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

From: Boris Gubenko (Boris.Gubenko_at_[hidden])
Date: 2006-12-08 08:53:13

Johan Rede wrote:

> 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?

Yes, based on the value of __IEEE_FLOAT macro.

On all the platforms: Tru64, AlphaVMS, iVMS and Linux, when IEEE
floating-point mode is in effect, the cxx compiler predefines
__IEEE_FLOAT macro as 1, otherwise it predefines it with a value
of zero.

While only VMS supports multiple floating-point modes, conditionalization
below will be correct for all the platforms the cxx compiler supports:

#if defined(__DECCXX) && !__IEEE_FLOAT
#  VAX FP mode
#endif

Or you can explicitly conditionalize for VMS:

#if defined(__DECCXX) && defined(__vms) && !__IEEE_FLOAT
#  VAX FP mode
#endif

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

Yes: /float=g_float and /float=d_float.

>> 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

Does it represent a problem on either platform or this is
implementation-defined?

> Wait, I'm a bit confused. Are you telling me that the Itanium processor
> supports the VAX FP modes?

No, it does not.

> Or is the VAX FP support done through software? Wouldn't that be very slow?

It is software emulation which is, obviously, slower than native IEEE, but
I don't have performance numbers at hand.

We encourage customers to use IEEE on both Alpha and Itanium. On Alpha,
most C++ customers are still using G_FLOAT, the default, but after the
IA64 port, there is a real push to use IEEE on both platforms, especially
in mixed-architecture environment.

> The VAX FP modes do not support denormals, infinity and NaN, right?

Right.

> We just need to able to detect the VAX FP modes at compile time.

In VAX FP modes, __IEEE_FLOAT macro is defined as zero.

Btw, there is another macro which can be used to distinguish between full
IEEE support (-ieee on Tru64 and /ieee=denorm on VMS) and fast IEEE mode
(which does not have NaNs and Infinity). This macro is _IEEE_FP which is
defined when IEEE support is enabled and not defined otherwise.

Based on the results I posted yesterday, is it correct to say that fpclassify
library passes the tests on both VMS platforms?

Thanks,
  Boris