如何使用MSVC内在函数来得到这个GCC code的相同呢？ [英] How to use MSVC intrinsics to get the equivalent of this GCC code?

问题描述

以下code调用内建函数在GCC CLZ / CTZ，并在其他系统上，已丙版本。很显然，将C版本有点不理想，如果系统有一个内建的CLZ / CTZ指令，像x86和ARM。

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 builtin clz/ctz instruction, like x86 and ARM.

#ifdef __GNUC__
#define clz(x) __builtin_clz(x)
#define ctz(x) __builtin_ctz(x)
#else
static uint32_t ALWAYS_INLINE popcnt( uint32_t x )
{
    x -= ((x >> 1) & 0x55555555);
    x = (((x >> 2) & 0x33333333) + (x & 0x33333333));
    x = (((x >> 4) + x) & 0x0f0f0f0f);
    x += (x >> 8);
    x += (x >> 16);
    return x & 0x0000003f;
}
static uint32_t ALWAYS_INLINE clz( uint32_t x )
{
    x |= (x >> 1);
    x |= (x >> 2);
    x |= (x >> 4);
    x |= (x >> 8);
    x |= (x >> 16);
    return 32 - popcnt(x);
}
static uint32_t ALWAYS_INLINE ctz( uint32_t x )
{
    return popcnt((x & -x) - 1);
}

#endif

我需要什么样的功能调用，这头我需要包括等添加适当的ifdef为MSVC在这里？我已经看了此页面，但我不完全知道什么使用#pragma是（需要它？）什么限制，它把对MSVC版本要求进行编译。正如有人谁不真正使用MSVC，我也不知道这些内在是否对其他架构ç的等价物或我是否#defining当他们的x86支持#ifdef / x86_64的为好。

What functions do I need to call, which headers do I need to include, etc to add a proper ifdef for MSVC here? I've already looked at this page, but I'm not entirely sure what the #pragma is for (is it required?) and what restrictions it puts on MSVC version requirements for compilation. As someone who doesn't really use MSVC, I also don't know whether these intrinsics have C equivalents on other architectures, or whether I have to #ifdef x86/x86_64 as well when #defining them.

推荐答案

从sh0dan code弹跳，实现应该这样进行修正：

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

由于在code评论说，无论CTZ和CLZ是不确定的，如果值是0。在我们的抽象，我们固定 __ builtin_clz（值）为（值__ builtin_clz（值）：32），但它是一个选择

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