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

[Peter Ross]

The following code

scale_x(S, p(X, Y)) = p(S*X, Y).

is represented in the HLDS by

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?

Pete
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