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The JIT compiler in JRuby is a relatively new creation for JRuby. Of course, the initial approach taken for the JRuby platform was the most straightforward: parse and interpret the code incrementally as the program executes, traversing the AST. As time went on, the JRuby team have taken a bunch of steps to improve the performance of JRuby, most of which have involved shortcutting the consistent, but also slow and indirect interpreter model. The JIT compiler is probably one of the most aggressive and technically complex steps taken to date.

JIT compilers are a novel idea: take some code in an “intermediate form”, and, given some heuristics, compile it into a “more native” representation, with the expectation that the more native version will perform faster, and allow for more optimizations by the underlying platform. If some optimizations can be thrown into the more native form in the process, all the better.

Java, as an example, has had a JIT compiler for several years now. In fact, Java was, for many developers, the first time they heard the term JIT; so much so that many developers I know think the “J” in JIT stands for “Java”. In fact, JIT stands for Just-In-Time. Smalltalker’s may recognize the term “Dynamic Translation” instead.

Anyway, when the Java runtime determines code is eligible for native compilation (frequency of execution is one of the primary parameters), it diverts the execution of the code, so it can perform some fancy hoop-jumping to turn the Java bytecode into platform-specific native instructions, thereby removing any cost of bytecode interpretation, and also throwing some nifty optimizations into the compiled code. From that point forward, the native code will be used for execution, unless and until it has been invalidated by changing assumptions.

JRuby’s JIT compiler is similar in nature, the primary difference being the source and destination formats. In Java the source format is Java bytecode, and the destination format is native machine instructions. Conversely, in JRuby, the source format is the JRuby AST, and the destination format is Java bytecode. Perhaps the most fascinating aspect of the JRuby JIT compiler is that it benefits from both the initial compilation into Java bytecode, and then later, when Java may attempt to translate the JRuby-generated bytecode into native machine instructions. So effectively, it is possible to get a double-JIT on your executing code.

Generating Java bytecode from interpreted Ruby code is no small feat, however; so, without further ado, let’s start the tour of JRuby’s JIT!

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One of the things that is always going on in any programming language is managing the scope of variables. Scope is central to both how we code, as well as how a program executes. Even just in methods, how variables are scoped can be a point of great contention (particularly in code reviews).

When it comes to implementing a programming language like JRuby, the concept of a scope permeates everything. After all, someone needs to track what variables are available at each level in the activation stack, and when the stack unwinds (either through normal use, or due to an exception), someone needs to unbind the variables in scope with it.

And, of course, Ruby has closures, which means that you have to carry (aka capture) free variables into scope of the closure. But, Ruby also has instance and class eval’ed code blocks. Those have an entirely different scope. When you get down to it, Ruby is bound to test a scope algorithm’s limits.

So how does JRuby do it?

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To better understand how JRuby combines the Java world with the Ruby world, I have recently been delving into the source code (available via git), and while the implementation there-in is always bound to evolve and change, it seemed that there would probably be some value in me documenting my journey through the guts of JRuby.

JRuby is a huge beast, so it can be hard to find a place to start – nonetheless, as the joke goes you eat an elephant one bite at a time, so I figured I’d start somewhere at least somewhat familiar.

One of the first areas I picked up was the method dispatch code. This is an area I have seen discussed through a number of blog entries by various JRuby committers; and given the criticality of the code in this area, I knew it was probably a fairly mainstream section of functionality; heavily used by a running JRuby application.

Unfortunately, it is also right in the middle of the implementation, so it’s a bit like starting to eat the elephant right in the middle. Nonetheless, I have made my way through a good bit of it, and learned a lot in the process, so let’s get started.
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