[Bug c/85800] A miscompilation bug with unsigned char

["rguenth at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85800

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |WAITING
   Last reconfirmed|                            |2018-05-16
                 CC|                            |rguenth at gcc dot gnu.org
     Ever confirmed|0                           |1

--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
Hmm, I can't reproduce it because the arrays happen to be not adjacent for me.
But if I change

    if (c1 == c2)
      // Always true if p and q have same integer representation.
      arr3[i] = arr1[i];
    else
      arr3[i] = arr2[i];

to

    if (c1 == c2)
      // Always true if p and q have same integer representation.
      arr3[i] = arr2[i];
    else
      arr3[i] = arr1[i];

I get 10 consistently.  This is because then arr3 will indeed be A.

Changing main to

int main() {
  struct { int B[4]; int A[4]; } a;
  printf("%p %p\n", a.A, &a.B[4]);
  store_10_to_p(a.A, &a.B[4]);
  printf("%d\n", a.A[0]);
  return 0;
}

also makes it work reliably for me.

I guess at -O3 you get store_10_to_p inlined.  You can try
changing the function to

void __attribute__((noinline,noclone)) store_10_to_p(int *p, int *q) {

to see if that makes any difference.

As said, I don't see any bug here.