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POIs inline assembly language slower than native C++ code?
 

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  1. POIs inline assembly language slower than native C++ code?
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    <p>I tried to compare the performance of inline assembly language and C++ code, so I wrote a function that add two arrays of size 2000 for 100000 times. Here's the code:</p> <pre><code>#define TIMES 100000 void calcuC(int *x,int *y,int length) { for(int i = 0; i &lt; TIMES; i++) { for(int j = 0; j &lt; length; j++) x[j] += y[j]; } } void calcuAsm(int *x,int *y,int lengthOfArray) { __asm { mov edi,TIMES start: mov esi,0 mov ecx,lengthOfArray label: mov edx,x push edx mov eax,DWORD PTR [edx + esi*4] mov edx,y mov ebx,DWORD PTR [edx + esi*4] add eax,ebx pop edx mov [edx + esi*4],eax inc esi loop label dec edi cmp edi,0 jnz start }; } </code></pre> <p>Here's <code>main()</code>:</p> <pre><code>int main() { bool errorOccured = false; setbuf(stdout,NULL); int *xC,*xAsm,*yC,*yAsm; xC = new int[2000]; xAsm = new int[2000]; yC = new int[2000]; yAsm = new int[2000]; for(int i = 0; i &lt; 2000; i++) { xC[i] = 0; xAsm[i] = 0; yC[i] = i; yAsm[i] = i; } time_t start = clock(); calcuC(xC,yC,2000); // calcuAsm(xAsm,yAsm,2000); // for(int i = 0; i &lt; 2000; i++) // { // if(xC[i] != xAsm[i]) // { // cout&lt;&lt;"xC["&lt;&lt;i&lt;&lt;"]="&lt;&lt;xC[i]&lt;&lt;" "&lt;&lt;"xAsm["&lt;&lt;i&lt;&lt;"]="&lt;&lt;xAsm[i]&lt;&lt;endl; // errorOccured = true; // break; // } // } // if(errorOccured) // cout&lt;&lt;"Error occurs!"&lt;&lt;endl; // else // cout&lt;&lt;"Works fine!"&lt;&lt;endl; time_t end = clock(); // cout&lt;&lt;"time = "&lt;&lt;(float)(end - start) / CLOCKS_PER_SEC&lt;&lt;"\n"; cout&lt;&lt;"time = "&lt;&lt;end - start&lt;&lt;endl; return 0; } </code></pre> <p>Then I run the program five times to get the cycles of processor, which could be seen as time. Each time I call one of the function mentioned above only.</p> <p>And here comes the result.</p> <h2>Function of assembly version:</h2> <pre><code>Debug Release --------------- 732 668 733 680 659 672 667 675 684 694 Average: 677 </code></pre> <h2>Function of C++ version:</h2> <pre><code>Debug Release ----------------- 1068 168 999 166 1072 231 1002 166 1114 183 Average: 182 </code></pre> <p>The C++ code in release mode is almost 3.7 times faster than the assembly code. Why?</p> <p>I guess that the assembly code I wrote is not as effective as those generated by GCC. It's hard for a common programmer like me to wrote code faster than its opponent generated by a compiler.Does that mean I should not trust the performance of assembly language written by my hands, focus on C++ and forget about assembly language?</p>
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    1. USAdriano Repetti
    1. USuser957121
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      1. POIs inline assembly language slower than native C++ code?
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    1. COPretty much. Handcoded assembly is appropriate in some circumstances, but care must be taken to ensure that the assembly version is indeed faster than what can be achieved with a higher level language.
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      1. POIs inline assembly language slower than native C++ code?
      1. USMagnus Hoff
    2. COYou might find it instructive to study the code generated by the compiler, and try to understand why it's faster than your assembly version.
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      1. POIs inline assembly language slower than native C++ code?
      1. USPaul R
    3. COYeah, looks like the compiler is better at writing asm than you. Modern compilers really are quite good.
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      1. USDavid Heffernan
 

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