Re: [boost] Optimized portable isnan, isfinite, fpclassify etc

From: Johan Råde (rade_at_[hidden])
Date: 2006-10-19 08:35:44

Boris Gubenko wrote:
>   I've got some additional comments about the endian'ness of HP-UX --
>   see attached. To our slight embarrassment, while HP-UX/ia64 does
>   predefine _BIG_ENDIAN macro, our other big-endian platform --
>   PA-RISC -- does not. However, the proposed fallback strategy is
>   guaranteed to generate the correct result for a platform where
>   neither _BIG_ENDIAN nor _LITTLE_ENDIAN macro is defined.
> 
>   If people think, that this is a good idea to check _BIG_ENDIAN/
>   _LITTLE_ENDIAN macro first and, then, fall back to OS/architecture
>   check, I can submit an obvious, trivial patch for endian.hpp.
> 
>   To me, the crux of the matter is that while little-endian HP-UX/ia64
>   is a remote possibility, it is not completely impossible. And if
>   this creature ever comes to existence, it will define _LITTLE_ENDIAN
>   macro, so the endian'ness check will continue to work.
> 
> "
>> A compile time check is adequate.
> 
> True, but the proper way to check at compile time is to test for one
> of the _BIG_ENDIAN or _LITTLE_ENDIAN macros. I believe most or all of
> the big-endian Unix platforms define _BIG_ENDIAN = 1, but few of the
> little-endian platforms define _LITTLE_ENDIAN. So I'd suggest
> checking for one of the two macros, and then, only if neither of them
> is set, fall back to checking the OS and architecture.
> 
> By the way, while unlikely, a little-endian HP-UX ABI on Integrity
> isn't completely out of the question at some point in the future. We
> already have the ability to compile little-endian code with our HP-UX
> compiler, and the _BIG_ENDIAN macro is the way to tell which option
> was used.
> "
> 

I think the file <endian.hpp> needs to be refactored.
It is turning into a mess.
We need to make it clearer that it is a decision tree:

1. Is it GCC? Use header <endian.h>

2. If not, does the processor have a unique endianness?

3. If not, does the OS / processor combo have a unique endianness?

4. If not, are any macros such as _LITTLE_ENDIAN defined.

But in the meantime, why not add the patch.

--Johan