[Bug c/63233] New: Valid out of bounds access leads to undefined behavior

["leis at in dot tum.de" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63233

            Bug ID: 63233
           Summary: Valid out of bounds access leads to undefined behavior
           Product: gcc
           Version: 4.9.1
            Status: UNCONFIRMED
          Severity: major
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: leis at in dot tum.de

Consider the following (minimized) program:

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

struct Foo {
   int a[1];
   int b;
};

int main(int argc, char** argv) {
   int index = atoi(argv[1]);

   struct Foo foo;
   foo.a[1] = 99;

   printf("%d\n", foo.a[index]);

   return 0;
}

When compiled with -O1 or higher and called with 1 as command line argument an
undefined value instead of 99 is printed. In my understanding of the standard,
foo.a is a pointer and foo.a[1] is simply (foo.a+1), which is a perfectly fine
memory address.

This program always prints the expected value (99) with clang (3.5) and icc
(14) on all optimization levels. In gcc 4.3.4 I get the expected result on -O0
and -O1 but not on -O2 or -O3.