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On Wednesday, 2013-04-10 at 01:20 -0300, Claudio Freire wrote:
Actually, the OP links show that it's not that they're "innaccurate"
per-se, it's stated on the IA docs (and I do remember reading that,
though I didn't find it now when I re-checked) that it's designed to
be used within the -Pi/2..Pi/2 range, and using the instructions
outside the range would yield less precise results, that reduction to
that range ought to be done in software, with the aid of fprem.
Ah, that makes sense.
Now, not sure what did libm does, but the "fast
and wrong" version
didn't even call fprem, so I guess the slowdown could come from it
(since it has to be invoked in a loop). If not, using fprem may be
much faster than invoking libm.
I guess this could be testing by calling sincos(fmod(x, M_PI)) on the
old, inaccurate gcc version, and checking the result.
In the forum thread I mentioned at the start there are some posts listing
other math libraries, some commercial, that give better results. But it
needs rebuild of the affected applications, of course.
Carlos E. R.
(from 12.1 x86_64 "Asparagus" at Telcontar)
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