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copied!<p>I took another look at this and realised that the best way to speed it up would be to rewrite</p> <pre><code> for (size_t i = 0; i < kRuns; ++i) { c = c + (a + b) * 0.5f; } </code></pre> <p>as</p> <pre><code>c = c + kRuns * ( a + b ) * 0.5f </code></pre> <p>which takes no time at all.</p> <p>When I hand code it using simple arrays, my optimizing compiler obviously does this for the 'loop' runs a million times in a time too short to measure.</p> <pre><code> float a[3], b[3], c[3]; a[0] = 1.0f, a[1] = 2.0f, a[2] = 3.0f; b[0] = 2.0f, b[1] = 3.0f, b[2] = 1.0f; c[0] = 6.0f, c[1] = 4.0f, c[2] = 5.0f; for (size_t i = 0; i < KRUNS; ++i) { c[0] = c[0] + ( a[0] + b[0] ) * 0.5; c[1] = c[1] + ( a[1] + b[1] ) * 0.5; c[2] = c[2] + ( a[2] + b[2] ) * 0.5; } </code></pre> <p>Doesn't yours?</p> <p>Using the ublas library prevents the optimizer doing its thing. Running this code</p> <pre><code> #define KRUNS 1000000 typedef boost::numeric::ublas::vector<float, boost::numeric::ublas::bounded_array<float, 3> > MYVECTOR3; MYVECTOR3 a(3), b(3), c(3); a[0] = 1.0f, a[1] = 2.0f, a[2] = 3.0f; b[0] = 2.0f, b[1] = 3.0f, b[2] = 1.0f; c[0] = 6.0f, c[1] = 4.0f, c[2] = 5.0f; for (size_t i = 0; i < KRUNS; ++i) { noalias(c) = c + (a + b) * 0.5f; } </code></pre> <p>takes 63 milliseconds. I cannot imagine why it should take 9400 milliseconds for you, no matter how slow your machine. I have ask again: are you sure you have switched on optimization and are linking to release libraries?</p>

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