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Re: indirect threading for bytecode interpreter

From: Tom Tromey
Subject: Re: indirect threading for bytecode interpreter
Date: Thu, 17 Sep 2009 15:06:46 -0600
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/23.1 (gnu/linux)

>>>>> "Helmut" == Helmut Eller <address@hidden> writes:

Helmut> 5% doesn't sound like a lot to some people.

Shrug.  Obviously I think the tradeoff is worth it, or I would not have
sent the patch.  I don't think the result is all that ugly.  And,
importantly, it is very low-hanging fruit.

Helmut> vmgen sounds like a good idea, but I fear that it makes the build
Helmut> process quite a bit more complicated.

You can check in the generated code.

vmgen is a nice idea.  I rejected writing this as a direct-threaded
interpreter because I assumed that the added memory use would be a bad
tradeoff.  But, if you are interested in that, perhaps I could take a
stab at it.

Helmut> I'm wondering why gcc can't perform this transformation from the
Helmut> switch based code.  Is there no compiler setting to skip the
Helmut> range check in the switch statement?

It isn't about range checking but about eliminating a jump during the

GCC could be taught to do this.  I imagine that it has always been
simpler for people to just update their interpreter than it has been to
try to fix GCC.

I don't think that some possible future GCC change should affect whether
this patch goes in.


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