返回不小于num的最小2的幂指数

nextpow2.c

static unsigned int nextpow2(unsigned int num) {
	--num;
	num |= num >> 1;
	num |= num >> 2;
	num |= num >> 4;
	num |= num >> 8;
	num |= num >> 16;
	return ++num;
}