[Bug tree-optimization/86886] Gcc miscompiles at -O3 on valid code

[helloqirun at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86886

--- Comment #2 from Qirun Zhang <helloqirun at gmail dot com> ---
(In reply to Andrew Pinski from comment #1)
> I don't think this is a bug.  You are taking the address of a field of an
> union and accessing via that address.  This is not tracked at all.

Here is a reduced case without "union".

$ gcc-6.1 -O3 abc.c; ./a.out
0

$ gcc abc.c; ./a.out
0

$ gcc-8.1 -O3 abc.c; ./a.out
1

$ gcc-trunk -O3 abc.c; ./a.out
1

$ cat abc.c
void printf();
short b, c;
int e, d;
int *f = &e, *g = &d;
unsigned short **h = (unsigned short **)&f;
void i(int a) {
  *f = 0;
  for (; b;)
    ;
  c = **h;
  d = b = e ? (unsigned char)c : 0;
  *g = b;
}
int main() {
  **h = 1;
  i(6);
  printf("%d\n", c);
}