In the C if-else statement, should the condition which is more likely to be true come first?

Question

I happened to write a if-else statement, the condition would be false at most time(check a static pointer is assigned or not). Which one would be better for the compiler to optimize? Or they are just equal?. The function would be called so many times, so it is critical to optimize its performance.

void foo() {
  static int * p = NULL;
  if (p == NULL) {
     p = (int *) malloc( SIZE * sizeof(int)); 
  }
  //do something here
} 

void foo() {
  static int * p = NULL;
  if (p != NULL) {
    //do something here 
  } else {
    p = (int *) malloc( SIZE * sizeof(int));  
    //do something
  }
}

Answer 1

Some compilers can allow the dev to specify which is condition is more likely or unlikely to occur. This is heavily used in the Linux kernel.

In gcc, there's the likely(x) or unlikely(x) macros. Example:

if (unlikely(p == NULL)) {
    p = malloc(10);
}

Answer 2

If the function is called often enough to not be evicted from the branch predictor, then the predictor will take care of everything for you, since it will learn very quickly which is the "likely" branch.

Some compilers allow you to decorate conditionals with prediction hints. Check your vendor's documentation, and add a hint if you can.

Specific platforms document the default predictor behaviour (e.g. see the Intel Optimization Manual for x86), but it's best left to your compiler to implement that via the aforementioned hints. In fact, you don't really have a choice, since you don't control your compilers code generation anyway, so the only last measure left to you would be to write the code in machine code yourself and implement the platform's advice on default prediction.