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

[Fergus Henderson]

On 14-Mar-2001, Ralph Becket <rbeck at microsoft.com> wrote:
> > 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,

(Did you mean "every var"?
The HLDS doesn't have "lvar"s, it only has "var"s.)

The LLDS back-end assumes that every HLDS variable occupies exactly one
machine word/register.  It makes use of that assumption for parameter
passing and when saving variables in stack slots.  Thanks for pointing
that out; in my previous mail I'd forgotten about the issue of saving
variables in stack slots for the LLDS back-end.  Probably the LLDS
back-end could be modified to deal with saving unboxed floats on the
stack, but I expect that would probaby be quite a bit of work.

The MLDS back-end does not assume that every HLDS variable occupies
exactly one machine word/register.  Instead, uses unboxed floats for
parameters and local variables, and inserts boxing/unboxing when
needed, e.g. when converting to/from a polymorphic type.

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

That sounds to me like a much easier way of solving the problem
that Pete is trying to solve.

-- 
Fergus Henderson <fjh at cs.mu.oz.au>  |  "I have always known that the pursuit
                                    |  of excellence is a lethal habit"
WWW: <http://www.cs.mu.oz.au/~fjh>  |     -- the last words of T. S. Garp.
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