[Bug c/69776] New: Wrong optimization with aliasing

[ch3root at openwall dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69776

            Bug ID: 69776
           Summary: Wrong optimization with aliasing
           Product: gcc
           Version: 5.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: ch3root at openwall dot com
  Target Milestone: ---

The program:

#include <stdlib.h>
#include <stdio.h>

int main()
{
  void *p = malloc(10);
  int *pi = p;
  double *pd = p;

  *pi = 1;
  printf("*pi = %d\n", *pi);

  int a = *pi;
  *pd = 0;
  *pi = a;

  printf("p = %p\n", p);
  printf("*pi = %d\n", *pi);
}

compiled with `gcc -std=c11 -pedantic -O2` prints this:

*pi = 1
p = 0x1165010
*pi = 0

The last value is wrong, it should be 1.

Tested on gcc 4.7.2, 4.9.2 and 5.2.0 -- all affected.