为什么 C 数组比 std::array 快得多?

我们目前正在用C++编写一些性能关键型代码，这些代码在许多大型矩阵和向量上运行。关于我们的研究，std::array和标准 C 阵列之间应该没有重大的性能差异 (请参阅此问题或此问题)。 但是，在测试时，我们通过使用 C 数组std::array经历了巨大的性能改进。 这是我们的演示代码：

#include <iostream>
#include <array>
#include <sys/time.h>
#define ROWS 784
#define COLS 100
#define RUNS 50
using std::array;
void DotPComplex(array<double, ROWS> &result, array<double, ROWS> &vec1, array<double, ROWS> &vec2){
for(int i = 0; i < ROWS; i++){
result[i] = vec1[i] * vec2[i];
}
}
void DotPSimple(double result[ROWS], double vec1[ROWS], double vec2[ROWS]){
for(int i = 0; i < ROWS; i++){
result[i] = vec1[i] * vec2[i];
}
}
void MatMultComplex(array<double, ROWS> &result, array<array<double, COLS>, ROWS> &mat, array<double, ROWS> &vec){
for (int i = 0; i < COLS; ++i) {
for (int j = 0; j < ROWS; ++j) {
result[i] += mat[i][j] * vec[j];
}
}
}
void MatMultSimple(double result[ROWS], double mat[ROWS][COLS], double vec[ROWS]){
for (int i = 0; i < COLS; ++i) {
for (int j = 0; j < ROWS; ++j) {
result[i] += mat[i][j] * vec[j];
}
}
}
double getTime(){
struct timeval currentTime;
gettimeofday(&currentTime, NULL);
double tmp = (double)currentTime.tv_sec * 1000.0 + (double)currentTime.tv_usec/1000.0;
return tmp;
}
array<double, ROWS> inputVectorComplex = {{ 0 }};
array<double, ROWS> resultVectorComplex = {{ 0 }};
double inputVectorSimple[ROWS] = { 0 };
double resultVectorSimple[ROWS] = { 0 };
array<array<double, COLS>, ROWS> inputMatrixComplex = {{0}};
double inputMatrixSimple[ROWS][COLS] = { 0 };
int main(){
double start;
std::cout << "DotP test with C array: " << std::endl;
start = getTime();
for(int i = 0; i < RUNS; i++){
DotPSimple(resultVectorSimple, inputVectorSimple, inputVectorSimple);
}
std::cout << "Duration: " << getTime() - start << std::endl;
std::cout << "DotP test with C++ array: " << std::endl;
start = getTime();
for(int i = 0; i < RUNS; i++){
DotPComplex(resultVectorComplex, inputVectorComplex, inputVectorComplex);
}
std::cout << "Duration: " << getTime() - start << std::endl;
std::cout << "MatMult test with C array : " << std::endl;
start = getTime();
for(int i = 0; i < RUNS; i++){
MatMultSimple(resultVectorSimple, inputMatrixSimple, inputVectorSimple);
}
std::cout << "Duration: " << getTime() - start << std::endl;
std::cout << "MatMult test with C++ array: " << std::endl;
start = getTime();
for(int i = 0; i < RUNS; i++){
MatMultComplex(resultVectorComplex, inputMatrixComplex, inputVectorComplex);
}
std::cout << "Duration: " << getTime() - start << std::endl;
}

编译者：icpc demo.cpp -std=c++11 -O0结果如下：

DotP test with C array: 
Duration: 0.289795 ms
DotP test with C++ array: 
Duration: 1.98413 ms
MatMult test with C array : 
Duration: 28.3459 ms
MatMult test with C++ array: 
Duration: 175.15 ms

带有-O3标志：

DotP test with C array: 
Duration: 0.0280762 ms
DotP test with C++ array: 
Duration: 0.0288086 ms
MatMult test with C array : 
Duration: 1.78296 ms
MatMult test with C++ array: 
Duration: 4.90991 ms

C 数组实现速度要快得多，无需优化编译器。为什么？ 使用编译器优化，点积同样快。但是对于矩阵乘法，使用 C 数组时仍然有显着的加速。 有没有办法在使用std::array时实现相同的性能？

更新：

使用的编译器：icpc 17.0.0

有了gcc 4.8.5我们的代码运行速度比使用任何优化级别的英特尔编译器慢得多。因此，我们主要对英特尔编译器的行为感兴趣。

根据 Jonas 的建议，我们使用以下结果(英特尔编译器)调整了RUNS 50.000：

带有-O0标志：

DotP test with C array: 
Duration: 201.764 ms
DotP test with C++ array: 
Duration: 1020.67 ms
MatMult test with C array : 
Duration: 15069.2 ms
MatMult test with C++ array: 
Duration: 123826 ms

带有-O3标志：

DotP test with C array: 
Duration: 16.583 ms
DotP test with C++ array: 
Duration: 15.635 ms
MatMult test with C array : 
Duration: 980.582 ms
MatMult test with C++ array: 
Duration: 2344.46 ms