upper_power_of_two.c

#include <stdio.h>

#if defined(DEBUG)
#define PRINT(_fmt_, ...) printf(_fmt_, __VA_ARGS__);
#else
#define PRINT(_fmt_, ...)
#endif

/* works in the range: [0x1, 0x7FFF FFFF FFFF FFFF] */
static size_t upper_power_of_two(size_t v)
{
	int i;

	PRINT("v:   0x%.16lx\n", v);
	v--;
	PRINT("v--: 0x%.16lx\n", v);
	for (i = 0; i < 6; i++) {
		v |= v >> (1 << i);

		PRINT("v |= v >> (1 << %d): 0x%.16lx\n", i, v);
	}
	v++;
	PRINT("v++: 0x%.16lx\n", v);

	return v;
}

int main(int argc, char **argv)
{
	size_t val = 0x7000000000000001;

	printf("upper_power_of_two(%lu) = %lu\n", val,
		upper_power_of_two(val));
	return 0;
}