fmod nan_mix usage

[Bruce Evans]

On Mon, 23 Jul 2018, enh wrote:

> On Mon, Jul 23, 2018 at 12:54 PM Bruce Evans <brde at optusnet.com.au> wrote:
> ...
> bionic doesn't have as many as it should, though i do add them any time we
> catch a regression. all our tests are in
> https://android.googlesource.com/platform/bionic/+/master/tests/ with
> complex_test.cpp and math_test.cpp being the interesting ones.
> (complex_test.cpp is laughably perfunctory right now, but sadly *did* catch
> bugs where historically the makefiles were broken and we weren't shipping
> all the functions for all the architectures.)

Are most of your systems arm?  I think libm doesn't get much testing on arm
in FreeBSD (I have never even run cc on an arm system), so it especially
useful to have tests for it on other systems.  This also partly explains
why my recent tests didn't see the bug -- x86 has fmod, remainder and
remquo in asm or builtins so the C versions are not normally used.  Maybe
arm should have a bit more in asm or builtins.

> ...

>>> it looks like e_remainder.c might have the same issue, but Android's
>> tests
>>> didn't catch that :-( i'll improve the tests...
>>
>> Indeed.  Also remquo* and ctanh* :-(.  ctanh* should be more like csinh*
>> and ccosh*, and it was.
>
> yeah, i caught remquo after i hit send (and have just uploading a CL with
> the missing tests). i'm glad to hear that ctanh* actually works because i'd
> failed to break it :-) i'll commit those extra tests too anyway.

ctanh* turned out to not need multiplicative NaN mixing.  It is both more
complicated and simpler than ccosh* and csinh*, since it has more complicated
expressions so needs more special cases for exceptional args, but then the
individual cases are simpler.

>...
>> The only other complicated case seems to be hypot[fl]().  This subtracts
>> instead of adds, since it wants to convert Inf-Inf to NaN.
>
> hypot seems okay from my testing. am i missing another test?

It passes my tests too, but uses a complicated method to pass.  I first
noticed difference related to precisions with it, and fixed them less
systematically than with nan_mix*.  I think it only uses subtractive
mixing because that worked to preserve the arg order in SSE because
SSE doesn't have reverse subtraction.  That is too special.

Bruce