Question About the Variable Allocator

Question About the Variable Allocator

[Noah Lavine]

Hello,

I've always been puzzled about part of the variable allocator. In
module/language/tree-il/analyze.scm, we deal with allocations, which
are hash tables that say where in the stack each local variable goes.
The maps are two level, symbol -> {lambda -> location}. The reason
given is that different lambdas could have different variables with
the same symbol.

But if I understand correctly, each variable also gets a gensym, and
the gensyms are globally unique. So wouldn't it be possible to use the
gensyms as keys instead, and only have a single level map? If so, why
don't we do it?

Thanks,
Noah

Re: Question About the Variable Allocator

[Andy Wingo]

Hi Noah,

On Sun 03 Jun 2012 16:20, Noah Lavine <noah.b.lavine@gmail.com> writes:

> I've always been puzzled about part of the variable allocator. In
> module/language/tree-il/analyze.scm, we deal with allocations, which
> are hash tables that say where in the stack each local variable goes.
> The maps are two level, symbol -> {lambda -> location}. The reason
> given is that different lambdas could have different variables with
> the same symbol.
>
> But if I understand correctly, each variable also gets a gensym, and
> the gensyms are globally unique. So wouldn't it be possible to use the
> gensyms as keys instead, and only have a single level map? If so, why
> don't we do it?

AFAIR the reason is not to permit multiple lexically bound variables
with the same gensym, but to provide for different allocations of the
same variable in different closures.

Thus a variable might be at local 5 in one closure, free variable 3 in
another, free variable 6 in another, etc.

Cheers,

Andy
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