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POTemplatized branchless int max/min function
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  1. POTemplatized branchless int max/min function
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    copied!<p>I'm trying to write a branchless function to return the MAX or MIN of two integers without resorting to if (or ?:). Using <a href="https://stackoverflow.com/questions/227383/how-do-i-programmatically-return-the-max-of-two-integers-without">the usual technique</a> I can do this easily enough for a given word size:</p> <pre><code>inline int32 imax( int32 a, int32 b ) { // signed for arithmetic shift int32 mask = a - b; // mask &lt; 0 means MSB is 1. return a + ( ( b - a ) &amp; ( mask &gt;&gt; 31 ) ); } </code></pre> <p>Now, assuming <a href="http://en.wikipedia.org/wiki/For_arguing" rel="nofollow noreferrer">arguendo</a> that I really am writing the kind of application on the kind of in-order processor where this is necessary, my question is whether there is a way to use C++ templates to generalize this to all sizes of int. </p> <p>The <strong>>>31</strong> step only works for int32s, of course, and while I could copy out overloads on the function for int8, int16, and int64, it seems like I should use a template function instead. But how do I get the size of a template argument in <em>bits</em>? </p> <p>Is there a better way to do it than this? Can I force the mask T to be signed? If T is unsigned the mask-shift step won't work (because it'll be a logical rather than arithmetic shift).</p> <pre><code>template&lt; typename T &gt; inline T imax( T a, T b ) { // how can I force this T to be signed? T mask = a - b; // I hope the compiler turns the math below into an immediate constant! mask = mask &gt;&gt; ( (sizeof(T) * 8) - 1 ); return a + ( ( b - a ) &amp; mask ); } </code></pre> <p>And, having done the above, can I prevent it from being used for anything but an integer type (eg, no floats or classes)?</p>
 

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