C++ 优化Microsoft Visual Studio编译器生成的程序集

我在做一个矩阵乘法的项目。我已经能够编写C代码，并且能够使用Microsoft visual studio 2012编译器为其生成汇编代码。编译器生成的代码如下所示。编译器使用了SSE寄存器，这正是我想要的，但它不是最好的代码。我想优化这段代码并用C代码内联编写，但我不理解汇编代码。基本上，汇编代码只适用于矩阵的一个维度，下面的代码只适用于4×4矩阵。我怎样才能使它适合n*n矩阵大小

< C++代码如下：

#define MAX_NUM 10
#define MAX_DIM 4

int main () {
    float mat_a [] = {1.0, 2.0, 3.0, 4.0, 1.0, 2.0, 3.0, 4.0, 1.0, 2.0, 3.0, 4.0, 1.0, 2.0, 3.0, 4.0};
    float mat_b [] = {1.0, 2.0, 3.0, 4.0, 1.0, 2.0, 3.0, 4.0, 1.0, 2.0, 3.0, 4.0, 1.0, 2.0, 3.0, 4.0};
    float result [] = {0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0};

    int num_row = 4;
    int num_col = 4;

    float sum;

    for (int i = 0; i < num_row; i++) {
       for (int j = 0; j < num_col; j++) {
          sum = 0.0;
          for (int k = 0; k < num_row; k++) {
             sum = sum + mat_a[i * num_col + k] * mat_b[k * num_col + j];
          }
          *(result + i * num_col + j) = sum; 
       }
    }

    return 0;
}

; Listing generated by Microsoft (R) Optimizing Compiler Version 17.00.50727.1 

    TITLE   C:\Users\GS\Documents\Visual Studio 2012\Projects\Assembly_InLine\Assembly_InLine\Source.cpp
    .686P
    .XMM
    include listing.inc
    .model  flat

