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    <p>There are a lot of answers mentioning SSE already… but since nobody has mentioned how to use it, I'll throw another in…</p> <p>Your code has most everything a vectorizer needs to work, except two constraints: aliasing and alignment.</p> <ul> <li><p>Aliasing is the problem of two names referring two the same object. For example, <code>my_point.dist2( my_point )</code> would operate on two copies of <code>my_point</code>. This messes with the vectorizer.</p> <p>C99 defines the keyword <code>restrict</code> for pointers to specify that the referenced object is referenced uniquely: there will be no other <code>restrict</code> pointer to that object in the current scope. Most decent C++ compilers implement C99 as well, and import this feature somehow.</p> <ul> <li>GCC calls it <code>__restrict__</code>. It may be applied to references or <code>this</code>.</li> <li>MSVC calls it <code>__restrict</code>. I'd be surprised if support were any different from GCC.</li> </ul> <p>(It is not in C++0x, though.)</p> <pre><code>#ifdef __GCC__ #define restrict __restrict__ #elif defined _MSC_VER #define restrict __restrict #endif   Base dist2(const Point&amp; restrict p) const restrict </code></pre></li> <li><p>Most SIMD units require alignment to the size of the vector. C++ and C99 leave alignment implementation-defined, but C++0x wins this race by introducing <code>[[align(16)]]</code>. As that's still a bit in the future, you probably want your compiler's semi-portable support, a la <code>restrict</code>:</p> <pre><code>#ifdef __GCC__ #define align16 __attribute__((aligned (16))) #elif defined _MSC_VER #define align16 __declspec(align (16)) #endif   struct Point { double align16 xyz[ 3 ]; // separate x,y,z might work; dunno … }; </code></pre></li> </ul> <p>This isn't guaranteed to produce results; both GCC and MSVC implement helpful feedback to tell you what wasn't vectorized and why. Google your vectorizer to learn more.</p>
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