[Bug tree-optimization/126048] New: Possible Missed Optimization at O3

[apowenq at gmail dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=126048

            Bug ID: 126048
           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 (!((1 <= v1_u8) && (v1_u8 <= 16))) __builtin_unreachable();
  int i0_u8 = v1_u8 << v1_u8;
  int i1_u8 = i0_u8 / v1_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/fhGWYT8xs

Possible: `(x << x) / x` ==> `1 << x`

GCC at O3:
"src":
        mov     ecx, edi
        mov     eax, edi
        sal     eax, cl
        cdq
        idiv    edi
        ret

LLVM at O3:
src:
        mov     ecx, edi
        mov     eax, 1
        shl     eax, cl
        ret