the actual standard.

[Bug middle-end/115954] Alignment of _Atomic structs incompatible between GCC and LLVM

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115954

--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Richard Biener from comment #1)
> Not sure what the x86 psABI says here (possibly nothing for aggregate
> _Atomic).

I've been asking for it to say something for years.
https://groups.google.com/g/ia32-abi/c/Tlu6Hs-ohPY

[Bug middle-end/115954] Alignment of _Atomic structs incompatible between GCC and LLVM

[wilco at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115954

--- Comment #5 from Wilco <wilco at gcc dot gnu.org> ---
(In reply to Richard Biener from comment #2)
> (In reply to Richard Biener from comment #1)
> > Not sure what the x86 psABI says here (possibly nothing for aggregate
> > _Atomic).
> 
> It doesn't consider _Atomic [influencing the ABI] at all.
> 
> Note I think your test queries actual object alignment which a compiler
> can of course increase vs. what the ABI requires as minimum alignment,
> you should possibly cross-check with alignof/sizeof of the type.
> 
> On x86 clang returns size 8 and align 8 for the atomic A7 type (GCC does
> not).

I tried using the type for sizeof/alignof, and it returns the same values. So
GCC overaligns structs that are an exact power of 2.

[Bug c/115954] Alignment of _Atomic structs incompatible between GCC and LLVM

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115954

--- Comment #6 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
https://gitlab.com/x86-psABIs/i386-ABI/-/issues/1 for x86_64 abi.

Aarch64 should most likely also do the same ...

[Bug target/115954] Alignment of _Atomic structs incompatible between GCC and LLVM

[wilco at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115954

--- Comment #7 from Wilco <wilco at gcc dot gnu.org> ---
(In reply to Andrew Pinski from comment #6)
> https://gitlab.com/x86-psABIs/i386-ABI/-/issues/1 for x86_64 abi.
> 
> Aarch64 should most likely also do the same ...

Yes, that's why I raised this - GCC only over aligning some sizes seems more an
accident rather than a designed ABI.

[Bug target/115954] Alignment of _Atomic structs incompatible between GCC and LLVM

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115954

--- Comment #8 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
https://gcc.gnu.org/legacy-ml/gcc/2019-08/msg00191.html

[Bug target/115954] Alignment of _Atomic structs incompatible between GCC and LLVM

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115954

--- Comment #9 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Note this was raised on the LLVM side back in 2016:
https://github.com/llvm/llvm-project/issues/26836

[Bug target/115954] Alignment of _Atomic structs incompatible between GCC and LLVM

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115954

--- Comment #10 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
https://gcc.gnu.org/legacy-ml/gcc/2019-08/msg00198.html

[Bug target/115954] Alignment of _Atomic structs incompatible between GCC and LLVM

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115954

--- Comment #11 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Can someone please raise this also to
https://github.com/ARM-software/abi-aa/issues ? Looks like the riscv folks are
ahead of the curve of defining the size/alignment here, see
https://github.com/riscv-non-isa/riscv-elf-psabi-doc/pull/112  and
https://github.com/riscv-non-isa/riscv-elf-psabi-doc/pull/117 .

[Bug target/115954] Alignment of _Atomic structs incompatible between GCC and LLVM

[wilco at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115954

--- Comment #12 from Wilco <wilco at gcc dot gnu.org> ---
This came out of the AArch64 Atomic ABI design work:
https://github.com/ARM-software/abi-aa/pull/256