C optimisations

Loop jamming

Never use two loops where one will suffice:
for(i=0; i<100; i++)
{
stuff();
}

for(i=0; i<100; i++)
{
morestuff();
}
It would be better to do:
for(i=0; i<100; i++)
{
stuff();
morestuff();
}
Note, however, that if you do a lot of work in the loop, it might not fit into your processor's instruction cache. In this case, two separate loops may actually be faster as each one can run completely in the cache.

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Faster for() loops

Simple but effective.
Ordinarily, you would code a simple for() loop like this:
for( i=0; i<10; i++){ ... }
i loops through the values 0,1,2,3,4,5,6,7,8,9
If you don't care about the order of the loop counter, you can do this instead:

for( i=10; i--; ) { ... }
Using this code, i loops through the values 9,8,7,6,5,4,3,2,1,0, and the loop should be faster.
This works because it is quicker to process "i--" as the test condition, which says "is i non-zero? If so, decrement it and continue.".
For the original code, the processor has to calculate "subtract i from 10. Is the result non-zero? if so, increment i and continue.". In tight loops, this make a considerable difference.
The syntax is a little strange, put is perfectly legal. The third statement in the loop is optional (an infinite loop would be written as "for( ; ; )" ). The same effect could also be gained by coding:

for(i=10; i; i--){}
or (to expand it further)
for(i=10; i!=0; i--){}
The only things you have to be careful of are remembering that the loop stops at 0 (so if you wanted to loop from 50-80, this wouldn't work), and the loop counter goes backwards.It's easy to get caught out if your code relies on an ascending loop counter.

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switch() instead of if...else...

For large decisions involving if...else...else..., like this:
if( val == 1)
dostuff1();
else if (val == 2)
dostuff2();
else if (val == 3)
dostuff3();
it may be faster to use a switch:
switch( val )
{
case 1: dostuff1(); break;

case 2: dostuff2(); break;

case 3: dostuff3(); break;
}
In the if() statement, if the last case is required, all the previous ones will be tested first. The switch lets us cut out this extra work. If you have to use a big if..else.. statement, test the most likely cases first.