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

From: Johan Råde (rade_at_[hidden])
Date: 2006-12-08 05:09:38

Boris Gubenko wrote:
> Johan Rede wrote:
> 
>> Try both with and without /float=ieee/ieee=denorm,
>> and let me know the results.
>>
> 
> /float=ieee is necessary on AlphaVMS because on this platform, the
> default floating point mode is G_FLOAT: one of so called VAX FP modes.

Interesting! I had no idea the VAX FP modes were still used.

It may be overkill to try to handle the VAX FP modes.
But maybe I should trigger a compile time assertion if they show up.
Is there any way to detect AlphaVMS without /float=ieee at compile time?

> With iVMS, we bit the bullet and made IEEE the default, although it
> also supports VAX FP modes.

Can you get the VAX FP modes on iVMS through compiler flags?

> As far as I can see, the only difference between the two platforms
> is -n.min/2 in fast IEEE mode for float and double:
> 
> Alpha
> ------
> float:  -n.min/2 00 00 00 00
> double: -n.min/2 00 00 00 00 00 00 00 00

= positive zero

> 
> IPF
> ---
> float:  -n.min/2 80 00 00 00
> double: -n.min/2 80 00 00 00 00 00 00 00

= negative zero

--Johan