this function never existed historically; since the float/double
functions it's based on are nonstandard and deprecated, there's really
no justification for its existence except that glibc has it. it can be
added back if there's ever really a need...

exp(inf), exp(-inf), exp(nan) used to raise wrong flags

The long double adjustment was wrong:
The usual check is
  mant_bits & 0x7ff == 0x400
before doing a mant_bits++ or mant_bits-- adjustment since
this is the only case when rounding an inexact ld80 into
double can go wrong. (only in nearest rounding mode)

After such a check the ++ and -- is ok (the mantissa will end
in 0x401 or 0x3ff).

fma is a bit different (we need to add 3 numbers with correct
rounding: hi_xy + lo_xy + z so we should survive two roundings
at different places without precision loss)

The adjustment in fma only checks for zero low bits
  mant_bits & 0x3ff == 0
this way the adjusted value is correct when rounded to
double or *less* precision.
(this is an important piece in the fma puzzle)

Unfortunately in this case the -- is not a correct adjustment
because mant_bits might underflow so further checks are needed
and this was the source of the bug.

this is silly, but it makes apps that read binary junk and interpret
it as ld80 "safer", and it gets gnulib to stop replacing printf...

this was fixed previously on i386 but the corresponding code on x86_64
was missed.

backported fix from freebsd:
http://svnweb.FreeBSD.org/base?view=revision&revision=233973

updated nextafter* to use FORCE_EVAL, it can be used in many other
places in the math code to improve readability.

apparently initializing a variable is not "using" it but assigning to
it is "using" it. i don't really like this fix, but it's better than
trying to make a bigger cleanup just before a release, and it should
work fine (tested against nsz's math tests).

make nexttoward, nexttowardf independent of long double representation.
fix nextafterl: it did not raise underflow flag when the result was 0.

old: 2*atan2(sqrt(1-x),sqrt(1+x))
new: atan2(fabs(sqrt((1-x)*(1+x))),x)
improvements:
* all edge cases are fixed (sign of zero in downward rounding)
* a bit faster (here a single call is about 131ns vs 162ns)
* a bit more precise (at most 1ulp error on 1M uniform random
samples in [0,1), the old formula gave some 2ulp errors as well)

this is a nonstandard function so it's not clear what conditions it
should satisfy. my intent is that it be fast and exact for positive
integral exponents when the result fits in the destination type, and
fast and correctly rounded for small negative integral exponents.
otherwise we aim for at most 1ulp error; it seems to differ from pow
by at most 1ulp and it's often 2-5 times faster than pow.

untested

use (1-x)*(1+x) instead of (1-x*x) in asin.s
the later can be inaccurate with upward rounding when x is close to 1

the int part was wrong when -1 < x <= -0 (+0.0 instead of -0.0)
and the size and performace gain of the asm version was negligible

cleaner implementation with unions and unsigned arithmetic

modfl(+-inf) was wrong on ld80 because the explicit msb
was not taken into account during inf vs nan check

previously a division was accidentally turned into integer div
(w = -i/NXT;) instead of long double div (w = -i; w /= NXT;)

It is probably not worth supporting gamma.
(it was already deprecated in 4.3BSD)

(fldl instruction was used instead of flds and fldt)

special care is made to avoid any inexact computations when either arg
is zero (in which case the exact absolute value of the other arg
should be returned) and to support the special condition that
hypot(±inf,nan) yields inf.

hypotl is not yet implemented since avoiding overflow is nontrivial.

(tgamma must be thread-safe, signgam is for lgamma* functions)

the old formula atan2(1,sqrt((1+x)/(1-x))) was faster but
could give nan result at x=1 when the rounding mode is
FE_DOWNWARD (so 1-1 == -0 and 2/-0 == -inf), the new formula
gives -0 at x=+-1 with downward rounding.

this has not been tested heavily, but it's known to at least assemble
and run in basic usage cases. it's nearly identical to the
corresponding i386 code, and thus expected to be just as correct or
just as incorrect.

old code saved/restored the fenv (the new code is only as slow
as that when inexact is not set before the call, but some other
flag is set and the rounding is inexact, which is rare)

before:
bench_nearbyint_exact              5000000 N        261 ns/op
bench_nearbyint_inexact_set        5000000 N        262 ns/op
bench_nearbyint_inexact_unset      5000000 N        261 ns/op

after:
bench_nearbyint_exact             10000000 N         94.99 ns/op
bench_nearbyint_inexact_set       25000000 N         65.81 ns/op
bench_nearbyint_inexact_unset     10000000 N         94.97 ns/op

fix comments about special cases

fix special cases, use multiplication instead of scalbnl

the fscale instruction is slow everywhere, probably because it
involves a costly and unnecessary integer truncation operation that
ends up being a no-op in common usages. instead, construct a floating
point scale value with integer arithmetic and simply multiply by it,
when possible.

for float and double, this is always possible by going to the
next-larger type. we use some cheap but effective saturating
arithmetic tricks to make sure even very large-magnitude exponents
fit. for long double, if the scaling exponent is too large to fit in
the exponent of a long double value, we simply fallback to the
expensive fscale method.

on atom cpu, these changes speed up scalbn by over 30%. (min rdtsc
timing dropped from 110 cycles to 70 cycles.)

this is a lot more efficient and also what is generally wanted.
perhaps the bit shuffling could be more efficient...