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From: "gabravier at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug rtl-optimization/94857] New: Failure to optimize load+add+store
 into add on memory when getting carry flag afterwards on x86
Date: Wed, 29 Apr 2020 17:45:57 +0000
X-Bugzilla-Reason: CC
X-Bugzilla-Type: new
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X-Bugzilla-Product: gcc
X-Bugzilla-Component: rtl-optimization
X-Bugzilla-Version: 10.0
X-Bugzilla-Keywords: 
X-Bugzilla-Severity: normal
X-Bugzilla-Who: gabravier at gmail dot com
X-Bugzilla-Status: UNCONFIRMED
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X-Bugzilla-Priority: P3
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D94857

            Bug ID: 94857
           Summary: Failure to optimize load+add+store into add on memory
                    when getting carry flag afterwards on x86
           Product: gcc
           Version: 10.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: rtl-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: gabravier at gmail dot com
  Target Milestone: ---

bool f(unsigned *p, unsigned x)
{
    unsigned u =3D *p;
    *p +=3D x;
    return u > *p;
}

With -O3, LLVM outputs :

f(unsigned int*, unsigned int): # @f(unsigned int*, unsigned int)
  add dword ptr [rdi], esi
  setb al
  ret

GCC outputs :

f(unsigned int*, unsigned int):
  add esi, DWORD PTR [rdi]
  mov DWORD PTR [rdi], esi
  setc al
  ret=