[Bug rtl-optimization/94857] New: Failure to optimize load+add+store into add on memory when getting carry flag afterwards on x86

[gabravier at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94857

            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 = *p;
    *p += 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