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    <p>I haven't found anything about changing the optimization flags for a loop, however according to the documentation you can use the attribute <code>optimize</code> (look <a href="http://gcc.gnu.org/onlinedocs/gcc/Function-Attributes.html" rel="nofollow">here</a> and <a href="http://www.acsu.buffalo.edu/~charngda/cc.html" rel="nofollow">here</a>) on a function to override the optimization settings for that function somewhat like this:</p> <pre><code>void foo() __attribute__((optimize("O2", "inline-functions"))) </code></pre> <p>If you want to change it for several functions, you can use <code>#pragma GCC optimize</code> to set it for all following functions (<a href="http://gcc.gnu.org/onlinedocs/gcc/Function-Specific-Option-Pragmas.html#Function-Specific-Option-Pragmas" rel="nofollow">look here</a>).</p> <p>So you should be able to compile the function containing crop with a different set of optimization flags, omitting the auto-vectorization. That has the disadvantage of hardcoding the compilation flags for that function, but is the best I found.</p> <p>With regards to restructuring for better performance the two points I already mentioned in the comments come to mind (assuming the ranges can't overlap):</p> <ul> <li><p>declaring the pointers as <code>__restrict</code> to tell the compiler that they don't alias (the area pointed to by one pointer won't be accessed by any other means inside the function). The possibility of pointer aliasing is a major stumbling block for the optimizer, since it can't easily reorder the accesses if it doesn't know if writing to <code>BufferD</code> will change the contents of <code>BufferS</code>.</p></li> <li><p>Replacing the inner loop with a call to copy:</p> <pre><code>std::copy(BufferS + rowOffsetS, BufferS + rowOffsetS + Destination.GetColumns(), BufferD + rowOffsetD); </code></pre> <p>The <code>copy</code> function is likely to be pretty well optimized (probably forwarding the arguments to <code>memmove</code>), so that might make your code faster, while also making your code shorter (always a plus).</p></li> </ul>
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    1. COUsing copy disables the auto vectorization of this loop **and** provides slightly better performance over what I had earlier. Thanks!
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