INCLUDELIB MSVCRTD
INCLUDELIB OLDNAMES

PUBLIC  _main
PUBLIC  __real@00000000
PUBLIC  __real@3f800000
PUBLIC  __real@40000000
PUBLIC  __real@40400000
PUBLIC  __real@40800000
EXTRN   @_RTC_CheckStackVars@8:PROC
EXTRN   @__security_check_cookie@4:PROC
EXTRN   __RTC_InitBase:PROC
EXTRN   __RTC_Shutdown:PROC
EXTRN   ___security_cookie:DWORD
EXTRN   __fltused:DWORD
;   COMDAT __real@40800000
CONST   SEGMENT
__real@40800000 DD 040800000r           ; 4
CONST   ENDS
;   COMDAT __real@40400000
CONST   SEGMENT
__real@40400000 DD 040400000r           ; 3
CONST   ENDS
;   COMDAT __real@40000000
CONST   SEGMENT
__real@40000000 DD 040000000r           ; 2
CONST   ENDS
;   COMDAT __real@3f800000
CONST   SEGMENT
__real@3f800000 DD 03f800000r           ; 1
CONST   ENDS
;   COMDAT __real@00000000
CONST   SEGMENT
__real@00000000 DD 000000000r           ; 0
CONST   ENDS
;   COMDAT rtc$TMZ
rtc$TMZ SEGMENT
__RTC_Shutdown.rtc$TMZ DD FLAT:__RTC_Shutdown
rtc$TMZ ENDS
;   COMDAT rtc$IMZ
rtc$IMZ SEGMENT
__RTC_InitBase.rtc$IMZ DD FLAT:__RTC_InitBase
rtc$IMZ ENDS
; Function compile flags: /Odtp /RTCsu /ZI
;   COMDAT _main
_TEXT   SEGMENT
_k$1 = -288                     ; size = 4
_j$2 = -276                     ; size = 4
_i$3 = -264                     ; size = 4
_sum$ = -252                        ; size = 4
_num_col$ = -240                    ; size = 4
_num_row$ = -228                    ; size = 4
_result$ = -216                     ; size = 64
_mat_b$ = -144                      ; size = 64
_mat_a$ = -72                       ; size = 64
__$ArrayPad$ = -4                   ; size = 4
_main   PROC                        ; COMDAT
; File c:\users\gs\documents\visual studio 2012\projects\assembly_inline\assembly_inline\source.cpp
; Line 4
    push    ebp
    mov ebp, esp
    sub esp, 484                ; 000001e4H
    push    ebx
    push    esi
    push    edi
    lea edi, DWORD PTR [ebp-484]
    mov ecx, 121                ; 00000079H
    mov eax, -858993460             ; ccccccccH
    rep stosd
    mov eax, DWORD PTR ___security_cookie
    xor eax, ebp
    mov DWORD PTR __$ArrayPad$[ebp], eax
; Line 5
    movss   xmm0, DWORD PTR __real@3f800000
    movss   DWORD PTR _mat_a$[ebp], xmm0
    movss   xmm0, DWORD PTR __real@40000000
    movss   DWORD PTR _mat_a$[ebp+4], xmm0
    movss   xmm0, DWORD PTR __real@40400000
    movss   DWORD PTR _mat_a$[ebp+8], xmm0
    movss   xmm0, DWORD PTR __real@40800000
    movss   DWORD PTR _mat_a$[ebp+12], xmm0
    movss   xmm0, DWORD PTR __real@3f800000
    movss   DWORD PTR _mat_a$[ebp+16], xmm0
    movss   xmm0, DWORD PTR __real@40000000
    movss   DWORD PTR _mat_a$[ebp+20], xmm0
    movss   xmm0, DWORD PTR __real@40400000
    movss   DWORD PTR _mat_a$[ebp+24], xmm0
    movss   xmm0, DWORD PTR __real@40800000
    movss   DWORD PTR _mat_a$[ebp+28], xmm0
    movss   xmm0, DWORD PTR __real@3f800000
    movss   DWORD PTR _mat_a$[ebp+32], xmm0
    movss   xmm0, DWORD PTR __real@40000000
    movss   DWORD PTR _mat_a$[ebp+36], xmm0
    movss   xmm0, DWORD PTR __real@40400000
    movss   DWORD PTR _mat_a$[ebp+40], xmm0
    movss   xmm0, DWORD PTR __real@40800000
    movss   DWORD PTR _mat_a$[ebp+44], xmm0
