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POC++ : comparing performance of pointers pointing at different locations in a char array (trying to learn alignment)
 

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  1. POC++ : comparing performance of pointers pointing at different locations in a char array (trying to learn alignment)
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    <p><strong>Context :</strong></p> <pre><code>char buffer[99]; int* ptr_int=(int*)(buffer+n); </code></pre> <p>Then i do some time consuming operations on *ptr_int and measure execution time using windows.h/QueryPerformanceCounter.</p> <p><strong>Confusion:</strong> For values of n : 0 to 4, execution time is about 12 secs For values of n : 5,6,7 execution time is about 20 secs For value of n : 32,33 execution time is again about 12 secs.</p> <p>This may be due to alignment but can someone please explain how exactly?</p> <p><em>Pentium dual core T2410/winxp/g++3.4.2(mingw-special)</em></p> <p><strong>Edit</strong> I am not trying to avoid the alignment issue by using better approaches, instead I am trying to find why I suddenly have alignment problem with <code>int* ptr_int=(int*)(buffer+5);</code> </p> <p>No issue with:<code>int* ptr_int=(int*)(buffer+3);</code> OR <code>int* ptr_int=(int*)(buffer+33);</code> </p>
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    1. COThat will not work on some architectures. You will get hardware faults. You seem be getting lucky that either your compiler is compensating or your hardware is OK with it. Alignment must be done to the size of the object.
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      1. USMartin York
    2. COIf you want to rule out alignment problems, measure the times of accessing char elements of the array (or use an int array)
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    3. COAlso, look at the address of 'buffer' and see how it's aligned.
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