Bug 31542 - Incorrect optization with nan values
Incorrect optization with nan values
Status: UNCONFIRMED
Product: gcc
Classification: Unclassified
Component: middle-end
3.4.6
: P3 normal
: ---
Assigned To: Not yet assigned to anyone
:
Depends on:
Blocks:
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Reported: 2007-04-11 21:55 UTC by Scott Minster
Modified: 2007-04-11 22:31 UTC (History)
2 users (show)

See Also:
Host: x86_64-redhat-linux
Target: x86_64-redhat-linux
Build: x86_64-redhat-linux
Known to work:
Known to fail:
Last reconfirmed:


Attachments
preprocessed example (27.67 KB, text/plain)
2007-04-11 21:56 UTC, Scott Minster
Details

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Description Scott Minster 2007-04-11 21:55:07 UTC
When building 64 bit with an optimization setting of -O2, -O3, or -Os, some binary comparisons with nan values fail to produce false.  All comparisons (==, >, <, etc) where one (or both) of the two values is nan should produce false.

There are no warnings or other error messages during compilation.  The attached test demonstrates the error with the final assert failing.  The max() function should return the maximum of the two values, ignoring nans.  i.e., max(5, nan) should be 5.  This fails with gcc3.4.6 -O2 -m64.

Strangely, switching the order of the final portion of the return to something like:

 return (a > b ? a : (b > a ? b : (isNaN(b) ? a : b)));

causes it to work.

Also, replacing isNaN() with a call to the slower, non-inline isnan() also makes the function work.
Comment 1 Scott Minster 2007-04-11 21:56:16 UTC
Created attachment 13353 [details]
preprocessed example
Comment 2 Andrew Pinski 2007-04-11 22:03:21 UTC
Are you sure you did not have -ffast-math on the command line?
Comment 3 Scott Minster 2007-04-11 22:31:01 UTC
-ffast-math was not used.  Adding that option actually makes the test work.