[LLVMdev] Function materializing in Java

Nicolas Geoffray nicolas.geoffray at lip6.fr
Fri Apr 18 13:40:09 PDT 2008

Hi everyone,

I would like to apply the following patch (java-materialize.patch) in 
order to materialize Java functions in vmkit. The current implementation 
is not satisfactory because the materializeFunction of a module provider 
is not supposed to do anything but read the bitcode, which is not the 
case in Java. In Java, materializing a function Foo can possibly trigger 
class loading (hence executing Java code), do static initialization of 
classes that may even invoke Foo. Hence after materializing a function, 
it's possible that the function has already been codegen'd.

So that's the first part of the patch: after materializing a function, 
the JIT checks if the function has already been codegened.

The second part of the patch involves multi-threading. Since 
materializing a Java function involves executing Java code, 
synchronizations may occur. And one can imagine a scenario where:

1) thread A requires the compilation of a function Bar.Foo, hence the 
LLVM JIT takes its lock and invokes matieralizeFunction on the Java 
module provider. Materializing Foo triggers the execution of Java code 
that will load the class Bar. During class loading, the code 
synchronizes on a object Obj already locked by another thread, B.

2) thread B is doing class loading and has locked Obj. It then calls a 
function that needs to be jitted. Since thread A already owns the lock 
of the JIT, thread A and B will be interlocked.

So the second part of the patch does not take the jit lock before 
materializing a function. The lock is taken after the materialization.

I hope I made myself clear on why Java needs this patch.

I also provide a patch for the BitcodeReader (the only module provider 
implemented in llvm, right?) in order to be thread-safe. Just tell me if 
you think this should be applied too.

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