floating point inconsistency

Vincent Lefevre vincent+gcc@vinc17.org
Wed Feb 17 14:28:00 GMT 2010


On 2010-02-16 15:39:31 +0100, Christoph Groth wrote:
> Thank you all very much for clarifying this issue.  I am very much
> surprised that x87 instructions behave differently on AMD and Intel
> processors but this seems indeed to be the case.

That's for elementary functions only (e.g. fsincos here). On

  http://www.vinc17.net/research/testlibm/

you can see that different platforms give different results.
Differencies may be due to the OS, the C library and/or the
hardware.

Note that on x86_64 machines, sin and cos are correctly rounded
(this is not proved, but probably true in practice, assuming no
bugs in the implementation). I wonder why GCC doesn't use these
functions by default instead of glibc's sincos (which could be
enabled only with an option like -ffast-math).

-- 
Vincent Lefèvre <vincent@vinc17.net> - Web: <http://www.vinc17.net/>
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Work: CR INRIA - computer arithmetic / Arénaire project (LIP, ENS-Lyon)



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