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POWhat is the fastest integer division supporting division by zero no matter what the result is?
 

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  1. POWhat is the fastest integer division supporting division by zero no matter what the result is?
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    <p><strong>Summary:</strong></p> <p>I'm looking for the fastest way to calculate</p> <pre><code>(int) x / (int) y </code></pre> <p>without getting an exception for <code>y==0</code>. Instead I just want an arbitrary result.</p> <hr> <p><strong>Background:</strong></p> <p>When coding image processing algorithms I often need to divide by an (accumulated) alpha value. The most simple variant is plain C code with integer arithmetic. My problem is that I typically get a division by zero error for result pixels with <code>alpha==0</code>. However this are exactly the pixels where the result doesn't matter at all: I don't care about color values of pixels with <code>alpha==0</code>.</p> <hr> <p><strong>Details:</strong></p> <p>I'm looking for something like:</p> <pre><code>result = (y==0)? 0 : x/y; </code></pre> <p>or</p> <pre><code>result = x / MAX( y, 1 ); </code></pre> <p>x and y are positive integers. The code is executed a huge number of times in a nested loop, so I'm looking for a way to get rid of the conditional branching.</p> <p>When y does not exceed the byte range, I'm happy with the solution</p> <pre><code>unsigned char kill_zero_table[256] = { 1, 1, 2, 3, 4, 5, 6, 7, [...] 255 }; [...] result = x / kill_zero_table[y]; </code></pre> <p>But this obviously does not work well for bigger ranges.</p> <p>I guess the final question is: Whats the fastest bit twiddling hack changing 0 to any other integer value, while leaving all other values unchanged?</p> <hr> <p><strong>Clarifications</strong></p> <p>I'm not 100% sure that branching is too expensive. However, different compilers are used, so I prefer benchmarking with little optimizations (which is indeed questionable).</p> <p>For sure, compilers are great when it comes to bit twiddling, but I can't express the "don't care" result in C, so the compiler will never be able to use the full range of optimizations.</p> <p>Code should be fully C compatible, main platforms are Linux 64 Bit with gcc &amp; clang and MacOS.</p>
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    1. COHow have you determined that the if-branch is too expensive?
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    2. COHow have you determined that there **is** a branch?
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    3. CO+1 for profiling, with modern day branch prediction you might not need this. Also, **why** are you coding your own image processing algorithms?
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