    movss   xmm0, DWORD PTR __real@3f800000
    movss   DWORD PTR _mat_a$[ebp+48], xmm0
    movss   xmm0, DWORD PTR __real@40000000
    movss   DWORD PTR _mat_a$[ebp+52], xmm0
    movss   xmm0, DWORD PTR __real@40400000
    movss   DWORD PTR _mat_a$[ebp+56], xmm0
    movss   xmm0, DWORD PTR __real@40800000
    movss   DWORD PTR _mat_a$[ebp+60], xmm0
; Line 6
    movss   xmm0, DWORD PTR __real@3f800000
    movss   DWORD PTR _mat_b$[ebp], xmm0
    movss   xmm0, DWORD PTR __real@40000000
    movss   DWORD PTR _mat_b$[ebp+4], xmm0
    movss   xmm0, DWORD PTR __real@40400000
    movss   DWORD PTR _mat_b$[ebp+8], xmm0
    movss   xmm0, DWORD PTR __real@40800000
    movss   DWORD PTR _mat_b$[ebp+12], xmm0
    movss   xmm0, DWORD PTR __real@3f800000
    movss   DWORD PTR _mat_b$[ebp+16], xmm0
    movss   xmm0, DWORD PTR __real@40000000
    movss   DWORD PTR _mat_b$[ebp+20], xmm0
    movss   xmm0, DWORD PTR __real@40400000
    movss   DWORD PTR _mat_b$[ebp+24], xmm0
    movss   xmm0, DWORD PTR __real@40800000
    movss   DWORD PTR _mat_b$[ebp+28], xmm0
    movss   xmm0, DWORD PTR __real@3f800000
    movss   DWORD PTR _mat_b$[ebp+32], xmm0
    movss   xmm0, DWORD PTR __real@40000000
    movss   DWORD PTR _mat_b$[ebp+36], xmm0
    movss   xmm0, DWORD PTR __real@40400000
    movss   DWORD PTR _mat_b$[ebp+40], xmm0
    movss   xmm0, DWORD PTR __real@40800000
    movss   DWORD PTR _mat_b$[ebp+44], xmm0
    movss   xmm0, DWORD PTR __real@3f800000
    movss   DWORD PTR _mat_b$[ebp+48], xmm0
    movss   xmm0, DWORD PTR __real@40000000
    movss   DWORD PTR _mat_b$[ebp+52], xmm0
    movss   xmm0, DWORD PTR __real@40400000
    movss   DWORD PTR _mat_b$[ebp+56], xmm0
    movss   xmm0, DWORD PTR __real@40800000
    movss   DWORD PTR _mat_b$[ebp+60], xmm0
; Line 7
    movss   xmm0, DWORD PTR __real@00000000
    movss   DWORD PTR _result$[ebp], xmm0
    movss   xmm0, DWORD PTR __real@00000000
    movss   DWORD PTR _result$[ebp+4], xmm0
    movss   xmm0, DWORD PTR __real@00000000
    movss   DWORD PTR _result$[ebp+8], xmm0
    movss   xmm0, DWORD PTR __real@00000000
    movss   DWORD PTR _result$[ebp+12], xmm0
    movss   xmm0, DWORD PTR __real@00000000
    movss   DWORD PTR _result$[ebp+16], xmm0
    movss   xmm0, DWORD PTR __real@00000000
    movss   DWORD PTR _result$[ebp+20], xmm0
    movss   xmm0, DWORD PTR __real@00000000
    movss   DWORD PTR _result$[ebp+24], xmm0
    movss   xmm0, DWORD PTR __real@00000000
    movss   DWORD PTR _result$[ebp+28], xmm0
    movss   xmm0, DWORD PTR __real@00000000
    movss   DWORD PTR _result$[ebp+32], xmm0
    movss   xmm0, DWORD PTR __real@00000000
    movss   DWORD PTR _result$[ebp+36], xmm0
    movss   xmm0, DWORD PTR __real@00000000
    movss   DWORD PTR _result$[ebp+40], xmm0
    movss   xmm0, DWORD PTR __real@00000000
    movss   DWORD PTR _result$[ebp+44], xmm0
    movss   xmm0, DWORD PTR __real@00000000
    movss   DWORD PTR _result$[ebp+48], xmm0
    movss   xmm0, DWORD PTR __real@00000000
    movss   DWORD PTR _result$[ebp+52], xmm0
    movss   xmm0, DWORD PTR __real@00000000
    movss   DWORD PTR _result$[ebp+56], xmm0
    movss   xmm0, DWORD PTR __real@00000000
    movss   DWORD PTR _result$[ebp+60], xmm0
