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?


#define iswap(x,y) (do uintptr_t __x=x,x=y,y=__x; while(0))
#define pswap(p,s) (do void *__p=p,p=s,s=__p; while(0))
#define fswap(f,g) (do long double __f=f,f=g,g=__f; while(0))