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");
}