[Bug c/31542] New: Incorrect optization with nan values

[Bug c/31542] New: Incorrect optization with nan values

[scott at minsters dot us]

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.


-- 
           Summary: Incorrect optization with nan values
           Product: gcc
           Version: 3.4.6
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: scott at minsters dot us
 GCC build triplet: x86_64-redhat-linux
  GCC host triplet: x86_64-redhat-linux
GCC target triplet: x86_64-redhat-linux


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=31542

[Bug c/31542] Incorrect optization with nan values

[scott at minsters dot us]

------- Comment #1 from scott at minsters dot us  2007-04-11 21:56 -------
Created an attachment (id=13353)
 --> (http://gcc.gnu.org/bugzilla/attachment.cgi?id=13353&action=view)
preprocessed example


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=31542

[Bug c/31542] Incorrect optization with nan values

[pinskia at gcc dot gnu dot org]

------- Comment #2 from pinskia at gcc dot gnu dot org  2007-04-11 22:03 -------
Are you sure you did not have -ffast-math on the command line?


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=31542

[Bug middle-end/31542] Incorrect optization with nan values

[scott at minsters dot us]

------- Comment #3 from scott at minsters dot us  2007-04-11 22:31 -------
-ffast-math was not used.  Adding that option actually makes the test work.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=31542

[Bug middle-end/31542] Incorrect optization with nan values

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=31542

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|---                         |4.5.0
         Resolution|---                         |FIXED
             Status|UNCONFIRMED                 |RESOLVED

--- Comment #6 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Just closing as fixed.

[Bug middle-end/31542] Incorrect optization with nan values

[scott at minsters dot us]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=31542

--- Comment #5 from Scott Minster <scott at minsters dot us> ---
Thanks for looking at this old bug.  I had totally forgotten about it myself!

I agree that it does not seem to affect any recent version of GCC.  It was
likely fixed in GCC 4, and doesn't seem to affect any later versions.

There's no good reason to spend time fixing such an old version, so this bug
can probably be closed as it is no longer relevant.

[Bug middle-end/31542] Incorrect optization with nan values

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=31542

--- Comment #4 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
I can't reproduce this in any recent versions of GCC. Even in GCC 4.7.3 and GCC
4.6.4 it works and we get no assert.