How to align a value to a given alignment

Question

I have a value that I want to align to a given alignment, ie increase the value to the next multiple of the alignment if it is not already aligned.

What is a concise way to do this in C++?

eg

int x;
int alignment;
int y = ???; // align x to alignment

Answer 1

If the alignment is a power of 2, and the machine uses complement of 2 for negative numbers then:

mask = alignment - 1;
aligned_val = unaligned_val + (-unaligned_val & mask);

Answer 2

Lets say alignment is a

---(k-1)a-----------x--------------ka---------
         <----r----><-----(a-r)--->

where k is an integer (so ka is a multiple of alignment)

First find the remainder

r = x%a

then increment x to next multiple

y = x + (a-r)

But if r = 0, then y = x

So finally

r = x%a;
y = r? x + (a - r) : x;