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if double precision r=x*y+z is not a half way case between two single
precision floats or it is an exact result then fmaf returns (float)r.
however the exactness check was wrong when x*y < z and could cause
incorrectly rounded result in nearest rounding mode when r is a half
way case.
fmaf(0x1.26524ep54, 0x1.cb7868p+11, 0x1.d10f5ep29)
was incorrectly rounded up to 0x1.d117ap29 instead of 0x1.d1179ep29.
(exact result is 0x1.d1179efffffffecp29, r is 0x1.d1179fp29)


the issue is described in commits 1e5eb73545ca6cfe8b918798835aaf6e07af5beb
and ffd8ac2dd50f99c3c83d7d9d845df9874ec3e7d5


The underflow exception is not raised correctly in some
cornercases (see previous fma commit), added comments
with examples for fmaf, fmal and nonx86 fma.
In fmaf store the result before returning so it has the
correct precision when FLT_EVAL_METHOD!=0




this is necessary to support archs where fenv is incomplete or
unavailable (presently arm). fma, fmal, and the lrint family should
work perfectly fine with this change; fmaf is slightly broken with
respect to rounding as it depends on nondefault rounding modes to do
its work.


thanks to the hard work of Szabolcs Nagy (nsz), identifying the best
(from correctness and license standpoint) implementations from freebsd
and openbsd and cleaning them up! musl should now fully support c99
float and long double math functions, and has nearcomplete complex
math support. tgmath should also work (fully on gcccompatible
compilers, and mostly on any c99 compiler).
based largely on commit 0376d44a890fea261506f1fc63833e7a686dca19 from
nsz's libm git repo, with some additions (dummy versions of a few
missing long double complex functions, etc.) by me.
various cleanups still need to be made, including readding (if
they're correct) some asm functions that were dropped.
