c – 加速一次短暂的浮动投射？

我有一个简短的浮动投射在C中是瓶颈我的代码.

该代码从硬件设备缓冲区转换,该缓冲区本身是短路的,这代表来自花式光子计数器的输入.

float factor=  1.0f/value;
for (int i = 0; i < W*H; i++)//25% of time is spent doing this
{
    int value = source[i];//ushort -> int
    destination[i] = value*factor;//int*float->float
}

一些细节

>值应从0到2 ^ 16-1,它表示高灵敏度相机的像素值
>我在配备i7处理器(i7 960,SSE 4.2和4.1)的多核x86机器上.
>源与8位边界对齐(硬件设备的要求)
> W * H总是可被8整除,大部分时间W和H可被8整除

这让我感到难过,有什么我可以做的吗？

我正在使用Visual Studios 2012 …

解决方法

这是一个基本的SSE4.1实现：

__m128 factor = _mm_set1_ps(1.0f / value);
for (int i = 0; i < W*H; i += 8)
{
    //  Load 8 16-bit ushorts.
    //  vi = {a,b,c,d,e,f,g,h}
    __m128i vi = _mm_load_si128((const __m128i*)(source + i));

    //  Convert to 32-bit integers
    //  vi0 = {a,0}
    //  vi1 = {e,h,0}
    __m128i vi0 = _mm_cvtepu16_epi32(vi);
    __m128i vi1 = _mm_cvtepu16_epi32(_mm_unpackhi_epi64(vi,vi));

    //  Convert to float
    __m128 vf0 = _mm_cvtepi32_ps(vi0);
    __m128 vf1 = _mm_cvtepi32_ps(vi1);

    //  Multiply
    vf0 = _mm_mul_ps(vf0,factor);
    vf1 = _mm_mul_ps(vf1,factor);

    //  Store
    _mm_store_ps(destination + i + 0,vf0);
    _mm_store_ps(destination + i + 4,vf1);
}

这假定：

>源和目标都与16个字节对齐.
> W * H是8的倍数.

通过进一步展开此循环可以做得更好. (见下文)

这里的想法如下：

>将8个短路装入单个SSE寄存器.
>将寄存器拆分为两个：一个是底部4个短裤,另一个是前4个短裤.
>将两个寄存器零扩展为32位整数.
>将它们转换为浮点数.
>乘以因子.
>将它们存放到目的地.

编辑：

我做了这种类型的优化已经有一段时间了,所以我继续展开循环.

酷睿i7 920 @ 3.5 GHz
Visual Studio 2012 – 发布x64：

Original Loop      : 4.374 seconds
Vectorize no unroll: 1.665
Vectorize unroll 2 : 1.416

进一步展开导致收益递减.

这是测试代码：

#include <smmintrin.h>
#include <time.h>
#include <iostream>
#include <malloc.h>
using namespace std;


void default_loop(float *destination,const short* source,float value,int size){
    float factor = 1.0f / value; 
    for (int i = 0; i < size; i++)
    {
        int value = source[i];
        destination[i] = value*factor;
    }
}
void vectorize8_unroll1(float *destination,int size){
    __m128 factor = _mm_set1_ps(1.0f / value);
    for (int i = 0; i < size; i += 8)
    {
        //  Load 8 16-bit ushorts.
        __m128i vi = _mm_load_si128((const __m128i*)(source + i));

        //  Convert to 32-bit integers
        __m128i vi0 = _mm_cvtepu16_epi32(vi);
        __m128i vi1 = _mm_cvtepu16_epi32(_mm_unpackhi_epi64(vi,vi));

        //  Convert to float
        __m128 vf0 = _mm_cvtepi32_ps(vi0);
        __m128 vf1 = _mm_cvtepi32_ps(vi1);

        //  Multiply
        vf0 = _mm_mul_ps(vf0,factor);
        vf1 = _mm_mul_ps(vf1,factor);

        //  Store
        _mm_store_ps(destination + i + 0,vf0);
        _mm_store_ps(destination + i + 4,vf1);
    }
}
void vectorize8_unroll2(float *destination,int size){
    __m128 factor = _mm_set1_ps(1.0f / value);
    for (int i = 0; i < size; i += 16)
    {
        __m128i a0 = _mm_load_si128((const __m128i*)(source + i + 0));
        __m128i a1 = _mm_load_si128((const __m128i*)(source + i + 8));

        //  Split into two registers
        __m128i b0 = _mm_unpackhi_epi64(a0,a0);
        __m128i b1 = _mm_unpackhi_epi64(a1,a1);

        //  Convert to 32-bit integers
        a0 = _mm_cvtepu16_epi32(a0);
        b0 = _mm_cvtepu16_epi32(b0);
        a1 = _mm_cvtepu16_epi32(a1);
        b1 = _mm_cvtepu16_epi32(b1);

        //  Convert to float
        __m128 c0 = _mm_cvtepi32_ps(a0);
        __m128 d0 = _mm_cvtepi32_ps(b0);
        __m128 c1 = _mm_cvtepi32_ps(a1);
        __m128 d1 = _mm_cvtepi32_ps(b1);

        //  Multiply
        c0 = _mm_mul_ps(c0,factor);
        d0 = _mm_mul_ps(d0,factor);
        c1 = _mm_mul_ps(c1,factor);
        d1 = _mm_mul_ps(d1,factor);

        //  Store
        _mm_store_ps(destination + i +  0,c0);
        _mm_store_ps(destination + i +  4,d0);
        _mm_store_ps(destination + i +  8,c1);
        _mm_store_ps(destination + i + 12,d1);
    }
}
void print_sum(const float *destination,int size){
    float sum = 0;
    for (int i = 0; i < size; i++){
        sum += destination[i];
    }
    cout << sum << endl;
}

int main(){

    int size = 8000;

    short *source       = (short*)_mm_malloc(size * sizeof(short),16);
    float *destination  = (float*)_mm_malloc(size * sizeof(float),16);

    for (int i = 0; i < size; i++){
        source[i] = i;
    }

    float value = 1.1;

    int iterations = 1000000;
    clock_t start;

    //  Default Loop
    start = clock();
    for (int it = 0; it < iterations; it++){
        default_loop(destination,source,value,size);
    }
    cout << (double)(clock() - start) / CLOCKS_PER_SEC << endl;
    print_sum(destination,size);

    //  Vectorize 8,no unroll
    start = clock();
    for (int it = 0; it < iterations; it++){
        vectorize8_unroll1(destination,unroll 2
    start = clock();
    for (int it = 0; it < iterations; it++){
        vectorize8_unroll2(destination,size);

    _mm_free(source);
    _mm_free(destination);

    system("pause");
}

c – 加速一次短暂的浮动投射？

解决方法

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