Can I safely assume that if(true) and if(false) will be optimized?

Question

In my code at some point I have a function with a bool value template parameter. The implementation of the function in the true and falsecases are pretty much identical, except for one line. So I thought I would just put an if(value) statement to avoid rewriting the same function twice. When the function is called, value is determined at compile time so the if statement reduces to an if(true) or an if(false). Now, I wonder: is it safe to assume that any reasonable compiler (say g++, clang++, ...) will optimize that statement?

Yes, performance is critical, yes, the function is immensely lightweight so that even a single if would affect speed, yes this function needs to be executed a gazillion of times on a real-time device on which many human lives depend.

Answer 1

The short answer is "yes". The compiler will evaluate constant expressions, and in branches, it will figure out if the branch is always or never taken.

Here's an example:

#include <stdio.h>
void test(void) {
  if (false) {
    puts("Hello, world!");
  }
}

Here's the output assembly:

test():
        push    rbp
        mov     rbp, rsp
        nop
        pop     rbp
        ret

Note that no call to puts is even emitted. This is GCC 6.3 with no optimizations enabled (optimization is turned off and this still happens the right way).

See: https://godbolt.org/g/PJ7lSf