我尝试按照 github std::simd 的实例操作,但我的向量化版本最终变得比原版慢 2-3 倍。如何正确使用?
文档 看起来缺乏足够的示例。未列出任何构造函数等等。我确定我可能在错误地使用它,但是由于文档有限,我不知道该如何继续。
g++ -o test test.cpp --std=c++2a -O0
#include <array>
#include <chrono>
#include <cstdlib>
#include <experimental/simd>
#include <iostream>
#include <random>
using std::experimental::native_simd;
using Vec3D_v = std::array<native_simd<float>, 3>;
native_simd<float> scalar_product(const Vec3D_v& a, const Vec3D_v& b) {
return a[0] * b[0] + a[1] * b[1] + a[2] * b[2];
}
using Vec3D = std::array<float, 3>;
float scalar_product(const std::array<float, 3>& a, const std::array<float, 3>& b) {
return a[0] * b[0] + a[1] * b[1] + a[2] * b[2];
}
int main(){
constexpr std::size_t VECREG_SIZE = native_simd<float>::size();
std::array<Vec3D, VECREG_SIZE * 1000> arr;
std::array<Vec3D_v, VECREG_SIZE * 1000> arr_v;
std::random_device rd;
std::mt19937 generator(rd());
std::uniform_real_distribution<float> distribution(0.f, 1.f);
for( std::size_t i = 0; i < arr.size(); ++i ){
arr[i] = {distribution(generator), distribution(generator), distribution(generator)};
arr_v[i] = {distribution(generator), distribution(generator), distribution(generator)};
}
float result = 0.f;
auto start = std::chrono::high_resolution_clock::now();
for( std::size_t i = 1; i < arr.size(); ++i ){
result += scalar_product(arr_v[i-1], arr_v[i])[0];
}
auto end = std::chrono::high_resolution_clock::now();
auto elapsed = end - start;
std::cout << "VC: " << elapsed.count() << '\n' << std::endl;
result = 0;
start = std::chrono::high_resolution_clock::now();
for( std::size_t i = 1; i < arr.size(); ++i ){
result += scalar_product(arr[i-1], arr[i]);
}
end = std::chrono::high_resolution_clock::now();
elapsed = end - start;
std::cout << "notVC: " << elapsed.count() << '\n';
return EXIT_SUCCESS;
}
-O0
呢?没有优化,你只是在测试所有本应被优化消除掉的开销。 - walnutscalar_product
函数在不同优化级别下的编译结果:https://godbolt.org/z/JPGPlo - walnut