Re: Native Code Again

[Andy Wingo <wingo@pobox.com>]

Heya Noah,

Replying out-of-order here.

On Sat 08 Jan 2011 18:27, Noah Lavine <noah.b.lavine@gmail.com> writes:

> Therefore, I think the path to a native-code VM is to leave the VM as
> it is (except maybe reserve an opcode for native-code calls). Then I
> write first a C parser for Guile and then a converter program that
> would take Guile's current VM and output a JIT VM like I've described.

Hah, it seems we concluded on the same thing after all... 

> Specifically, you could construct the VM using the JITcode generator,
> either on Guile startup or whenever the user decided to enable native
> code generation.

This is a very interesting possibility.  It scares me, in its
complexity, but it does seem to handle all of the objections that I had.

> The ideal method of calling natively-generated code would be for the
> VM to tail call the JITed code, which would then tail call the VM when
> it was done. The only two ways of doing this are asking GCC to add a
> tail call intrinsic (unportable) or building a library of tail-call
> ASM for different platforms (more generally useful, but also basically
> unportable).

I actually don't think that either of these are bad ideas.

And also... why not rely on gcc's tail-call optimization, in the case
where it works?  You can check for it at configure-time.  I just ran
some small tests for tail-calls between functions in separate
compilation units and it shows that indeed, gcc does the right thing.

I guess that given this circumstance, things are a lot easier.  The JIT
library still needs to know how to tail-call a C function, but that is
more tractable.  If you don't have tail-calls, perhaps the JIT compiler
uses trampolines as you and Ludovic proposed; and that becomes a case
that only gets exercised with -O0, and possibly not even then if we add
-foptimize-sibling-calls when available.

My apologies again for going back and forth on this issue!  It's
important, and there are a few options, so I guess more discussion is
better here.

Let us know your thoughts :)

Regards,

Andy
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