[Bug target/96189] Failure to use eflags from cmpxchg on x86

["rguenther at suse dot de" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96189

--- Comment #6 from rguenther at suse dot de <rguenther at suse dot de> ---
On July 16, 2020 9:05:52 AM GMT+02:00, ubizjak at gmail dot com
<gcc-bugzilla@gcc.gnu.org> wrote:
>https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96189
>
>Uroš Bizjak <ubizjak at gmail dot com> changed:
>
>           What    |Removed                     |Added
>----------------------------------------------------------------------------
>              CC|                            |jakub at gcc dot gnu.org,
>               |                            |rguenth at gcc dot gnu.org
>             Status|RESOLVED                    |REOPENED
>         Resolution|FIXED                       |---
>
>--- Comment #5 from Uroš Bizjak <ubizjak at gmail dot com> ---
>Hm...
>
>Please note that peephole2 scanning require exact RTL sequences, and
>already
>fails for e.g.:
>
>_Bool
>foo (unsigned int *x, unsigned int z)
>{
>  unsigned int y = 0;
>  __atomic_compare_exchange_n (x, &y, z, 0, __ATOMIC_RELAXED,
>__ATOMIC_RELAXED);
>  return y == 0;
>}
>
>(which is used in a couple of places throughout glibc), due to early
>peephole2
>optimization that converts:
>
>(insn 7 4 8 2 (set (reg:SI 0 ax [90])
>        (const_int 0 [0])) "cmpx0.c":5:3 75 {*movsi_internal}
>
>to:
>
>(insn 31 4 8 2 (parallel [
>            (set (reg:DI 0 ax [90])
>                (const_int 0 [0]))
>            (clobber (reg:CC 17 flags))
>
>Other than that, the required sequence is broken quite often by various
>reloads, due to the complexity of CMPXCHG insn.
>
>However, __atomic_compare_exchange_n returns a boolean value that is
>exactly
>what the first function is testing, so the following two functions are
>equivalent:
>
>--cut here--
>_Bool
>foo (unsigned int *x, unsigned int y, unsigned int z)
>{
>  unsigned int old_y = y;
>  __atomic_compare_exchange_n (x, &y, z, 0, __ATOMIC_RELAXED,
>__ATOMIC_RELAXED);
>  return y == old_y;
>}
>
>_Bool
>bar (unsigned int *x, unsigned int y, unsigned int z)
>{
>  return __atomic_compare_exchange_n (x, &y, z, 0, __ATOMIC_RELAXED,
>__ATOMIC_RELAXED);
>}
>--cut here--
>
>I wonder, if the above transformation can happen on the tree level, so
>it would
>apply universally for all targets, and would also handle CMPXCHG[8,16]B
>doubleword instructions on x86 targets.
>
>Let's ask experts.

In principle value numbering can make the comparison available at the cmpxchg
and replace the later comparison. We've pondered with this trick for memcpy
results for example. 

Richard.