What's the most efficient way to make bitwise operations in a C array

Question

I have a C array like:

char byte_array[10];

And another one that acts as a mask:

char byte_mask[10];

I would

Answer 1

If you want to make it faster, make sure that byte_array has length that is multiple of 4 (8 on 64-bit machines), and then:

char byte_array[12];
char byte_mask[12];
/* Checks for proper alignment */
assert(((unsigned int)(void *)byte_array) & 3 == 0);
assert(((unsigned int)(void *)byte_mask) & 3 == 0);
for (i = 0; i < (10+3)/4; i++) {
  ((unsigned int *)(byte_array))[i] &= ((unsigned int *)(byte_mask))[i];
}

This is much faster than doing it byte per byte.

(Note that this is in-place mutation; if you want to keep the original byte_array also, then you obviously need to store the results in another array instead.)