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copied!<p><a href="http://gcc.godbolt.org/#%7B%22version%22:3,%22filterAsm%22%3a%7B%22labels%22%3atrue,%22directives%22%3atrue,%22commentOnly%22%3atrue,%22intel%22%3atrue,%22colouriseAsm%22%3atrue%7D,%22compilers%22%3a%5B%7B%22source%22%3a%22//%20Preprocessor%5Cn#include%20%3Ciostream%3E%5Cn#include%20%3Cvector%3E%5Cn%5Cn//%20Internal%20branching%5Cnvoid%20f1%28std%3a%3avector%3Cint%3E&%20v,%5Cn%20%20%20%20%20%20%20%20const%20int%20x%20=%200%29%5Cn%7B%5Cn%20%20%20%20for%20%28unsigned%20int%20i%20=%201;%5Cn%20%20%20%20%20%20%20%20%20i%20%3C%20v.size%28%29;%5Cn%20%20%20%20%20%20%20%20%20++i%5Cn%20%20%20%20%29%20%7B%5Cn%20%20%20%20%20%20%20%20v%5Bi%5D%20=%20%28x%20!=%200%29%20?%5Cn%20%20%20%20%20%20%20%20%20%20%5Ct%20%20%20%28v%5Bi-1%5D%2ax%29%20%3a%5Cn%20%20%20%20%20%20%5Ct%5Ct%20%20%20%28v%5Bi-1%5D%2bv%5Bi-1%5D%29;%5Cn%20%20%20%20%7D%5Cn%7D%5Cn%5Cn//%20Main%5Cnint%20main%28%29%5Cn%7B%5Cn%20%20%20%20std%3a%3avector%3Cint%3E%20v%2810,%202%29;%5Cn%20%20%20%20f1%28v%29;%5Cn%20%20%20%20return%200;%5Cn%7D%22,%22compiler%22%3a%22/usr/bin/g%2b%2b-4.8%22,%22options%22%3a%22-O3%22%7D%5D%7D" rel="nofollow">Godbolt page</a> tells me following things:</p> <p><code>#6</code>,<code>#7</code> Exit immediately if the loop won't ever be entered (<code>size()</code> <= 1).</p> <p><code>#8</code>,<code>#9</code> Check if <code>x</code> is not 0, if it's true, then one loop will be chosen (<code>.L3</code> + <code>.L6</code>), otherwise second loop will be taken (<code>.L5</code>).</p> <p>As you can see two optimizations were performed including the one you were querying about.</p>

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