Re: [boost] Ann: Floating Point Utilities Review starts today

From: John Maddock (john_at_[hidden])
Date: 2008-02-26 09:04:23

John Maddock wrote:
> 1) Floating point classification routines: these are optimised
> implementations of the C99 and C++ TR1 functions fpclassify, isinf,
> isnan, isnormal and isfinite.  From Boost-1.35 onwards these are
> already a part of Boost.Math (see
> http://svn.boost.org/svn/boost/trunk/libs/math/doc/sf_and_dist/html/math_toolkit/special/fpclass.html)
> so if accepted the two implementations will get merged.
>
> The review here should focus on the implementation used, and testing
> on whatever platforms you have available - in particular are there any
> circumstances (compiler optimisation settings etc) where this
> implementation breaks?

These look fine to me, and as Johan mentioned, any breakages should be fixed 
in endian.hpp.

Sorry to bring this up again (!), but I've been trying to get my head around 
the aliasing rules and what is and isn't allowed, in particular by following 
the rather useful article here: 
http://www.cellperformance.com/mike_acton/2006/06/understanding_strict_aliasing.html

>From this it appears that your current code is perfectly fine, however I 
still can't help feeling that we could do better than call an external 
function (memcpy) to move 4 or 8 bytes.  The suggestion in the article is to 
use a union to move from one type to another:

union float_to_int32
{
   float f;
   boost::uint32_t i;
};

boost::uint32_t get_bits(float f)
{
   float_to_int32 u;
   u.f = f;
   return u.i;
}

I can't claim to have tested exhaustively, but modifying get_bits and 
set_bits to use this method seems to work OK for VC8&9 and Minwg GCC.

Thoughts?

John.