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POWhy would the same code yield different numeric results on 32 vs 64-bit machines?
 

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  1. POWhy would the same code yield different numeric results on 32 vs 64-bit machines?
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    <p>We are working on a library of numeric routines in C. We are not sure yet whether we will work with single precision (<code>float</code>) or double (<code>double</code>), so we've defined a type <code>SP</code> as an alias until we decide:</p> <pre><code>typedef float SP; </code></pre> <p>When we run our unit tests, they all pass on my machine (a 64-bit Ubuntu) but they fail on my colleague's (a 32-bit Ubuntu that was mistakenly installed on a 64-bit machine).</p> <p>Using Git's <code>bisect</code> command, we found the exact diff that began yielding different results between his machine and mine:</p> <pre><code>-typedef double SP; +typedef float SP; </code></pre> <p>In other words, going from double precision to single precision yields numerically different results on our machines (about 1e-3 relative difference in the worst cases).</p> <p>We are fairly certain that we are never comparing unsigned ints to negative signed ints anywhere.</p> <p><strong>Why would a library of numeric routines yield different results on a 32-bit operating system and on a 64-bit system?</strong></p> <p><strong>CLARIFICATION</strong></p> <p>I'm afraid I might not have been clear enough: we have Git commit <code>2f3f671</code> that uses double precision, and where the unit tests pass equally well on both machines. Then we have Git commit <code>46f2ba</code>, where we changed to single precision, and here the tests <em>still</em> pass on the 64-bit machine but <em>not</em> on the 32-bit machine.</p>
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    1. COAre you sure it's the bittage of the OS and not hardware differences?
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    2. COLikely because of this: http://stackoverflow.com/questions/1076190/64-bit-floating-point-porting-issues , (the 32 bit code is using the 387 coprocessor, the 64 bit code might be using sse)
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    3. COnos is the only one who got this one right. The given answers are wrong.
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