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POSwitched compiler - performance dropped - Trying to understand why
 

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  1. POSwitched compiler - performance dropped - Trying to understand why
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    <p>I'm writing a Ruby C Extension that computes the "soft selection" of a set of vertices. It's many iteration where the distance between 3d points are computed.</p> <p>Initially I used Pelles C IDE - based it on a template I had found.</p> <p>I then made an update, where I switched to using <code>nmake</code> that comes with Visual Studio C++ Express 2010. What I found was a performance drop - which was odd because if anything it should have been faster.</p> <p>I then reverted back to the original code I had written in Pelles C and compiled it with <code>nmake</code> and found that the exact same code was slower.</p> <p><strong>Pelles C</strong></p> <pre><code>&gt; Updating soft selection took 0.741 seconds (12176 of 21692 Vertices) &gt; Updating soft selection took 0.751 seconds (10911 of 21692 Vertices) &gt; Updating soft selection took 0.859 seconds (10765 of 21692 Vertices) &gt; Updating soft selection took 0.753 seconds (10653 of 21692 Vertices) &gt; Updating soft selection took 0.75 seconds (10747 of 21692 Vertices) &gt; Updating soft selection took 0.751 seconds (10822 of 21692 Vertices) </code></pre> <p><strong>Visual Studio</strong></p> <pre><code>&gt; Updating soft selection took 1.282 seconds (11853 of 21692 Vertices) &gt; Updating soft selection took 1.273 seconds (12204 of 21692 Vertices) &gt; Updating soft selection took 1.286 seconds (11720 of 21692 Vertices) &gt; Updating soft selection took 1.248 seconds (12996 of 21692 Vertices) &gt; Updating soft selection took 1.293 seconds (10705 of 21692 Vertices) &gt; Updating soft selection took 1.276 seconds (12204 of 21692 Vertices) </code></pre> <p>I'm very inexperienced with C and compiling - but I assume that the performance difference is due to differences between the compiler and the compile instructions?</p> <p>For the <code>nmake</code> version I used the Makefile produced by <code>extconf.rb</code> - for the Pelles C version I used whatever the setting was for the sample project I found.</p> <p>Am I right in that it's the <code>CFLAGS</code> that's important here?</p> <p><strong>CFLAGS?</strong></p> <p>For the Pelles C project is is: <code>CCFLAGS = -Tx86-coff -MD -Ot -Ox -W1 -Gd -Ze -Zl#</code></p> <p>For the <code>nmake</code> project it is: <code>CFLAGS = -MD -Zi -O2b2xg- -G6</code></p> <p>When I looked up <code>CFLAGS</code> and performance it usually mentioned the flags <code>O</code>, <code>O2</code> and <code>O3</code> - now I see an <code>O2</code> in the <code>nmake</code> Makefile, but with an odd set of trailing characters.</p> <p>The Pelles C project has <code>Ot</code> and <code>Ox</code> ... ?</p> <p>I was unable to work out the meaning of these. The extension will be compiled under Windows and OSX (PPC and Intel). What configuration of the compiler can I do to get the most performance out of it? At least restore the performance I had.</p> <p><strong>Makefile and Pelles C configuration</strong></p> <p>Here is a Pastie of the <code>nmake</code> Makefile: <a href="http://pastie.org/3543595" rel="nofollow">http://pastie.org/3543595</a></p> <p>Here is a Pastie of the Pelles C project file: <a href="http://pastie.org/3543597" rel="nofollow">http://pastie.org/3543597</a></p>
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    1. COThe problem with the Microsoft C++ compiler is that it might not be compiling your program down into machine code but rather into the CLR (Common Language Runtime) which is then "interpreted" (I know it's a politcally charged word) in a way much like java Bytecodes are interpreted. This could explain the differences - because there is an overhead of starting the CLR every time you run the program. (I"m not sure how you force the compiler to compile into native machine code - but I think it can be done for C and C++)
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      1. USZeke Hansell
    2. COPerhaps Pelles just compiles your code better.
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      1. USDavid Heffernan
    3. CO@Zeke That's pretty unlikely. You have to take extra steps to get C++/CLI.
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