What is the effect of ordering if…else if statements by probability?

Question

Specifically, if I have a series of if...else if statements, and I somehow know beforehand the relative probability that each statement will evalua

Answer 1

Also depends on your compiler and the platform you’re compiling for.

In theory, the most likely condition should make the control jump as less as possible.

Typically the most likely condition should be first:

if (most_likely) {
     // most likely instructions
} else …

The most popular asm’s are based on conditional branches that jump when condition is true. That C code will be likely translated to such pseudo asm:

jump to ELSE if not(most_likely)
// most likely instructions
jump to end
ELSE:
…

This is because jumps make the cpu cancel the execution pipeline and stall because the program counter changed (for architectures that support pipelines which are really common). Then it’s about the compiler, which may or may not apply some sophisticated optimizations about having the statistically most probably condition to get the control make less jumps.