[Bug tree-optimization/125814] New: Possible Missed Optimization at O3

[apowenq at gmail dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=125814

            Bug ID: 125814
           Summary: Possible Missed Optimization at O3
           Product: gcc
           Version: 17.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: apowenq at gmail dot com
  Target Milestone: ---

cat prog.c
int src(int v1_u8) {
  if (!((2 <= v1_u8) && (v1_u8 <= 3))) __builtin_unreachable();
  int i0_u8 = 5 - v1_u8;
  int i1_u8 = v1_u8 ^ i0_u8;
  return i1_u8;
}

GCC truck at O3 cannot optimize the C program (not a regression), but Clang
truck at O3 can.
Reproducer: https://godbolt.org/z/3jzG36v7r

GCC at O3:
"src":
        mov     eax, 5
        sub     eax, edi
        xor     eax, edi
        ret

Clang at O3:
src:
        mov     eax, 1
        ret