[Bug math/29193] sincos produces a different output than sin/cos

["dangelog at gmail dot com" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=29193

--- Comment #5 from Giuseppe D'Angelo <dangelog at gmail dot com> ---
Hello,

In a nutshell, there are a few things crossing each other:

1) `__sin_fma` returns slightly different results than `__sin_avx` or
`__sin_sse2`. I don't think that this is a problem at all, but I might be
wrong, I just lack the necessary knowledge about what glibc specifically
promises here.

2) `sincos` seems to be consistent with `__sin_sse2`, and therefore
inconsistent with `__sin_fma`. Is this inconsistency OK? Should it be
documented?

3) In general, this opens the question: within the very same "environment"
(same process, CPU, libm, fpenv, etc.), is there any guarantee that `sin(x)`
always  yields the very same result for identical values of `x`? Is `sin(x)`
meant to be perfectly reproducible? If yes, should GCC stop turning pairs of
`sin(x)` and `cos(x)` calls into one call to `sincos`?

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