C programming tricks

While programming optimization that makes ones' code completely unreadable is often a bad thing, there are places for it, like that one inner loop of your code that takes up 98% of the program's running time.

What are you tricks for improving performance in C, other than the obvious inline assembly or the like?

>>50
I'm not >>49, but I agree with him, although the results come out the same. printf() is extraordinarily slow, pushing any interesting results into the noise. Same can be said for rand(), although it's not as bad as printf(). Using time from the shell includes the startup time, so it reduces accuracy.

Here's the results I get with what I wrote:

C:\Devel\src>gcc t.c -std=c99 -o t.exe

C:\Devel\src>t
& 1: 15515 ticks
% 2: 25168 ticks

C:\Devel\src>gcc t.c -std=c99 -o t.exe -O2

C:\Devel\src>t
& 1: 2658 ticks
% 2: 2649 ticks

And here's the code. Note that it's still far from ideal, but at least the interesting bits are completely vanishing into the margin of error:

#include <stdlib.h>
#include <stdio.h>
#include <time.h>
#include <limits.h>

#define ITER 10

int main(void)
{
  int tmp;     // prevent certain optimizers from eliminating the loops
  int stub[2];
  clock_t start_time, avg_time;

  // timing for & 1
  start_time = clock();
  for(int i=0; i < ITER; i++) {
    for(int j=0; j < INT_MAX; j++) {
      tmp = stub[j & 1];    // cheaper than zOMG rand()
    }
  }
  avg_time = (clock() - start_time) / ITER;

  printf("& 1: %i ticks\n", avg_time);
 
  // timing for % 2
  start_time = clock();
  for(int i=0; i < ITER; i++) {
    for(int j=0; j < INT_MAX; j++) {
      tmp = stub[j % 2];
    }
  }
  avg_time = (clock() - start_time) / ITER;

  printf("%% 2: %i ticks\n", avg_time);

  return 0;
}

So, it's clear the result is the same after optimization. Modulus is slower, but the optimizer changes %2 to & 1.