[m-dev.] introduce box/unbox float operators in HLDS

[Ralph Becket]

> From: Peter Ross [mailto:peter.ross at miscrit.be]
> 
> scale(S, P0) = P :-
>     PO => p(X, Y),
>     NewX := S * X,
>     P <= p(NewX, Y).
> 
> I would argue that we should represent it as the following in the HLDS,
> thus allowing structure reuse to see the box/unboxing, and reuse the
> cells where possible.  Of course when floats are unboxed then we revert
> back to the original HLDS representation.
> 
> scale(S, P0) = P :-
>     P0 => p(BoxedX, BoxedY),
>     BoxedX => box(X),
>     BoxedY => box(Y),
>     NewX := S * X,
>     NewBoxedX <= box(NewX),         % Reuse the BoxedX cell>
>     NewBoxedY <= box(Y),            % Reuse the BoxedY cell.
>     P <= p(NewBoxedX, NewBoxedY).   % Reuse the P0 cell.
> 
> What are the problems with this suggestion?

(Forgive me if I'm revealing my ignorance here...)

I thought that the idea behind the HLDS was that every lvar occupied
exactly one machine word/register, hence for `NewX := S * X' on
floats all the variables there are pointers to machine floats, rather 
than machine floats themselves.

If that's the case, wouldn't it be simpler to just add SR versions
of the float operations?

Cheers,

Ralph
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