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?

1. Use fprintf ("fast printf") instead of printf.
2. ++i is faster than both i++ and i = i + 1.
3. void main(void) is faster than int main(void) or int main(int, char **) since no value needs to be returned to the OS.
4. Swapping with exclusive-or (a^=b^=a^=b swaps a and b) is faster than using a temporary. This works for all types (including structures), but not on all compilers. Some compilers may also give you a harmless warning.
5. Static storage duration objects are faster than automatic storage duration objects because the CPU doesn't have to set aside storage on the stack every time a function is called. Make your loop indexes global so that you can use them everywhere:
int i;
void func(void) { for (i = 0; i < 10; i++) ; /* ... */ }
void func2(void) { for (i = 0; i < 20; i++) ; /* ... */ }
/* ... */
6. Compilers often give more memory to arrays than you asked for. Here's how to check how big an array actually is (memset returns a null pointer if the size you passed to it is bigger than the size of the array you passed to it):
int arr[256];
size_t realsize;
for (realsize = 0; realsize <= SIZE_MAX; ++realsize)
        if (!memset(arr, 0, realsize)) break;
/* now you know that arr actually has realsize / sizeof (int) elements */
If you combine this with #5, your program will be faster in the long run (but this usually doesn't work for short programs).