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Date: Mon, 23 Jul 2018 14:59:28 -0700
From: Steve Kargl <sgk@troutmask.apl.washington.edu>
To: Bruce Evans <brde@optusnet.com.au>
Cc: enh via freebsd-numerics <freebsd-numerics@freebsd.org>
Subject: Re: fmod nan_mix usage
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On Tue, Jul 24, 2018 at 07:41:17AM +1000, Bruce Evans wrote:
> On Mon, 23 Jul 2018, Steve Kargl wrote:
> >
> > 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.
> 

Whoops.  I should know better!  I have -fno-builtins hardcoded
in my development trees and completely forgot about constant
folding.

-- 
Steve