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POreason why custom loop is faster? bad compiler? unsafe custom code? luck?(lucky cache hits)
 

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  1. POreason why custom loop is faster? bad compiler? unsafe custom code? luck?(lucky cache hits)
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    <p>i just started learning assembly and making some custom loop for swapping two variables using C++ 's asm{} body with Digital-Mars compiler in C-Free 5.0</p> <p>Enabled the -o (optimization)</p> <p>And got the results:</p> <pre><code> time of for-loop(cycles) 844 time of while-loop(cycles) 735 time of custom-loop-1(cycles) 562 time of custom-loop-2(cycles) 469 </code></pre> <p>i couldnt find Digital-Mars compiler "asm output" option to compare. There is no other optimisation options in the build options. Should i change my compiler? if yes, which one? Can you look at the codes below and tell me why custom loops are faster?</p> <p>here is the standard for loop:</p> <pre><code>t1=clock(); for(int i=0;i&lt;200000000;i++) { temp=a;//instruction 1 a=b;//instruction 2 b=temp;//3 instructions total } t2=clock(); printf("\n time of for-loop(increasing) %i \n",(t2-t1)); </code></pre> <p>here is the standard while loop:</p> <pre><code>t1=clock(); while(j&lt;200000000) { temp=a;//again it is three instructions a=b; b=temp; j++; } t2=clock(); printf("\n time of while-loop(cycles) %i \n",(t2-t1)); </code></pre> <p>here is my custom loop 1:</p> <pre><code>t1=clock(); j=200000000;//setting the count __asm { pushf //backup push eax //backup push ebx //backup push ecx //backup push edx //backup mov ecx,0 //init of loop range(0 to 200000000) mov edx,j do_it_again: //begin to loop mov eax,a //basic swap steps between cpu and mem(cache) mov ebx,b mov b,eax mov a,ebx //four instructions total inc ecx // j++ cmp ecx,edx //i&lt;200000000 ? jb do_it_again // end of loop block pop edx //rolling back to history pop ecx pop ebx pop eax popf } t2=clock(); printf("\n time of custom-loop-1(cycles) %i \n",(t2-t1)); </code></pre> <p>here is my second custom loop:</p> <pre><code>t1=clock(); j=200000000;//setting the count __asm { pushf //backup push eax push ebx push ecx push edx mov ecx,0 //init of loop range(0 to 200000000) mov edx,j mov eax,a //getting variables to registers mov ebx,b do_it_again2: //begin to loop //swapping with using only 2 variables(only in cpu) sub eax,ebx //a is now a-b add ebx,eax //b is now a sub eax,ebx //a is now -b xor eax,80000000h //a is now b and four instructions total inc ecx // j++ cmp ecx,edx //i&lt;200000000 ? jb do_it_again2 // end of loop block pop edx //rollback pop ecx pop ebx pop eax popf } t2=clock(); printf("\n time of custom-loop-2(cycles) %i \n",(t2-t1)); </code></pre> <p>full code:</p> <pre><code>#include&lt;stdio.h&gt; #include&lt;stdlib.h&gt; #include&lt;time.h&gt; int main() { int j=0; int a=0,b=0,temp=0; srand(time(0)); time_t t1=0; time_t t2=0; t1=clock(); for(int i=0;i&lt;200000000;i++) { temp=a;//instruction 1 a=b;//instruction 2 b=temp;//3 instructions total } t2=clock(); printf("\n time of for-loop(cycles) %i \n",(t2-t1)); t1=clock(); while(j&lt;200000000) { temp=a;//again it is three instructions a=b; b=temp; j++; } t2=clock(); printf("\n time of while-loop(cycles) %i \n",(t2-t1)); t1=clock(); j=200000000;//setting the count __asm { pushf //backup push eax //backup push ebx //backup push ecx //backup push edx //backup mov ecx,0 //init of loop range(0 to 200000000) mov edx,j do_it_again: //begin to loop mov eax,a //basic swap steps between cpu and mem(cache) mov ebx,b mov b,eax mov a,ebx //four instructions total inc ecx // j++ cmp ecx,edx //i&lt;200000000 ? jb do_it_again // end of loop block pop edx //rolling back to history pop ecx pop ebx pop eax popf } t2=clock(); printf("\n time of custom-loop-1(cycles) %i \n",(t2-t1)); t1=clock(); j=200000000;//setting the count __asm { pushf //backup push eax push ebx push ecx push edx mov ecx,0 //init of loop range(0 to 200000000) mov edx,j mov eax,a //getting variables to registers mov ebx,b do_it_again2: //begin to loop //swapping with using only 2 variables(only in cpu) sub eax,ebx //a is now a-b add ebx,eax //b is now a sub eax,ebx //a is now -b xor eax,80000000h //a is now b and four instructions total inc ecx // j++ cmp ecx,edx //i&lt;200000000 ? jb do_it_again2 // end of loop block pop edx //rollback pop ecx pop ebx pop eax popf } t2=clock(); printf("\n time of custom-loop-2(cycles) %i \n",(t2-t1)); return 0; } </code></pre> <p>i am just learning c++ and assembly and wondered how things going on. Thank you</p> <p>windows xp, pentium 4 (2 GHz) Digital-Mars in C-Free </p>
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    1. UShuseyin tugrul buyukisik
    1. UShuseyin tugrul buyukisik
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      1. POreason why custom loop is faster? bad compiler? unsafe custom code? luck?(lucky cache hits)
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      1. POreason why custom loop is faster? bad compiler? unsafe custom code? luck?(lucky cache hits)
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      1. POreason why custom loop is faster? bad compiler? unsafe custom code? luck?(lucky cache hits)
      1. UShuseyin tugrul buyukisik
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    1. CODid you disassemble the code the compiler generated? That'd be at least a hint as to what it's spending its cycles on that your code isn't.
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      1. POreason why custom loop is faster? bad compiler? unsafe custom code? luck?(lucky cache hits)
      1. UScHao
    2. CODo you really build with optimizations on? I once did similar expirements with memcmp and expirienced that the compiled code was slightly faster besides calls etc.. Such questions are barely answereable and always specific for the concrete case.
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      1. POreason why custom loop is faster? bad compiler? unsafe custom code? luck?(lucky cache hits)
      1. USSebastian Hoffmann
    3. COYa, this is a difficult question to answer without a disassembly, but it may just be a eh compiler.
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      1. POreason why custom loop is faster? bad compiler? unsafe custom code? luck?(lucky cache hits)
      1. USsean
 

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