我创建了以下代码以测试我对 sse 内在函数的理解。代码可以正确编译和运行,但是 sse 的改进不是很明显。使用 sse 内在函数大约是。快 20%。它不应该快大约 4 倍或速度提高 400% 吗?编译器是否优化了标量循环?如果是这样,如何禁用它?我写的sse_mult()函数有问题吗?
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#include <emmintrin.h>
// gcc options -mfpmath=sse -mmmx -msse -msse2 \ Not sure if any are needed have been using -msse2
/*--------------------------------------------------------------------------------------------------
* SIMD intrinsics header files
*
* <mmintrin.h> MMX
*
* <xmmintrin.h> SSE
*
* <emmintrin.h> SSE2
*
* <pmmintrin.h> SSE3
*
* <tmmintrin.h> SSE3
*
* <smmintrin.h> SSE4.1
*
* <nmmintrin.h> SSE4.2
*
* <ammintrin.h> SSE4A
*
* <wmmintrin.h> AES
*
* <immintrin.h> AVX
*------------------------------------------------------------------------------------------------*/
#define n 1000000
// Global variables
float a[n]; // array to hold random numbers
float b[n]; // array to hold random numbers
float c[n]; // array to hold product a*b for scalar multiply
__declspec(align(16)) float d[n] ; // array to hold product a*b for sse multiply
// Also possible to use __attribute__((aligned(16))); to force correct alignment
// Multiply using loop
void loop_mult() {
int i; // Loop index
clock_t begin_loop, end_loop; // clock_t is type returned by clock()
double time_spent_loop;
// Time multiply operation
begin_loop = clock();
// Multiply two arrays of doubles
for(i = 0; i < n; i++) {
c[i] = a[i] * b[i];
}
end_loop = clock();
// Calculate time it took to run loop. Type int CLOCK_PER_SEC is # of clock ticks per second.
time_spent_loop = (double)(end_loop - begin_loop) / CLOCKS_PER_SEC;
printf("Time for scalar loop was %f seconds\n", time_spent_loop);
}
// Multiply using sse
void sse_mult() {
int k,i; // Index
clock_t begin_sse, end_sse; // clock_t is type returned by clock()
double time_spent_sse;
// Time multiply operation
begin_sse = clock();
// Multiply two arrays of doubles
__m128 x,y,result; // __m128 is a data type, can hold 4 32 bit floating point values
result = _mm_setzero_ps(); // set register to hold all zeros
for(k = 0; k <= (n-4); k += 4) {
x = _mm_load_ps(&a[k]); // Load chunk of 4 floats into register
y = _mm_load_ps(&b[k]);
result = _mm_mul_ps(x,y); // multiply 4 floats
_mm_store_ps(&d[k],result); // store result in array
}
int extra = n%4; // If array size isn't exactly a multiple of 4 use scalar ops for remainder
if(extra!=0) {
for(i = (n-extra); i < n; i++) {
d[i] = a[i] * b[i];
}
}
end_sse = clock();
// Calculate time it took to run loop. Type int CLOCK_PER_SEC is # of clock ticks per second.
time_spent_sse = (double)(end_sse - begin_sse) / CLOCKS_PER_SEC;
printf("Time for sse was %f seconds\n", time_spent_sse);
}
int main() {
int i; // Loop index
srand((unsigned)time(NULL)); // initial value that rand uses, called the seed
// unsigned garauntees positive values
// time(NULL) uses the system clock as the seed so values will be different each time
for(i = 0; i < n; i++) {
// Fill arrays with random numbers
a[i] = ((float)rand()/RAND_MAX)*10; // rand() returns an integer value between 0 and RAND_MAX
b[i] = ((float)rand()/RAND_MAX)*20;
}
loop_mult();
sse_mult();
for(i=0; i<n; i++) {
// printf("a[%d] = %f\n", i, a[i]); // print values to check
// printf("b[%d] = %f\n", i, b[i]);
// printf("c[%d] = %f\n", i, c[i]);
// printf("d[%d] = %f\n", i, d[i]);
if(c[i]!=d[i]) {
printf("Error with sse multiply.\n");
break;
}
}
return 0;
}
最佳答案
您的程序受内存限制。 SSE 没有太大区别,因为大部分时间都花在从 RAM 读取那些大数组上。减小这些数组的大小,以便它们适合缓存。而是增加遍数。当所有数据都已在缓存中时,SSE 版本的执行速度应该明显更快。
请记住,可能还涉及其他因素:
- GCC 可以(在某种程度上)自动向量化循环。 (不过我认为它需要 -O3)
- 您测试的第一个方法会比较慢,因为缓存尚未填满。您可能希望多次交替运行这两种方法。
关于c - 数组乘法与 sse 内在函数乘法的时间?,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/26494785/