[Bug rtl-optimization/128055] New: wrong code at -O1 and above: store is deleted although it is read back through an __attribute__((alias)) of the variable

[kyuwoncho18 at gmail dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=128055

            Bug ID: 128055
           Summary: wrong code at -O1 and above: store is deleted although
                    it is read back through an __attribute__((alias)) of
                    the variable
           Product: gcc
           Version: 17.0
            Status: UNCONFIRMED
          Keywords: wrong-code
          Severity: normal
          Priority: P3
         Component: rtl-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: kyuwoncho18 at gmail dot com
  Target Milestone: ---
            Target: x86_64-pc-linux-gnu

int a = 0;
extern int b __attribute__((alias("a")));

__attribute__((noipa)) int f(void) {
  a = 1;
  int x = b;
  a = 2;
  return x;
}

int main(void) {
  if (f() != 1)
    __builtin_abort();
}

godbolt: https://godbolt.org/z/66s8zYosG

b is an alias of a, so f must return 1 and the program must exit normally, as
it does at -O0 and with -O2 -fno-dse. At -O1, -O2, -O3 and -Os the store a = 1
is deleted, f returns 0, and the program aborts.