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From: "ubizjak at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug target/96189] Failure to use eflags from cmpxchg on x86
Date: Thu, 16 Jul 2020 07:05:52 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D96189

Uro=C5=A1 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 Uro=C5=A1 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 =3D 0;
  __atomic_compare_exchange_n (x, &y, z, 0, __ATOMIC_RELAXED,
__ATOMIC_RELAXED);
  return y =3D=3D 0;
}

(which is used in a couple of places throughout glibc), due to early peepho=
le2
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 =3D y;
  __atomic_compare_exchange_n (x, &y, z, 0, __ATOMIC_RELAXED,
__ATOMIC_RELAXED);
  return y =3D=3D 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 w=
ould
apply universally for all targets, and would also handle CMPXCHG[8,16]B
doubleword instructions on x86 targets.

Let's ask experts.=