#include <stdio.h>
int isMinValue (int x) {
  return (x-1) > x;
}
int main () {
  int i = -2000000000;  
  while (!isMinValue(i))
    i--;
  printf ("Min value is %d\n", i);
}

$ gcc -O1 undefined.c && ./a.out 
Min value is -2147483648

$ gcc -O2 undefined.c && ./a.out 
^C #infinite loop

#include <stdio.h>
int count = 0;
int f () {
  count += 1;
  return count;
}
int main () {
  int z = f() + f();
  printf ("%d\n", z);
  z = (z += 1) + (z = z*z);
  printf ("%d\n", z);
}

$ clang -Wall undefined3.c 
undefined3.c:11:21: warning: unsequenced modification and access to 'z'
  z = (z += 1) + (z = z*z);
         ~~         ^
1 warning generated.
$ ./a.out 
3
20

• z=z+1; z=z+z*z

#include <stdio.h>
int count = 0;
int f () {
  count += 1;
  return count;
}
int main () {
  int z = f() + f();
  printf ("%d\n", z);
  z = (z += 1) + (z = z*z);
  printf ("%d\n", z);
}

$ gcc -Wall -O3 undefined3.c 
undefined3.c: In function ‘main’:
undefined3.c:11:5: warning: operation on ‘z’ may be undefined
   z = (z += 1) + (z = z*z);
     ^
$ ./a.out 
3
32

• z=z+1; z=z*z; z=z+z;