; Line 9
    mov DWORD PTR _num_row$[ebp], 4
; Line 10
    mov DWORD PTR _num_col$[ebp], 4
; Line 14
    mov DWORD PTR _i$3[ebp], 0
    jmp SHORT $LN9@main
$LN8@main:
    mov eax, DWORD PTR _i$3[ebp]
    add eax, 1
    mov DWORD PTR _i$3[ebp], eax
$LN9@main:
    mov eax, DWORD PTR _i$3[ebp]
    cmp eax, DWORD PTR _num_row$[ebp]
    jge $LN7@main
; Line 15
    mov DWORD PTR _j$2[ebp], 0
    jmp SHORT $LN6@main
$LN5@main:
    mov eax, DWORD PTR _j$2[ebp]
    add eax, 1
    mov DWORD PTR _j$2[ebp], eax
$LN6@main:
    mov eax, DWORD PTR _j$2[ebp]
    cmp eax, DWORD PTR _num_col$[ebp]
    jge $LN4@main
; Line 16
    movss   xmm0, DWORD PTR __real@00000000
    movss   DWORD PTR _sum$[ebp], xmm0
; Line 17
    mov DWORD PTR _k$1[ebp], 0
    jmp SHORT $LN3@main
$LN2@main:
    mov eax, DWORD PTR _k$1[ebp]
    add eax, 1
    mov DWORD PTR _k$1[ebp], eax
$LN3@main:
    mov eax, DWORD PTR _k$1[ebp]
    cmp eax, DWORD PTR _num_row$[ebp]
    jge SHORT $LN1@main
; Line 18
    mov eax, DWORD PTR _i$3[ebp]
    imul    eax, DWORD PTR _num_col$[ebp]
    add eax, DWORD PTR _k$1[ebp]
    mov ecx, DWORD PTR _k$1[ebp]
    imul    ecx, DWORD PTR _num_col$[ebp]
    add ecx, DWORD PTR _j$2[ebp]
    movss   xmm0, DWORD PTR _mat_a$[ebp+eax*4]
    mulss   xmm0, DWORD PTR _mat_b$[ebp+ecx*4]
    addss   xmm0, DWORD PTR _sum$[ebp]
    movss   DWORD PTR _sum$[ebp], xmm0
; Line 19
    jmp SHORT $LN2@main
$LN1@main:
; Line 20
    mov eax, DWORD PTR _i$3[ebp]
    imul    eax, DWORD PTR _num_col$[ebp]
    lea ecx, DWORD PTR _result$[ebp+eax*4]
    mov edx, DWORD PTR _j$2[ebp]
    movss   xmm0, DWORD PTR _sum$[ebp]
    movss   DWORD PTR [ecx+edx*4], xmm0
; Line 21
    jmp $LN5@main
$LN4@main:
; Line 22
    jmp $LN8@main
$LN7@main:
; Line 24
    xor eax, eax
; Line 25
    push    edx
    mov ecx, ebp
    push    eax
    lea edx, DWORD PTR $LN16@main
    call    @_RTC_CheckStackVars@8
    pop eax
    pop edx
    pop edi
    pop esi
    pop ebx
    mov ecx, DWORD PTR __$ArrayPad$[ebp]
    xor ecx, ebp
    call    @__security_check_cookie@4
    mov esp, ebp
    pop ebp
    ret 0
    npad    1
$LN16@main:
    DD  3
    DD  $LN15@main
$LN15@main:
    DD  -72                 ; ffffffb8H
    DD  64                  ; 00000040H
    DD  $LN12@main
    DD  -144                    ; ffffff70H
    DD  64                  ; 00000040H
    DD  $LN13@main
    DD  -216                    ; ffffff28H
    DD  64                  ; 00000040H
    DD  $LN14@main
$LN14@main:
    DB  114                 ; 00000072H
    DB  101                 ; 00000065H
    DB  115                 ; 00000073H
    DB  117                 ; 00000075H
    DB  108                 ; 0000006cH
    DB  116                 ; 00000074H
    DB  0
$LN13@main:
    DB  109                 ; 0000006dH
    DB  97                  ; 00000061H
    DB  116                 ; 00000074H
    DB  95                  ; 0000005fH
    DB  98                  ; 00000062H
    DB  0
$LN12@main:
    DB  109                 ; 0000006dH
    DB  97                  ; 00000061H
    DB  116                 ; 00000074H
    DB  95                  ; 0000005fH
    DB  97                  ; 00000061H
    DB  0
_main   ENDP
_TEXT   ENDS
END

