c - Do temp variables slow down my program?

Question

Suppose I have the following C code:

int i = 5;
int j = 10;
int result = i + j;

If I'm looping over this many times, would it be faster to use int result = 5 + 10? I often create temporary variables to make my code more readable, for example, if the two variables were obtained from some array using some long expression to calculate the indices. Is this bad performance-wise in C? What about other languages?

Answer 1

A modern optimizing compiler should optimize those variables away, for example if we use the following example in godbolt with gcc using the -std=c99 -O3 flags (see it live):

#include <stdio.h>

void func()
{
  int i = 5;
  int j = 10;
  int result = i + j;

  printf( "%d
", result ) ;
}

it will result in the following assembly:

movl    $15, %esi

for the calculation of i + j, this is form of constant propagation.

Note, I added the printf so that we have a side effect, otherwise func would have been optimized away to:

func:
  rep ret

These optimizations are allowed under the as-if rule, which only requires the compiler to emulate the observable behavior of a program. This is covered in the draft C99 standard section 5.1.2.3 Program execution which says:

In the abstract machine, all expressions are evaluated as specified by the semantics. An actual implementation need not evaluate part of an expression if it can deduce that its value is not used and that no needed side effects are produced (including any caused by calling a function or accessing a volatile object).

Also see: Optimizing C++ Code : Constant-Folding