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From Stefano Mazzocchi <stef...@apache.org>
Subject Re: [arch] VM Candidate : JikesRVM http://jikesrvm.sourceforge.net/
Date Fri, 20 May 2005 03:51:47 GMT
Nick Lothian wrote:
>>>Why should it be so? I guess the platform dependent code 
>>emission code 
>>>is err ... not platform independent anyway. Also, if the reference 
>>>platform is for instance LLVM, or some other, off the 
>>shelf, low-level 
>>>intermediate representation, then there is no more platform 
>>>to take care of at the JVM level (I suppose)...
>>Andy is right: "writing in Java" *above* the JVM interface 
>>means you are creating bytecode and all the portability 
>>efforts were taken care for you by the JVM makers. "writing 
>>in Java" *below* the JVM interface means that you have to do, 
>>at some point, some native code transformations yourselfs, 
>>for every single different CPU architecture.
> This is true, but doesn't actually seem too hard. I'm looking at the
> "arch" section of the Jikes RVM
> (http://cvs.sourceforge.net/viewcvs.py/jikesrvm/rvm/src/vm/) and the two
> architectures there (intel & PowerPC) certianly give a good starting
> point for other architectures. There appears to be an (unfinished) ARM
> backend available for it, too
> (http://www.cs.man.ac.uk/~jsinger/armrvm.html) 

Ok. I admit I never looked into that part myself so I can't tell you how
hard it is to port a java VM onto another CPU architecture.

>>Writing a JVM in a compilable higher language means that the 
>>compiler will do all that for you.
> No it doesn't (unless I'm missing something). It's true you will be able
> to compile your VM on other architectures, but you still need to write
> the code generator for those architectures yourself (ie, the bit that
> converts bytecode into native instructions). In a C-based JVM you'd need
> to write that in C whereas in Jikes RVM it is in Java.

D'oh, you're absolutely right!

> JCVM has a nice feature in this area in that it converts bytecode to C
> and then lets GCC compile that. That would appear to make it very
> portable, but it isn't the way most VMs work. (Ironicaly, the C
> genetation code in JC VM is written in Java - see
> http://cvs.sourceforge.net/viewcvs.py/jcvm/jcvm/java/org/dellroad/jc/cge
> n/)

Hmmm, interesting... I wonder if we could hook directly into the
compiler intermediate representation instead of having to write a big
string using C syntax and fire a parser on it.

Does anybody know if GCC allows such a thing?

Keep in mind I know squad about GCC and friends.

Stefano, who should really do his homework some day.

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