1. Write your own
JVM compiler
OSCON Java 2011
Ian Dees
@undees
Hi, I’m Ian. I’m here to show that there’s never been a better time than now to write your own
compiler.

2. the three paths
Abstraction
compilers
Time
Why would someone want to do this? That depends on the direction you’re coming from.

3. Abstraction
compilers
xor eax, eax
e-
Time
If, like me, you came from a hardware background, compilers are the next logical step up the ladder
of abstraction from programming.

4. logic
λx.x grammars
Abstraction
compilers
xor eax, eax
e-
Time
If you come from a computer science background, compilers and parsers are practical tools to get
things done (such as reading crufty ad hoc data formats at work).

5. logic
λx.x grammars
Abstraction
compilers
xor eax, eax
e-
Time
If you’re following the self-made path, compilers are one of many computer science topics you may
come to naturally on your travels.

6. where we’re headed
By the end of this talk, we’ll have seen the basics of how to use JRuby to create a compiler for a
fictional programming language.

7. Thnad
a fictional programming language
Since all the good letters for programming languages are taken (C, D, K, R, etc.), let’s use a fictional
letter. “Thnad,” a letter that comes to us from Dr. Seuss, will do nicely.

8. function factorial(n) {
if(eq(n, 1)) {
1
} else {
times(n, factorial(minus(n, 1)))
}
}
print(factorial(4))
Here’s a simple Thnad program. As you can see, it has only integers, functions, and conditionals.
No variables, type definitions, or anything else. But this bare minimum will keep us plenty busy for
the next half hour.

9. hand-waving
before we see the code
Before we jump into the code, let’s look at the general view of where we’re headed.

10. minus(n, 1)
{ :funcall =>
{ :name => 'minus' },
{ :args => [ { :arg => { :name => 'n' } },
{ :arg => { :number => '1' } } } ] }
:funcall
:name :args
‘minus’ :arg :arg
:name :number
‘n’ ’1’
We need to take a piece of program text like “minus(n, 1)” and break it down into something like a
sentence’s parts of speech: this is a function call, this is a parameter, and so on. The code in the
middle is how we might represent this breakdown in Ruby, but really we should be thinking about it
graphically, like the tree at the bottom.

11. minus(n, 1)
Funcall.new 'minus', Usage.new('n'), Number.new(1)
Funcall
@name @args
‘minus’ Usage Number
@name @value
‘n’ 1
The representation on the previous slide used plain arrays, strings, and so on. It will be helpful to
transform these into custom Ruby objects that know how to emit JVM bytecode. Here’s the kind of
thing we’ll be aiming for. Notice that the tree looks really similar to the one we just saw—the only
differences are the names in the nodes.

12. 1. parse
2. transform
3. emit
Each stage of our compiler will transform one of our program representations—original program
text, generic Ruby objects, custom Ruby objects, JVM bytecode—into the next.

13. 1. parse
First, let’s look at parsing the original program text into generic Ruby objects.

14. describe Thnad::Parser do
before do
@parser = Thnad::Parser.new
end
it 'reads a number' do
expected = { :number => '42' }
@parser.number.parse('42').must_equal expected
end
end
Here’s a unit test written with minitest, a unit testing framework that ships with Ruby 1.9 (and
therefore JRuby). We want the text “42” to be parsed into the Ruby hash shown here.

15. tool #1: Parslet
http://kschiess.github.com/parslet
How are we going to parse our programming language into that internal representation? By using
Parslet, a parsing tool written in Ruby. For the curious, Parslet is a PEG (Parsing Expression
Grammar) parser.

16. require 'parslet'
module Thnad
class Parser < Parslet::Parser
# gotta see it all
rule(:space) { match('s').repeat(1) }
rule(:space?) { space.maybe }
# numeric values
rule(:number) { match('[0-9]').repeat(1).as(:number) >> space? }
end
end
Here’s a set of Parslet rules that will get our unit test to pass. Notice that the rules read a little bit
like a mathematical notation: “a number is a sequence of one or more digits, possibly followed by
trailing whitespace.”

17. 2. transform
The next step is to transform the Ruby arrays and hashes into more sophisticated objects that will
(eventually) be able to generate bytecode.

18. input = { :number => '42' }
expected = Thnad::Number.new(42)
@transform.apply(input).must_equal expected
Here’s a unit test for that behavior. We start with the result of the previous step (a Ruby hash) and
expect a custom Number object.

19. tool #2: Parslet
(again)
How are we going to transform the data? We’ll use Parslet again.

20. module Thnad
class Transform < Parslet::Transform
rule(:number => simple(:value)) { Number.new(value.to_i) }
end
end
This rule takes any simple Ruby hash with a :number key and transforms it into a Number object.

21. require 'parslet'
module Thnad
class Number < Struct.new(:value)
# more to come...
end
end
Of course, we’ll need to define the Number class. Once we’ve done so, the unit tests will pass.

22. 3. emit
Finally, we need to emit JVM bytecode.

23. bytecode
ldc 42
Here’s the bytecode we want to generate when our compiler encounters a Number object with 42 as
the value. It just pushes the value right onto the stack.

24. describe 'Emit' do
before do
@builder = mock
@context = Hash.new
end
it 'emits a number' do
input = Thnad::Number.new 42
@builder.expects(:ldc).with(42)
input.eval @context, @builder
end
end
And here’s the unit test that specifies this behavior. We’re using mock objects to say in effect,
“Imagine a Ruby object that can generate bytecode; our compiler should call it like this.”

25. require 'parslet'
module Thnad
class Number < Struct.new(:value)
def eval(context, builder)
builder.ldc value
end
end
end
Our Number class will now need an “eval” method that takes a context (useful for carrying around
information such as parameter names) and a bytecode builder (which we’ll get to on the next slide).

26. tool #3: BiteScript
https://github.com/headius/bitescript
A moment ago, we saw that we’ll need a Ruby object that knows how to emit JVM bytecode. The
BiteScript library for JRuby will give us just such an object.

27. iload 0
ldc 1
invokestatic example, 'minus', int, int, int
ireturn
This, for example, is a chunk of BiteScript that writes a Java .class file containing a function call—in
this case, the equivalent of the “minus(n, 1)” Thnad code we saw earlier.

28. $ javap -c example
Compiled from "example.bs"
public class example extends java.lang.Object{
public static int doSomething(int);
Code:
0: iload_0
1: iconst_1
2: invokestatic #11; //Method minus:(II)I
5: ireturn
If you run that through the BiteScript compiler and then use the standard Java tools to view the
resulting bytecode, you can see it’s essentially the same program.

29. enough hand-waving
let’s see the code!
Armed with this background information, we can now jump into the code.

30. a copy of our home game
https://github.com/undees/thnad
At this point in the talk, I fired up TextMate and navigated through the project, choosing the
direction based on whim and audience questions. You can follow along at home by looking at the
various commits made to this project on GitHub.

31. more on BiteScript
http://www.slideshare.net/CharlesNutter/
redev-2010-jvm-bytecode-for-dummies
There are two resources I found helpful during this project. The first was Charles Nutter’s primer on
Java bytecode, which was the only JVM reference material necessary to write this compiler.

32. see also
http://createyourproglang.com
Marc-André Cournoyer
The second helpful resource was Marc-André Cournoyer’s e-book How to Create Your Own
Freaking Awesome Programming Language. It’s geared more toward interpreters than compilers,
but was a great way to check my progress after I’d completed the major stages of this project.

33. hopes
My hope for us as a group today is that, after our time together, you feel it might be worth giving
one of these tools a shot—either for a fun side project, or to solve some protocol parsing need you
may encounter one day in your work.