---

VisualStudio和SSE是这里的一个红色鲱鱼（以及C++和C的胡说）。假设您在发布模式下编译，那么代码效率低下还有其他原因，特别是对于大型矩阵。主要原因是它的缓存不友好。为了使您的代码对任意n*n矩阵有效，您需要对大小矩阵进行优化

在使用SIMD或线程之前，优化缓存非常重要。在下面的代码中，我使用块乘法，仅使用单个线程而不使用SSE/AVX，将1024x1204矩阵的代码加速10倍以上（旧代码为7.1s，新代码为0.6s）。如果您的代码内存有限，那么使用SIMD不会有任何好处

我已经在这里描述了使用转置对矩阵乘法的一阶改进。

但让我来描述一种更为缓存友好的方法。假设您的硬件有两种类型的内存：

又小又快

又大又慢

事实上，现代CPU实际上有几个级别（L1小而快，L2大而慢，L3更大而慢，主内存更大而慢。一些CPU甚至有一个L4），但这种只有两个级别的简单模型仍然会导致性能的大幅提高

使用此模型和两种类型的内存，您可以通过将矩阵划分为适合小型快速内存的方形块，并执行块矩阵乘法，来显示您将获得最佳性能。接下来，您需要重新排列内存，以便每个磁贴的元素都是连续的

下面是一些代码，展示了如何做到这一点。我在1024x1024矩阵上使用了64x64的块大小你的代码需要7秒，我的代码需要0.65秒。矩阵大小必须是64x64的倍数，但很容易将其扩展到任意大小的矩阵。如果您想查看如何优化块的示例，请参见

#包括
#包括
#包括
#包括
无效重新排序（浮动*a、浮动*b、整数n、整数bs）{
int nb=n/bs；
int-cnt=0；
对于（It i＝0；IF如果是C代码，为什么C++标签而不是C？这不是最好的代码吗？这些天不太容易胜过编译器/编译器编写者。通常他们比你知道的主题更多，尤其是在流水线方面。你使用正确的编译器标志吗？C++是从C派生的。我在写这个代码。C++。这是.CPP文件。“我不理解汇编代码”……“它不是最好的代码”。…我不明白如果你甚至都不理解它，你怎么能断定它不是最好的代码…你能够手工优化代码的几率可能没有你想象的那么高…发布的程序集看起来像一个调试版本。请尝试查看优化版本的输出.
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <omp.h>

void reorder(float *a, float *b, int n, int bs) {
    int nb = n/bs;
    int cnt = 0;
    for(int i=0; i<nb; i++) {
        for(int j=0; j<nb; j++) {
            for(int i2=0; i2<bs; i2++) {
                for(int j2=0; j2<bs; j2++) {
                    b[cnt++] = a[bs*(i*n+j) + i2*n + j2];
                }
            }
        }
    }
}

void gemm_slow(float *a, float *b, float *c, int n) {
    for(int i=0; i<n; i++) {
        for(int j=0; j<n; j++) {
            float sum = c[i*n+j];
            for(int k=0; k<n; k++) {
                sum += a[i*n+k]*b[k*n+j];
            }
            c[i*n+j] += sum;
        }
    }
}

void gemm_block(float *a, float *b, float *c, int n, int n2) {
    for(int i=0; i<n2; i++) {
        for(int j=0; j<n2; j++) {
            float sum = c[i*n+j];
            for(int k=0; k<n2; k++) {
                sum += a[i*n+k]*b[k*n2+j];
            }
            c[i*n+j] = sum;
        }
    }
}

void gemm(float *a, float*b, float*c, int n, int bs) {
    int nb = n/bs;
    float *b2 = (float*)malloc(sizeof(float)*n*n);
    reorder(b,b2,n,bs);
    for(int i=0; i<nb; i++) {
        for(int j=0; j<nb; j++) {   
            for(int k=0; k<nb; k++) {
                gemm_block(&a[bs*(i*n+k)],&b2[bs*bs*(k*nb+j)],&c[bs*(i*n+j)], n, bs);
            }
        }
    }
    free(b2);
}


int main() {
    const int bs = 64;
    const int n = 1024;
    float *a = new float[n*n];
    float *b = new float[n*n];
    float *c1 = new float[n*n]();
    float *c2 = new float[n*n]();
    for(int i=0; i<n*n; i++) {
        a[i] = 1.0*rand()/RAND_MAX;
        b[i] = 1.0*rand()/RAND_MAX;
    }
    double dtime;

    dtime = omp_get_wtime();
    gemm_slow(a,b,c1,n);
    dtime = omp_get_wtime() - dtime;
    printf("%f\n", dtime);

    dtime = omp_get_wtime();
    gemm(a,b,c2,n,64);
    dtime = omp_get_wtime() - dtime;
    printf("%f\n", dtime);

    printf("%d\n", memcmp(c1,c2, sizeof(float)*n*n));

}