fmod nan_mix usage

[Bruce Evans]

On Mon, 23 Jul 2018, Steve Kargl wrote:

> On Mon, Jul 23, 2018 at 11:28:08AM -0700, enh via freebsd-numerics wrote:
>> the recent change from
>>
>>   return (x*y)/(x*y);
>>
>> to
>>
>>   return nan_mix(x, y)/nan_mix(x, y);
>>
>> in e_fmod.c broke some of our unit tests. for example, fmod(3.f, 0.f) in
>> one of the VM tests.
>>
>> bionic/tests/math_test.cpp:(784) Failure in test
>> math_h_force_long_double.fmod
>> Value of: isnan(fmod(3.0, 0.0))
>>   Actual: false
>> Expected: true
>
> Can you share the code for the relevant tests?
> This simple program gives the expected results
> on amd64.
>
> #include <math.h>
> #include <stdio.h>
>
> int
> main(void)
> {
> 	printf("%e %d\n", fmodf(3.f, 0.f), isnan(fmodf(3.f, 0.f)));
> 	printf("%le %d\n", fmod(3.0, 0.0),  isnan(fmod(3.0, 0.0)));
> 	printf("%Le %d\n", fmodl(3.L, 0.L),  isnan(fmodl(3.L, 0.L)));
> 	return 0;
> }
>
> % cc -o z -O a.c -lm && ./z
> nan 1
> nan 1
> nan 1

clang normally evaluates this at compile, so it doesn't test the libary.
This is arguably a bug in clang, since it doesn't set the exception flags.
#pragma FENV_ACCESS should control this, but it is hard to use and rarely
works.

The test data needs to be non-literal and perhaps even volatile to prevent
the compiler evaluating it at compile time.

Bruce