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From: "hjl.tools at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug target/103069] cmpxchg isn't optimized
Date: Mon, 24 Jan 2022 23:55:41 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D103069

H.J. Lu <hjl.tools at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|REOPENED                    |NEW

--- Comment #9 from H.J. Lu <hjl.tools at gmail dot com> ---
nptl/nptl_setxid.c in glibc has

  do=20=20
    {=20=20=20
      flags =3D THREAD_GETMEM (self, cancelhandling);
      newval =3D THREAD_ATOMIC_CMPXCHG_VAL (self, cancelhandling,
                                          flags & ~SETXID_BITMASK, flags);
    }=20=20=20
  while (flags !=3D newval);

GCC 12 generates:

         899f0: 64 8b 14 25 08 03 00    mov    %fs:0x308,%edx
         899f8: 89 d6                   mov    %edx,%esi
         899fa: 89 d0                   mov    %edx,%eax
         899fc: 83 e6 bf                and    $0xffffffbf,%esi
         899ff: f0 0f b1 31             lock cmpxchg %esi,(%rcx)=20=20=20=
=20=20=20=20
         89a03: 75 eb                   jne    899f0
<__GI___nptl_setxid_sighand
ler+0x90>=