At 06:25 PM 7/31/2002 +0200, Jerome Vouillon wrote:
>Scratchpads
>
> We need to allocate an area in the heap for each lexical variable.
> Instead of allocating this area one variable at a time, we can
> allocate a single "scratchpad" value for all variables of a block:
> this is more efficient.
>
> The compiler can keep track of the index of the variables in the
> scratchpad. So, the scratchpad can be implemented as an array of
> PMCs. (We will probably need some faster opcodes to access the
> array: there is no need to perform a method call, nor to do any
> bound checking.)
That is exactly what we are doing. However, the scratchpads themselves
are kept in a data structure, currently a stack, so discarding one
scratchpad activates the previous.
>MY
>
> To implement MY, we need to be able to access to a variable by its
> name. For this, the compiler can generate statically a hash mapping
> the variable name to its index in the scratchpad. Then,
> MY{"foo"} will look up the index of the variable "foo" in the
> hash and use the index to get the variable PMC in the scratchpad.
I think this is the plan.
> To access the scratchpad of an outer block, we need a way to move
> from a scratchpad to its parent scratchpad. This is trivial if we
Currently it is as a linked-list.
>Closures
>
> A subroutine must have access to the scratchpads of all the
> englobing blocks. As the scratchpads are linked, it is sufficient
> to add a pointer to the immediately englobing scratchpads to the
> closure (Sub class).
And they need to be COW, as closures have access to their
own copies of lexicals. I asked Jonathan to reuse the stack code
I had already written because it was using the same semantics
with COW as control and user stacks.
>Conclusion
>
> It seems to me that to implement lexical variables, we only need to
> implement the set_pmc method and to extend the Sub class so that it
> contains both a code pointer and a scratchpad.
I agree with you. It can be done without special ops that we just added.
I see no reason why we can't add an op that allows you to get the
scratchpad into a Px reg and use it just as a Hash or Array. I expect
we might want to.
> In particular, we don't need any special support for scratchpads, at
> least for the moment.
>
> What do you think?
I think you have the same idea that we do. We chose to implement
the access as ops, and you prefer using a PMC Array directly. I can
at least see one advantage to the explicit ops: they don't require
a register to use them in the general case.
-Melvin
