C quick calculation of next multiple of 4?

Question

What\'s a fast way to round up an unsigned int to a multiple of 4?

A multiple of 4 has the two least significant bits 0, right? So I could

Answer 1

I assume that what you are trying to achieve is the alignment of the input number, i.e. if the original number is already a multiple of 4, then it doesn't need to be changed. However, this is not clear from your question. Maybe you want next multiple even when the original number is already a multiple? Please, clarify.

In order to align an arbitrary non-negative number i on an arbitrary boundary n you just need to do

i = i / n * n;

But this will align it towards the negative infinity. In order to align it to the positive infinity, add n - 1 before peforming the alignment

i = (i + n - 1) / n * n;

This is already good enough for all intents and purposes. In your case it would be

i = (i + 3) / 4 * 4;

However, if you would prefer to to squeeze a few CPU clocks out of this, you might use the fact that the i / 4 * 4 can be replaced with a bit-twiddling i & ~0x3, giving you

i = (i + 3) & ~0x3;

although it wouldn't surprise me if modern compilers could figure out the latter by themselves.