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From: "gabravier at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug target/96189] New: Failure to use eflags from cmpxchg on x86
Date: Mon, 13 Jul 2020 19:44:23 +0000
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X-Bugzilla-Version: 11.0
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D96189

            Bug ID: 96189
           Summary: Failure to use eflags from cmpxchg on x86
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: gabravier at gmail dot com
  Target Milestone: ---

bool f(unsigned char* addr, unsigned char old_val, unsigned char new_val)
{
    auto old_val_cpy =3D old_val;
    __atomic_compare_exchange_n(addr, &old_val, new_val, 0, 0, 0);
    return old_val =3D=3D old_val_cpy;
}

With -O3, LLVM outputs this :

f(unsigned char*, unsigned char, unsigned char): # @f(unsigned char*, unsig=
ned
char, unsigned char)
  mov eax, esi
  lock cmpxchg byte ptr [rdi], dl
  sete al
  ret

GCC outputs this :

f(unsigned char*, unsigned char, unsigned char):
  mov eax, esi
  movzx edx, dl
  lock cmpxchg BYTE PTR [rdi], dl
  cmp al, sil
  sete al
  ret

GCC could use the EFLAGS generated from cmpxchg, but it does not.=