Comment by abainbridge

The examples are fun, but rather than yet another article saying how amazing optimizing compilers are (they are, I already know), I'd probably benefit more from an article explaining when obvious optimizations are missed and what to do about it.

Some boring examples I've just thought of...

eg 1:

    int bar(int num) { return num / 2; }

Doesn't get optimized to a single shift right, because the that won't work if num is negative. In this case we can change the ints to unsigneds to tell the compiler we know the number isn't negative. But it isn't always easy to express to the compiler everything you know about your data and use case. There is an art in knowing what kinds of things you need to tell the compiler in order to unlock optimizations.

eg 2:

    int foo(void) { return strlen("hello"); }

We all know that strlen will return 5, but some compilers don't: https://godbolt.org/z/M7x5qraE6

eg 3:

    int foo(char const *s) {
      if (strlen(s) < 3) return 0;
      if (strcmp(s, "hello") == 0)
        return 1;
      return 0;
    }

This function returns 1 if s is "hello". 0 otherwise. I've added a pointless strlen(). It seems like no compiler is clever enough to remove it. https://godbolt.org/z/Koj65eo5K. I can think of many reasons the compiler isn't able to spot this.