>>24
1. Use
fprintf (
"fast printf") instead of
printf. (recent studies revealed that the fastest printing function is
vasprintf (the
very accellerated sprintf)
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).