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Re: RTL Question

From: Andy Wingo
Subject: Re: RTL Question
Date: Wed, 20 Jun 2012 09:31:34 +0200
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/23.4 (gnu/linux)

On Tue 19 Jun 2012 05:52, Noah Lavine <address@hidden> writes:

> Time for my next RTL question - how do I use the toplevel-ref
> instruction? It looks like I need to make a variable object in the
> instruction stream, or at a known offset from it. I think I should use
> either make-non-immediate or the linker to do that, but I don't quite
> know how.

I haven't yet wired that up.  You have it right: a variable object in
the object file.  It's not actually in the instruction stream: it's in
another section.  The other section will be writable and end up on a
different page in memory, so that we share the text but not the writable
data.  Also the writable data section will be added as a GC root.

I think the thing to do is to add a macro-instruction that emits a
toplevel ref, adds a variable object to the constant table, and
adds a relocation so that the offset is fixed up at link time.

Another option would be to implement inline caches for toplevel
references, and also toplevel calls.  A reference would be a thunk call.
The first reference would update the cache.  You could share the caches
for accesses to the same toplevel within a function.  They have to be
very few (bytecode) instructions in the hot case though, otherwise that
would be a lose.

Dunno.  That part is still unsettled, basically!


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