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POFill with/without intrinsics C++
 

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  1. POFill with/without intrinsics C++
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    <p>I'm studying intrinsic functions impact on performance, and I'm a little bit confused: they seem to have no impact at all! I'm trying to fill an array of doubles with two different functions and I see no differences. I allocated the array with a call to _aligned_malloc with alignment parameter set to 8. I use Visual Studio 2008 and I compiled in Release mode, both with and without optimizations (/O2 - /Od) and both with and without intrinsics (/Oi) - all the four combinations. Two different versions follow:</p> <pre><code>#ifdef _NO_INTRIN void my_fill(double* vett, double value, int N) { double* last = vett + N; while( vett != last) { *vett++ = value; } } #else void my_fill(double* vett, double value, int N) { double* last = vett + N; // set "classically" unaligned data, if any while( (0xF &amp; (uintptr_t)vett) &amp;&amp; vett != last ) *vett++ = value; __m128d* vett_ = (__m128d*)vett; uintptr_t fff0 = ~0 &lt;&lt; 4; // round address to nearest aligned data setting to zero least significant 4 bits __m128d* last_ = (__m128d*)( fff0 &amp; (uintptr_t)last); // process until second-last element to manage odd values of N for( ; vett_ &lt; last_-1; vett_++ ) { *vett_ = _mm_set1_pd(value); } vett = (double*)vett_; while(vett != last) *vett++ = value; } #endif </code></pre> <p>As a last specification, I aligned my data to 8B and not to 16 because I plan to execute this function in a multi-threaded way on different portions of the array. So, also aligning data to 16B I couldn't be sure that all the portions of the array would be aligned (es. 303 elements, 3 threads, 101 element per thread, 1st portion aligned to 16B, 2nd portion starting @ vett+101*8 ==> unaligned). That's why I tried to implement an alignment-agnostic function. I tried to fill an array of 1M elements on my Intel Atom CPU N570 @ 1.66 GHz and I got always the same execution time. So... what's wrong with my approach? Why I see no differences? Thank you all in advance.</p>
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    1. USbiagiop1986
    1. USbiagiop1986
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    1. COHow exactly are you measuring the execution time?
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      1. USWildCrustacean
    2. COPost the benchmarking code as well.
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      1. USdan3
    3. COIs this C or C++?
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      1. USLuchian Grigore
 

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