half datatype?

[Dmitry Olshansky]

11/18/2012 3:21 PM, Manu пишет:
> I've often wondered about having an official 'half' type.
> It's very common in rendering/image processing, supported by most video
> cards (so compression routines interacting with this type are common),
> and it's also supported in hardware by some cpu's.
>
> ARM for instance supports 'half's in hardware, and GCC has an __fp16
> type which would map nicely if D supported the type in the front end.
>
> The alternative us to use ushort everywhere, which is awkward, because
> it is neither unsigned, nor is it an integer, and it's not typesafe
> (allows direct assignment to ints and stuff)...
> It would be nice if: cast(half)someFloat would yield the proper value,
> even if it is performed in software in most architectures, it could be
> mapped to hardware for those that do it.

I guess half(someFloat) will do for conversion.

>
> It could be done in a library, but then GCC couldn't map it properly to
> the hardware type, and since D has no way to describe implicit casts
> (that I know of?) it becomes awkward to use.

alias this should work.
Just tried it, works wonders:

import std.math;

struct half{
     ushort data; //this one should be __fp16 where it works
//other overloaded ops, etc.

     alias getFloat this;
     //***only for the purpose of showing implicit conversion ***
     float getFloat(){ return data * 0.1; }
}

void main(){
     float x = half(12);
     assert(abs(x - 1.2) < 1e-6);
}

> someFloat = someHalf <- doesn't work, because a cast operator expects an
> explicit cast, even though this is a lossless conversion and should be
> exactly the same as someDouble = someFloat.
>
> Thoughts?

Everything but hardware support is doable as is. I'm not sure if it's 
possible and/or feasible to make _efficient_ wrapper type that uses 
hardware support.

The easiest path seems to be:
- convince GDC to add __fp16 type as an extension that maps to GCC's one
- use it on GDC, and fallback to emulation on DMD

That being said I personally have no objections to add half type to 
built-ins in DMD.

-- 
Dmitry Olshansky