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