$ cat >test.c
#include <stdio.h>

int main(void) {
  int i = 0;
  i = i++;
  printf("%d\n", i);
  return 0;
}
$ gcc-3.3 -Wall -O0 -g1 -o test test.c
test.c: In function `main':
test.c:5: warning: operation on `i' may be undefined
$ ./test
1
$ gcc-4.0 -Wall -O0 -g1 -o test test.c
test.c: In function 'main':
test.c:5: warning: operation on 'i' may be undefined
$ ./test
1
$ gcc-3.3 -Wall -O3 -g0 -o test test.c
test.c: In function `main':
test.c:5: warning: operation on `i' may be undefined
$ ./test
1
$ gcc-4.0 -Wall -O3 -g0 -o test test.c
test.c: In function 'main':
test.c:5: warning: operation on 'i' may be undefined
$ ./test
1