[llvm-dev] A "Java Backend"

[David Chisnall via llvm-dev]

On 19 Jul 2016, at 04:06, Lorenzo Laneve via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> 
> My idea was to create a complete backend treating Java as a normal platform, to enable LLVM to compile programs to Java Bytecode (.class) and Java Archive files (.jar). This could be useful in situations where we need to compile a program for a platform still not natively supported by LLVM.
> 
> I don't know if it exists already, I've heard about this "LLJVM" but I don't think it does the same thing as my idea.
> What do you think?

I think that it will be difficult.  Java bytecode is intrinsically designed to be memory safe, whereas LLVM IR is not.  There is no equivalent of inttoptr or ptrtoint in Java bytecode and the closest equivalent of a GEP is to retrieve a field from an object (though that’s only really for GEP + load/store).

You could potentially do something a bit ugly and treat all of LLVM memory as one big ByteBuffer object, and make pointers indexes into this, but then you’d make it very hard for your LLVM-originating code to interoperate with Java-originating code and so you’d have to write a lot of code to take the place of the system call layer.

Oh, and I doubt that you’ll find many more platforms that have a fully functional JVM than are LLVM targets.  Even big-endian MIPS64 is not well-supported by Java (JamVM - a pure interpreter - is the only thing that we’ve managed to find that works).

David

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