[LLVMdev] Performance of JNI in VMKit

[Bruno Daniel]

Hi Nicolas,

thanks for your detailed answer! Now I understand the issues a little better.
I'm going to install LLVM + Vmkit and try it out using JNI.

Regarding the garbage collector, do you think that the Garbage Collection ABI
of C++11 could be implemented in a way compatible to the JVM's garbage collector?
http://www2.research.att.com/~bs/C++0xFAQ.html#gc-abi

Perhaps this could make the C/C++/Java integration quite seamless and the
two worlds of the C-ABI languages and the JVM languages could be united.

All the best
  Bruno Daniel

Nicolas Geoffray wrote:
> Hi Bruno,
> 
> On Wed, Jun 20, 2012 at 3:04 PM, Bruno Daniel <abml at mailoo.org> wrote:
> 
>> Dear developers,
>>
>> Are there any benchmarks for the performance of Java Native Interface (JNI)
>> calls in VMKit? Since VMKit is based on LLVM which can also run C++ code
>> (maybe in the same just-in-time compiler?) I guess calls from Java to C++
>> and
>> back could be much faster than in Sun's JVM which has extremely slow C++ ->
>> Java callbacks.
>>
>> If this was the case, this would be a big advantage of VMKit over all other
>> JVMs and mixing existing C/C++ libraries with Java would be as easy as in
>> the
>> glue language Python.
>>
> 
> While mixing C/C++/Java with the same compiler seems appealing for
> performance, Java has a garbage collector, and a garbage collector does not
> cope well with native frames. It requires object indirection when leaving
> Java boundaries, as well as being able to interrupt the execution of a
> thread for initiating a collection.
> 
> Note that it is possible to do it: VMKit is written in C/C++ and objects
> are being passed directly, and a C++ frame, since it is compiled by clang
> can be interrupted. However, it is very subtle how it works, and a slight
> omission of a GC object declaration leads to crashes that are hard to
> detect. It's always possible to do better, like defining a convention that
> will tell clang which objects are being used by the GC, but it hasn't been
> worked on yet.
> 
> If you don't care about a GC, then sure, you can compiler C/C++/Java into a
> single native executable, where performance of inter-language calls are the
> same as intra-language calls.
> 
> Also, note that there were some works on trying to optimize JNI calls
> through inlining at runtime (http://dl.acm.org/citation.cfm?id=1064997). I
> don't know if the optimization is actually being used in existing JVMs.
> 
> Hope this helps!
> Nicolas
> 
> 
>>
>> All the best
>>   Bruno Daniel
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