use Perl

I've been playing with Parrot some more recently, and thinking about converting Java bytecode to Parrot bytecode. My thoughts went something like this: Parrot is a register machine. The Java virtual machine is a stack machine. Parrot is also a stack machine (it's only a register machine for speed, there is still a stack). Instead of converting Java bytecode to Parrot bytecode, I can make Parrot into a JVM. I can achieve this by adding new JVM-like opcodes to Parrot (really quite trivial, all I coded up was a new jvm.ops file and reconfigured and typed "make". The 14 new opcodes where generated, added to Parrot, and the assembler automagically supported them.)

Have I confused you already? Well, let me explain it. Here is a piece of Java source code:

public static void spin() {
  int i;
  for(i = 0, i < 12345678; i++);
  System.out.print(i);
}

javac turns that into the following bytecode:

   0 iconst_0
   1 istore_0
   2 goto 8
   5 iinc 0 1
   8 iload_0
   9 sipush 1000
  12 if_icmplt 5
  15 getstatic #3 <Field java.io.PrintStream out>
  18 iload_0
  19 invokevirtual #4 <Method void print(int)>
  22 return

Now, after I've added my jvm.ops file to Parrot, I can code this in Parrot assembler as the following code:

# This is Parrot assembler:
      iconst_0
      istore_0
      goto    IN
REDO: iinc      0, 1
IN:   iload_0
      sipush    12345678
      if_icmplt REDO
      iload_0
      jvm_print
      end

... and the usual ./assemble.pl jvm.pasm > jvm.pbc; ./parrot jvm.pbc will compile and run the JVM-like Parrot opcodes.

That is, I've implemented "iconst_0" (a JVM bytecode which pushes zero onto the stack) and so on in Parrot, and Parrot is running JVM-like bytecode as well as Parrot bytecode (of course, I've only implemented a couple of JVM opcodes). This shows how flexible Parrot is, and is the method that will be used when multiple language target Parrot I imagine.

So I posted the code to perl6-internals and got little response back, apart from the "you're crazy" and "wow". It's a cute idea and it leads on to issues raised by Dan such as sharing objects between Java and other languages hosted on Parrot, or maybe doing wild JNI stuff. [Note: we can't go much further at the moment as Parrot doesn't have a symbol table, objects, methods, or method calls yet - but it will ;-]

This is all research for my Targeting Parrot talk at OSCON. Is there anything you'd like to know about targeting languages at Parrot? Ask away!

Anyway, off to a London.pm social meeting. I'm wearing my Parrot tshirt today...