[Bug tree-optimization/128053] New: wrong code at -O2 and -O3 for a vectorized read of an unsigned bit-field that contains the sign bit of a union's int member

[kyuwoncho18 at gmail dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=128053

            Bug ID: 128053
           Summary: wrong code at -O2 and -O3 for a vectorized read of an
                    unsigned bit-field that contains the sign bit of a
                    union's int member
           Product: gcc
           Version: 17.0
            Status: UNCONFIRMED
          Keywords: wrong-code
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: kyuwoncho18 at gmail dot com
  Target Milestone: ---
            Target: x86_64-pc-linux-gnu

union U { int i; struct { unsigned lo : 24; unsigned hi : 8; } s; };
int in[8] = { (int) 0xff000000u };
unsigned out[8];
int main(void) {
  for (int k = 0; k < 8; k++) {
    union U u;
    u.i = in[k];
    out[k] = u.s.hi;
  }
  if (out[0] != 255)
    __builtin_abort();
}

godbolt: https://godbolt.org/z/95zTzhser

u.s.hi is bits 24-31 of 0xff000000, so out[0] must be 255; -O0, -O1 and -O2
-fno-tree-vectorize get that, but -O2 and -O3 give 4294967295 and abort.