Pointer alignment optimized for power of 2 alignment

#include <stdio.h>
#include <stdint.h>
#include <stdlib.h>
#include <assert.h>


static
bool is_pow2(uint64_t a)
{
	return a && !(a & (a - 1));
}

static
size_t ceil_align(size_t ptr, size_t alignment)
{
  	assert(alignment);
  	
  	if(is_pow2(alignment))
    {
        size_t result = ptr & -alignment;
        if(result != ptr) result = result + alignment;

        return result;
    }
  	else
    {
        size_t remain = ptr % alignment;
        if(remain == 0) return ptr;
        return (ptr - remain) + alignment;
    }
}

static
void *ceil_align(void *ptr, size_t alignment)
{
  	return (void *)ceil_align((size_t)ptr, alignment);
}

int main(void)
{
    printf("%lu\n", ceil_align(65, 32));
    return 0;
}


// Align stack pointer to 16. Stack must already be aligned by 8.
//mov r10, rsp
//and r10, 1000b
//add rsp, r10