How to use MSVC intrinsics to get the equivalent of this GCC code?

Question

The following code calls the builtin functions for clz/ctz in GCC and, on other systems, has C versions. Obviously, the C versions are a bit suboptimal if the system has a

Answer 1

There are two intrinsics "_BitScanForward" and "_BitScanReverse", which suits the same purpose for MSVC. Include . The functions are:

#ifdef _MSC_VER
#include <intrin.h>

static uint32_t __inline ctz( uint32_t x )
{
   int r = 0;
   _BitScanReverse(&r, x);
   return r;
}

static uint32_t __inline clz( uint32_t x )
{
   int r = 0;
   _BitScanForward(&r, x);
   return r;
}
#endif

There are equivalent 64bit versions "_BitScanForward64" and "_BitScanReverse64".

Read more here:

x86 Intrinsics on MSDN

Answer 2

I find it in a korean website https://torbjorn.tistory.com/317 In msvc compiler, you can use __lzcnt(unsigned int) to replace __builtin_clz(unsigned int) in gcc compiler.

Answer 3

If MSVC has a compiler intrinsic for this, it'll be here:

Compiler Intrinsics on MSDN

Otherwise, you'll have to write it using __asm

Answer 4

Bouncing from sh0dan code, the implementation should be corrected like this :

#ifdef _MSC_VER
#include <intrin.h>

uint32_t __inline ctz( uint32_t value )
{
    DWORD trailing_zero = 0;

    if ( _BitScanForward( &trailing_zero, value ) )
    {
        return trailing_zero;
    }
    else
    {
        // This is undefined, I better choose 32 than 0
        return 32;
    }
}

uint32_t __inline clz( uint32_t value )
{
    DWORD leading_zero = 0;

    if ( _BitScanReverse( &leading_zero, value ) )
    {
       return 31 - leading_zero;
    }
    else
    {
         // Same remarks as above
         return 32;
    }
}
#endif

As commented in the code, both ctz and clz are undefined if value is 0. In our abstraction, we fixed __builtin_clz(value) as (value?__builtin_clz(value):32) but it's a choice

Answer 5

Tested on linux and windows (x86) :

#ifdef WIN32
    #include <intrin.h>
    static uint32_t __inline __builtin_clz(uint32_t x) {
        unsigned long r = 0;
        _BitScanReverse(&r, x);
        return (31-r);
    }
#endif

uint32_t clz64(const uint64_t x)
{
    uint32_t u32 = (x >> 32);
    uint32_t result = u32 ? __builtin_clz(u32) : 32;
    if (result == 32) {
        u32 = x & 0xFFFFFFFFUL;
        result += (u32 ? __builtin_clz(u32) : 32);
    }
    return result;
}