[Bug target/98977] New: [x86] Failure to optimize consecutive sub flags usage

[gabravier at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98977

            Bug ID: 98977
           Summary: [x86] Failure to optimize consecutive sub flags usage
           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: ---

extern bool z, c;

uint8_t f(uint8_t dest, uint8_t src)
{
    u8 res = dest - src;
    z = !res;
    c = src > dest;
    return res;
}

With -O3, LLVM outputs this:

f(unsigned char, unsigned char):
  mov eax, edi
  sub al, sil
  sete byte ptr [rip + z]
  setb byte ptr [rip + c]
  ret

GCC outputs this:

f(unsigned char, unsigned char):
  mov eax, edi
  sub al, sil
  sete BYTE PTR z[rip]
  cmp dil, sil
  setb BYTE PTR c[rip]
  ret

It seems desirable to eliminate the `cmp`, unless there's some weird flag stall
thing I'm not aware of.