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PONaN handled differently in different g++ versions
 

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    <p>Consider the following program, which is obviously buggy:</p> <pre><code>#include &lt;cstdio&gt; double test(int n) { if (n % 2 == 0) return 0.0; // warning: control reaches end of non-void function } int main() { printf("%.9lf\n", test(0)); printf("%.9lf\n", test(1)); printf("%.9lf\n", test(2)); printf("%.9lf\n", test(3)); return 0; } </code></pre> <p>When compiled with g++, version 4.2.4 (Ubuntu 4.2.4-1ubuntu4) on a 32-bit Ubuntu 8.04, it produces the following output:</p> <pre><code>0.000000000 nan 0.000000000 nan </code></pre> <p>When compiled with g++, version 4.4.3 (Ubuntu 4.4.3-4ubuntu5) on a 64-bit Ubuntu 10.04, it produces the following output:</p> <pre><code>0.000000000 0.000000000 0.000000000 0.000000000 </code></pre> <p>It seems that the older compiler does some extra work in order to return NaN instead of garbage, while the newer compiler simply returns whatever there is in memory. What exactly causes this difference in behavior and how to control it and make it predictable across different compiler versions?</p> <p>EDIT: Sorry for not mentioning undefined behavior earlier. I know that the difference comes from the fact that this program has undefined behavior. What I'd like to know <strong>why</strong> the former gcc version seems to put some effort and produce code that consistently returns NaN and <strong>when</strong> this behavior changed to the one observed in the latter gcc version. Also, by "predictable" I meant not how to write good programs in C++, but how to control this gcc behavior (maybe with some command line options?).</p>
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    1. COHow can you make undefined behavior more predictable?
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      1. USConstantinius
    2. CO@Jasiu: look at the emitted code to see whether one compiler is doing extra work, not the output of the program. It could be that neither of them is doing any work, and hence returning garbage, but some little difference in how they use registers or how they lay out the stack means that the garbage value was 0 on one version and NaN on the other. Or it could be the result is unreliable, and sometimes the one that produced 0 this time will produce NaN and vice-versa. I don't have either of those versions installed myself.
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      1. USSteve Jessop
    3. COTry different optimisation flags, too!
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      1. USKerrek SB
 

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