64位雙向量的向量比32位無符號整數的向量更快？

Question

我有兩種代碼迭代大小為500的矢量設計。其中一個設計包含64位雙精度數組，第二個設計使用包含32位整數的數組。 我期待32位設計更快，因為更多有用的數據可以打包在緩存中。

編譯器MSVC，CPU Ivy Bridge，編譯64位模式。

這是代碼1，使用32位整數（在2600個 CPU周期中運行）：

#include <vector>
#include <iostream>

int main(){

    std::vector<unsigned int> x1;
    std::vector<unsigned int> x2;
    std::vector<unsigned int> x3;
    x1.resize(500);
    x2.resize(500);
    x3.resize(500);

    for(int i =0; i<500; i++){
        x1[i] = i;
        x2[i] = 2*i;
        x3[i] = 4*i;
    }


    int counter = 0;
    while(counter < 1000){
        unsigned long long start = 0;
        unsigned long long end = 0;

        double m = 0;
        double n = 0;

        start = __rdtsc();

        for(int i=0; i < 500; i++){
            unsigned int a = x1[i];
            unsigned int b = x2[i];
            unsigned int g = x3[i];
            m = m + (a * g);
            n = n + (b * g);
        }

        end = __rdtscp();

        std::cout << (end-start) << "\t\t"<<m << n << std::endl;
        counter++;
    }
}

產生這個asm（-Os）：

start = __rdtscp(&p);
 rdtscp  
 lea         r8,[rbp+6Fh]  
 mov         dword ptr [r8],ecx  
 shl         rdx,20h  
 or          rax,rdx  
 mov         r10,rax  
        unsigned int p;
        unsigned int q;
        unsigned long long start = 0;
        unsigned long long end = 0;

        double m = 0;
 mov         r8,rbx  
 mov         r9d,1F4h  
            unsigned int a = x1[i];
            unsigned int b = x2[i];
            unsigned int g = x3[i];
 mov         edx,dword ptr [r8+r15]  
            m = m + (a * g);
 mov         ecx,edx  
 imul        ecx,dword ptr [r8+r14]  
 xorps       xmm0,xmm0  
 cvtsi2sd    xmm0,rcx  
 addsd       xmm7,xmm0  
            n = n + (b * g);
 imul        edx,dword ptr [r8]  
 mov         eax,edx  
 xorps       xmm0,xmm0  
 cvtsi2sd    xmm0,rax  
 addsd       xmm8,xmm0  

        for(int i=0; i < 500; i++){
 add         r8,4  
 dec         r9  
 jne         main+0E5h (013F681261h)  
        }

        end = __rdtscp(&q);
 rdtscp  
        }

        end = __rdtscp(&q);
 lea         r8,[rbp+6Fh]  
 mov         dword ptr [r8],ecx  
 shl         rdx,20h  
 or          rdx,rax  

這是代碼2，使用64位雙精度（代碼在2000個 CPU周期中運行）：

#include <vector>
#include <iostream>

int main(){

    std::vector<double> x1;
    std::vector<double> x2;
    std::vector<unsigned long long> x3;
    x1.resize(500);
    x2.resize(500);
    x3.resize(500);

    for(int i =0; i<500; i++){
        x1[i] = i;
        x2[i] = 2*i;
        x3[i] = 4*i;
    }

    int counter = 0;
    while(counter < 1000){
        unsigned int p;
        unsigned int q;
        unsigned long long start = 0;
        unsigned long long end = 0;

        double m = 0;
        double n = 0;

        start = __rdtscp(&p);

        for(int i=0; i < 500; i++){
            double a = x1[i];
            double b = x2[i];
            unsigned long long g = x3[i];
            m = m + (a * g);
            n = n + (b * g);
        }

        end = __rdtscp(&q);

        std::cout << (end-start) << "\t\t"<<m << n << std::endl;
        counter++;
    }
}

這里是asm（-Os）產生的：

start = __rdtscp(&p);
 rdtscp  
 lea         r8,[rbp+6Fh]  
 mov         dword ptr [r8],ecx  
 shl         rdx,20h  
 or          rax,rdx  
 mov         r9,rax  
        unsigned int p;
        unsigned int q;
        unsigned long long start = 0;
        unsigned long long end = 0;

        double m = 0;
 mov         rdx,rbx  
 mov         r8d,1F4h  
            double a = x1[i];
            double b = x2[i];
            unsigned long long g = x3[i];
 mov         rcx,qword ptr [rdx+r15]  
 xorps       xmm1,xmm1  
            m = m + (a * g);
 cvtsi2sd    xmm1,rcx  
 test        rcx,rcx  
 jns         main+120h (013F32129Ch)  
 addsd       xmm1,xmm9  
 movaps      xmm0,xmm1  
 mulsd       xmm0,mmword ptr [rdx+r14]  
 addsd       xmm6,xmm0  
            n = n + (b * g);
 mulsd       xmm1,mmword ptr [rdx]  
 addsd       xmm7,xmm1  

        for(int i=0; i < 500; i++){
 add         rdx,8  
 dec         r8  
 jne         main+10Ah (013F321286h)  
        }

        end = __rdtscp(&q);
 rdtscp  
        }

        end = __rdtscp(&q);
 lea         r8,[rbp+6Fh]  
 mov         dword ptr [r8],ecx  
 shl         rdx,20h  
 or          rdx,rax

Answer 1

區別在於第一個代碼中整數到雙精度的轉換（向量包含unsigned int ，產品是整數運算，但積累使用double ，在匯編程序中，這會將cvtsi2sd指令添加到代碼中）。

在第二個代碼中，您在任何地方都使用雙打，因此您沒有轉換，代碼運行得更快。

這種差異將是更加明顯具有定點和浮點處理單元之間的嚴格區分在CPU上（在POWER平台是一個這樣的例子）。 在這方面，X86平台非常寬容。