Re: [boost] [math] boost::math::isfinite fails with --fast-math

Subject: Re: [boost] [math] boost::math::isfinite fails with --fast-math
From: Paul A. Bristow (pbristow_at_[hidden])
Date: 2012-11-26 09:47:38

> -----Original Message-----
> From: boost-bounces_at_[hidden] [mailto:boost-bounces_at_[hidden]] On Behalf Of Gaetano
> Mendola
> Sent: Monday, November 26, 2012 1:34 PM
> To: boost_at_[hidden]
> Cc: John Maddock
> Subject: Re: [boost] [math] boost::math::isfinite fails with --fast-math
> 
> On 11/26/2012 02:13 PM, John Maddock wrote:
> >> I'm wandering if this is to be expected, the following code fails if
> >> compiled with --fast-math:
> >>
> >> //====================================================
> >> #include <boost/math/special_functions/fpclassify.hpp>
> >> int main() {
> >>   const float a = 0.0f/0.0f;
> >>   assert(not boost::math::isfinite(a)); }
> >
> > Which fails - does the 0/0 actually yield an infinity?  Or is it isfinite that fails?
> >
> > Either way is hard to make guarentees once you enable an option like that.
> 
> The following code:
> 
> #include <boost/math/special_functions/fpclassify.hpp>
> int main() {
>    const float a = 0.0f/0.0f;
>    std::cout << "a => " << a << std::endl;
>    std::cout << "boost::math::isfinite(a) => " << boost::math::isfinite(a) << std::endl;
>    std::cout << "a != a:  " << (a!=a ? "true" : "false") << std::endl; }
> 
> gives:
> 
> a => nan
> boost::math::isfinite(a) => 1
> a != a:  true

This appears to perhaps not conform to this 

-ffinite-math-only,  <<<<<<<<<<<<  so the IEEE pattern for infinity (or NaN?) will never be set.

"
-ffast-math
Sets -fno-math-errno, -funsafe-math-optimizations, -fno-trapping-math, 

-ffinite-math-only,  <<<<<<<<<<<<  so the IEEE pattern for infinity will never be set.

 -fno-rounding-math, 

-fno-signaling-nans

Or does this mean quiet NaNs *are* supported?

And do the specific tests boost::math:: isnan and isinf help you any more?  (They shouldn't, I
think).

Can you achieve what you want (-funsafe-math-optimizations for speed ?) by specifying individual
options?

And provide some test cases and raise a Trac ticket (No promises - the logic is more than byzantine
enough already).  

Paul