Re: [boost] is_nan

From: Johan Råde (rade_at_[hidden])
Date: 2006-07-25 08:44:25

John Maddock wrote:
> Martin Bonner wrote:
>> ----Original Message----
>> From: Johan Råde
>>
>>> template<class T> bool is_nan(T a)
>>> {
>>> return a != a;
>>> }
>>>
>>>
>>> 1. Do these implementations work an all platforms?
>> I believe is_nan is vunerable to over-eager optimizers for built-in
>> types.
> 
> If implemented that way, then yes: only compilers in strict IEEE mode and 
> probably with optimisations turned off will do the right thing.  It won't 
> work at all on platforms with non-IEEE arithmetic :-(
> 
> Which I why I routed everything through fpclassify: try get it right once in 
> one place and hopefully it doesn't turn into a maintenence problem :-)
> 
> Of course if the platform has native floating point classification macros 
> then these should be used: they're likely to be much more efficient than 
> "generic" solutions.  Unfortunately I found several platforms with native 
> macros that didn't actually do the right thing in all cases during testing 
> of that code :-(
> 
> John. 
> 
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> 

John,

You seem to have thought through these issues carefully.
When will you submit the floating point properties library to Boost?
I need it.

Based on your document and input from Paul Giaconne
I suggest the following interface:

     bool is_finite();
     bool is_normal();
     bool is_subnormal();

     bool is_infinite();
     bool is_plus_infinity();
     bool is_minus_infinity();

     bool is_nan();
     bool is_quiet_nan();
     bool is_signalling_nan();

The IEEE754 standard uses the term "denormal",
but the draft for the next revision of the IEEE754 standard uses the 
term "subnormal", so "subnormal" is probably better.

--Johan Råde