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We are in the process of upgrading to Scala 2.10. We seem to have fixed all the show stoppers, but for some of our numerical calculations the answers are a little different in a small number of cases. Are there any known changes introduced between 2.9.1 and 2.10.2 that might cause this or has anybody else seen something like this before?

Since all our small tests pass this should be some cumulative effect in iterative calculations, and even then it only affects a small percentage of cases. These are complex calculations and this is the only system for performing them, so we do not have an independent way to verify which version is right, short of doing a clean room reimplementation.

Daniel Mahler
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  • Is it possible to run a debugger on both the 2.9 version and the 2.10 version to try and track down specifically which calculation is behaving differently? – theon Aug 22 '13 at 08:23
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    I.e. would it be possible to post an example? – theon Aug 22 '13 at 08:24
  • @theon, unfortunately since the effect is rare and even then occurs only as a small effect in as a result of long cumulative computations involving third party libraries (which we have not changed) so we have not been able to isolate it to a self contained example. This also makes isolating the source of the difference by stepping through the code in parallel using a debugger is a bit of a project in itself. That is why I am hoping a high level description will ring a bell with somebody and help us narrow down the search. – Daniel Mahler Aug 22 '13 at 19:53

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Sorry, can't post just a comment yet. What is the magnitude of the differences you're seeing, and what kind of calculation are you doing in your code? It may not be meaningful to ask which is correct, it's FP, after all. Could you try compiling your code with strictfp? Slight differences in code gen and optimization can result in differences in when/how often 80-bit intermediates are truncated to 64 -- assuming you're running on an x86 architecture. If you have access to hardware that doesn't have 80-bit fp registers you might try running both versions on that. Even then, simple expressions like LL + S1 + S2, where LL is much greater than S1 or S2, can give different results depending on the order of the adds.

djh
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