Re: [boost] is_nan

From: John Maddock (john_at_[hidden])
Date: 2006-07-26 13:17:57

Johan Råde wrote:
> Impressive. You seem to have thought of everything. I'm just a bit
> concerned about efficiency. For instance, if I call your isnan and I
>   use VC++, then the following happens:
>
> 1. _fpclass is called
> 2. a switch statement is called to translate the result
> 3. the translated result is compared against FP_NAN
>
> Just doing x != x must be a lot faster, and works, at least on VC++
> 7.1,
> even with compiler optimizations turned on.
>
> I have situations where I want to check every element in an array with
> more than 100,000,000 elements, so I care about efficiency.

Understood, it's a question of getting a portable version - maybe an overly 
pessimistic version - and then adding optimisations for specific compilers 
where required.  Actually MSVC has _isnan, so I really should be calling 
that :-)  Likewise platforms that have an isnan macro should use it etc...

John.