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From Tom Tromey <tro...@redhat.com>
Subject Re: [arch] Interpreter vs. JIT for Harmony VM
Date Wed, 21 Sep 2005 15:11:11 GMT
>>>>> "Geir" == Geir Magnusson <geirm@apache.org> writes:

>> On the other hand, a fast code-generating JIT can call runtime
>> helpers and native methods without additional glue code whereas an
>> interpreter has to have special glue code to make it work in a JIT
>> environment.

Geir> I believe you, but I don't understand this.  Can you explain in more
Geir> detail?

It is about handling calling conventions.

There are conceptually (at least) 2 cases to consider when
implementing the java 'invoke' family of opcodes in an interpreter.

In the first case, suppose you're invoking another method that you
know is interpreted.  In this case you might simply make a recursive
call to the interpreter function itself, passing in new locals as an
array or something.  The interpreter itself might look something like
(I'm just making this up, but it is reasonably close to, e.g., what
libgcj does):

  void interpret (jclass declaringClass, jmethodID method,
                  union jslot *locals)

... where jslot corresponds to a single stack or local variable slot
as discussed in the JVM spec.

So to make your call you would look up the method and pass slots from
your current stack as the 'locals' argument.

(Note that you aren't required to do things this way; in libgcj we
only use the native ABI and we don't special case calls to interpreted
methods at all.  We probably pay some performance penalty for
this... though the interpreter is plenty slow on its own :-)


In the second case, you're calling some function that is not an
interpreted function, e.g. a native method.  In this case the
underlying function will be using whatever low-level function calling
ABI is defined by the current platform (and implemented in the C
compiler).

There is no standard way in C to make such calls.  Instead you end up
having to use something like libffi -- a piece of code that translates
from some array-of-arguments view to the low-level register twiddling
required to make an arbitrary C call.


For a JIT the situation is different.  A JIT already understands a lot
about register twiddling.  I don't know whether it is common to use
the C ABI when writing a JIT, but in any case it would seem that
putting this in there as well is no big deal.  Then instead of
figuring out at call time how to make a given call, you simply
determine it at compile time and generate the appropriate code.

>> Our experience is that a fast, zero optimizing JIT can yield low-
>> enough response time. So, I think at least Harmony has the option
>> of having a decent system without an interpreter. Thoughts?

Geir> Basic thought is yes, I always figured we'd have this pluggable, with
Geir> an interpreter for ease of porting, and then platform-specific JIT.

It seems to me that there's a design question here.  For instance, if
you want to eventually take interpreted code and compile it (when it
is "hot"), for full pluggability your JIT(s) and your interpreter need
to agree on some set of bookkeeping details in order to make this
possible.  OTOH, you could make other decisions that make this problem
go away, for instance having a single choice of execution engine up
front; so the "fast JIT" and the "optimizing JIT" are just part of the
same code base and only need to talk to each other, and can be built
in an ad hoc way.


Personally I'd be just as happy if we only had a JIT.  There are
already plenty of interpreters out there.

Tom

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