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POAlgorithm for determining Alignment of elements in C/C++ structs
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  1. POAlgorithm for determining Alignment of elements in C/C++ structs
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    copied!<p>Okay, Allow me to re-ask the question, as none of the answers got at what I was really interested in (apologies if whole-scale editing of the question like this is a faux-paus).</p> <p>A few points:</p> <ul> <li>This is offline analysis with a different compiler than the one I'm testing, so SIZEOF() or similar won't work for what I'm doing.</li> <li>I know it's implementation-defined, but I happen to know the implementation that is of interest to me, which is below.</li> </ul> <p>Let's make a function called pack, which takes as input an integer, called alignment, and a tuple of integers, called elements. It outputs another integer, called size.</p> <p>The function works as follows:</p> <pre><code>int pack (int alignment, int[] elements) { total_size = 0; foreach( element in elements ) { while( total_size % min(alignment, element) != 0 ) { ++total_size; } total_size += element; } while( total_size % packing != 0 ) { ++total_size; } return total_size; } </code></pre> <p>I think what I want to ask is "what is the inverse of this function?", but I'm not sure whether inversion is the correct term--I don't remember ever dealing with inversions of functions with multiple inputs, so I could just be using a term that doesn't apply.</p> <p>Something like what I want (sort of) exists; here I provide pseudo code for a function we'll call determine_align. The function is a little naive, though, as it just calls pack over and over again with different inputs until it gets an answer it expects (or fails).</p> <pre><code>int determine_align(int total_size, int[] elements) { for(packing = 1,2,4,...,64) // expected answers. { size_at_cur_packing = pack(packing, elements); if(actual_size == size_at_cur_packing) { return packing; } } return unknown; } </code></pre> <p>So the question is, is there a better implementation of determine_align?</p> <p>Thanks,</p>
 

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