BitScanReverse64Test.cpp

#include <intrin.h>
#include <cstdint>
#include <iostream>

#pragma intrinsic(_BitScanReverse64)
#pragma intrinsic(_BitScanReverse)
using namespace std;

unsigned long old_version(int64_t value)
{
	unsigned long leading_zero = 0;
	_BitScanReverse64(&leading_zero, value);
	return leading_zero;
}
unsigned long new_version(int64_t value)
{
	unsigned long leading_zero = 0;
	uint32_t high = value >> 32;
	if (_BitScanReverse(&leading_zero, high) != 0)
	{
		leading_zero += 32;
	}
	else 
	{
		uint32_t low = value & 0x00000000FFFFFFFF;
		_BitScanReverse(&leading_zero, low);		
	}
	return leading_zero;
}

int main()
{
	cout << "Bit\tOld\tNew\tSame?\n";
	for (int i = 0; i <= 64; ++i)
	{
		int64_t value = 1;
		value <<= i;
		auto old_lz = old_version(value);
		auto new_lz = new_version(value);
		cout << i << "\t" << old_lz << "\t" << new_lz << "\t" << (old_lz == new_lz) << "\n";
	}
}