#include 
  #include 

  int shift(int x, int n) { return x << n; }

  int main(int argc, char **argv) {
      printf("%d\n", shift(1, 32));   /* n == width: UB */
      return 0;
  }

This program exhibits UB in Fil-C, and you can see that the optimizer does different things at -O0 (outputs 1) and -O1/-O2/-O3 (outputs 0). Since this creates poison, which Fil-C doesn't remove, you can use it to construct all kinds of weird things.

    static void loop(void) {
        int s = shift(1, 32);
        int n = 0;
        for (int i = 0; i < s + 3; i++)
            n++;
        printf("[loop] iterations=%d (s+3=%d)\n", n, s + 3);
    }
    
    static void sw(void) {
        switch (shift(1, 32)) {
        case 0:  puts("[switch] case 0"); break;
        case 1:  puts("[switch] case 1"); break;
        default: puts("[switch] default"); break;
        }
    }
    
    int main(int argc, char **argv) {
        loop();
        sw();
        return 0;
    }

In Fil-C -O0, this gives 4 iterations of the loop and executes sw(). At any higher optimization level, it turns loop() into an infinite loop and drops sw() from the binary entirely.