MMX代码运行速度比C ++代码慢 [英] MMX Codes running slower then C++ codes

问题描述

为什么我的MMX代码运行速度慢于c ++代码(绿色)?结果是一样的.仅有速度差

Why are my MMX codes running slower than the c++ codes(in green)? result is the same. only speed difference

void tom::add(void* btr)
{

         __declspec(align(8))short* b =(short*)btr;

         int j;


/*
for(j = 0; j < 4; j++)
    {
    /// 1st stage add.
    int s0 = (int)(b[j]     + b[j+3]);
    int s3 = (int)(b[j]     - b[j+3]);
    int s1 = (int)(b[j+1] + b[j+2]);
    int s2 = (int)(b[j+1] - b[j+2]);

    /// 2nd stage add.

    b[j]        = (short)(s0 + s1);
    b[j+8]  = (short)(s0 - s1);
    b[j+4]  = (short)(s2 + (s3 << 1));
    b[j+12] = (short)(s3 - (s2 << 1));
    }//end for j...

   */
    __m64*b1 = (__m64*)b;
        j=0;
    __m64 f0 = _mm_set_pi16(b[j+12],b[j+8],b[j+4],b[j]);
   __m64 f1 = _mm_set_pi16(b[j+13],b[j+9],b[j+5],b[j+1]);
    __m64 f2 = _mm_set_pi16(b[j+14],b[j+10],b[j+6],b[j+2]);
    __m64 f3 = _mm_set_pi16(b[j+15],b[j+11],b[j+7],b[j+3]);
    for(j = 0; j < 4; j+=4)
       {

         // stage one add
       __m64 s0 =_mm_add_pi16(f0,f3);
    __m64 s3 =_mm_sub_pi16(f0,f3);
    __m64 s1 =_mm_add_pi16(f1,f2);
     __m64 s2 =_mm_sub_pi16(f1,f2);
         // stage two add
    *(&b1[j]) =_mm_add_pi16(s0,s1);
    *(&b1[j+2]) =_mm_sub_pi16(s0,s1);
    *(&b1[j+1]) =_mm_add_pi16(s2,_mm_slli_pi16(s3, 1));  
    *(&b1[j+3]) =_mm_sub_pi16(s3,_mm_slli_pi16(s2, 1));
    }
      _mm_empty();
}

推荐答案

在您的 MMX 版本中，您有一个循环声明为for(j = 0; j < 4; j += 4)；不需要它，它只执行一次，然后可以将其删除并假定为j=0.
您是否正在使用一些编译器优化和/或启用了 SSE 指令?一般来说，由编译器进行的优化往往比您可以使用汇编程序手动编写的代码好得多.

In your MMX version you have a loop declared as for(j = 0; j < 4; j += 4); this is not needed, it executes just once, then you could remove it and assume j=0.
Are you using some compiler optimization and/or enabled the SSE instructions? Generally speaking, optimizations made by the compiler tends to be too much better than the code that you can manually write using assembly.

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