#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() { return ++count; }
int g() { return ++count >= 2 ? 5 : 3; }
int h(int a, int b) { return a+b; }
int main() {
  int x = h(f(), g());
  printf ("%d\n", x);
  
  int y = 3;
  y = (y += 1) + (y = y*y);
  printf ("%d\n", y);
}

$ clang -Wall undefined.c 
undefined.c:13:21: warning: unsequenced modification and access to 'y' [-Wunsequenced]
  y = (y += 1) + (y = y*y);
         ~~         ^
1 warning generated.
$ ./a.out 
6
20

• a=f(), b=g(), a+b
• y=y+1; y=y+y*y

#include <stdio.h>
int count = 0;
int f() { return ++count; }
int g() { return ++count >= 2 ? 5 : 3; }
int h(int a, int b) { return a+b; }
int main() {
  int x = h(f(), g());
  printf ("%d\n", x);
  
  int y = 3;
  y = (y += 1) + (y = y*y);
  printf ("%d\n", y);
}

$ gcc -Wall -O3 undefined.c
undefined.c:13:21: warning: unsequenced modification and access to 'y' [-Wunsequenced]
   y = (y += 1) + (y = y*y);
     ^
$ ./a.out 
5
32

• b=g(), a=f(), a+b
• y=y+1; y=y*y; y=y+y;