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From: "gabravier at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/94846] New: Failure to optimize jnc+inc into
 adc
Date: Wed, 29 Apr 2020 12:05:38 +0000
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X-Bugzilla-Component: tree-optimization
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D94846

            Bug ID: 94846
           Summary: Failure to optimize jnc+inc into adc
           Product: gcc
           Version: 10.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: gabravier at gmail dot com
  Target Milestone: ---

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

    return *p;
}

This is what LLVM outputs with -O3 :=20

f(unsigned int*, unsigned int): # @f(unsigned int*, unsigned int)
  mov eax, esi
  add eax, dword ptr [rdi]
  adc eax, 0
  mov dword ptr [rdi], eax
  ret

This is what GCC outputs :

f(unsigned int*, unsigned int):
  mov eax, esi
  add eax, DWORD PTR [rdi]
  jnc .L6
  add eax, 1
.L6:
  mov DWORD PTR [rdi], eax
  ret

GCC should most likely be optimizing this to the same thing as LLVM.=