[LLVMdev] JVM bytecode generation vs. LLVM

[Nicolas Geoffray]

Hi Joachim,

On Sat, Aug 27, 2011 at 12:10 PM, Joachim Durchholz <jo at durchholz.org>wrote:

> Sorry if I'm repeating something that was already said.
>
> I was just thinking "why the heck do I seem headed for JVM generation if
> what I want to use is LLVM", and this is the result:
>
> I'm coming from a Java background. I'm using Eclipse, I'm used to the
> syntax highlighting, cross referencing and refactoring support that
> Eclipse offers.
> I know I will want to have the same infrastructure for my language, and
> I want it written in my language. I WILL need a JVM backend, no matter
> what.
>
> Now, I'd still love to use LLVM. It has a lot to offer for the phases
> "above" code generation. I don't need register allocation, but I'd like
> to make use of common constant elimination, loop unrolling, inlining, or
> the pass management infrastructure; that's a whole lot of code I don't
> need to write.
> And when it comes to generating raw machine code, I can confidently say:
> develop in Eclipse and run the stuff as JVM code, but deploy using the
> machine-code backend provided by LLVM.
>
> So my conclusion is:
> To make LLVM attractive for us Java-based language designers, we need
> the means to write a JVM backend.
> The actual backend is easy, libraries for class file and JAR generation
> exist.
> I'd need help for:
> * Determining where exactly the line is drawn between "this LLVM
> component is useful for JVM bytecode generation" and "this LLVM
> component isnt". (Constant folding would be, register allocation would
> not, but there's a lot of gray areas between these two.)
>

I guess the existing line between opt/llc is similar to yours. You're just
interested in llvm bitcode optimizations that produce a different, hopefully
optimized bitcode. That's what the "opt" binary does. Register allocation is
target specific and does not "produce" any bitcode.


> * Not being a JNI or C++ expert, building the JNI infrastructure that
> would allow calling LLVM components from Java.
> * Not being a true Eclipse expert, wrapping LLVM binaries as Eclipse
> plugins. Eclipse expects plugins to be available for download via HTTP,
> with some XML that describes dependencies. Setting this up would be
> easy, getting the details right would be work.
>
> That's just my specific skillset, other language designers might have
> different ones, but I guess it is not very likely that the exact right
> combination will come up easily. There aren't many people around who're
> experts in C++, Java, and Eclipse.
>
> Oh, and the question I'm having is: Is LLVM for me?
>
> Regards,
> Jo
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