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LLVM: Implementing a Language

Tutorial Introduction

Welcome to the “Implementing a language with LLVM” tutorial. This tutorial runs through the implementation of a simple language, showing how fun and easy it can be. This tutorial will get you up and started as well as help to build a framework you can extend to other languages. The code in this tutorial can also be used as a playground to hack on other LLVM specific things.

The goal of this tutorial is to progressively unveil our language, describing how it is built up over time. This will let us cover a fairly broad range of language design and LLVM-specific usage issues, showing and explaining the code for it all along the way, without overwhelming you with tons of details up front.

It is useful to point out ahead of time that this tutorial is really about teaching compiler techniques and LLVM specifically, not about teaching modern and sane software engineering principles. In practice, this means that we’ll take a number of shortcuts to simplify the exposition. For example, the code leaks memory, uses global variables all over the place, doesn’t use nice design patterns like visitors, etc... but it is very simple. If you dig in and use the code as a basis for future projects, fixing these deficiencies shouldn’t be hard.

I’ve tried to put this tutorial together in a way that makes chapters easy to skip over if you are already familiar with or are uninterested in the various pieces. The structure of the tutorial is:

  • Chapter #1: Introduction to the Kaleidoscope language, and the definition of its Lexer - This shows where we are going and the basic functionality that we want it to do. In order to make this tutorial maximally understandable and hackable, we choose to implement everything in C++ instead of using lexer and parser generators. LLVM obviously works just fine with such tools, feel free to use one if you prefer.
  • Chapter #2: Implementing a Parser and AST - With the lexer in place, we can talk about parsing techniques and basic AST construction. This tutorial describes recursive descent parsing and operator precedence parsing. Nothing in Chapters 1 or 2 is LLVM-specific, the code doesn’t even link in LLVM at this point. :)
  • Chapter #3: Code generation to LLVM IR - With the AST ready, we can show off how easy generation of LLVM IR really is.
  • Chapter #4: Adding JIT and Optimizer Support - Because a lot of people are interested in using LLVM as a JIT, we’ll dive right into it and show you the 3 lines it takes to add JIT support. LLVM is also useful in many other ways, but this is one simple and “sexy” way to show off its power. :)
  • Chapter #5: Extending the Language: Control Flow - With the language up and running, we show how to extend it with control flow operations (if/then/else and a ‘for’ loop). This gives us a chance to talk about simple SSA construction and control flow.
  • Chapter #6: Extending the Language: User-defined Operators - This is a silly but fun chapter that talks about extending the language to let the user program define their own arbitrary unary and binary operators (with assignable precedence!). This lets us build a significant piece of the “language” as library routines.
  • Chapter #7: Extending the Language: Mutable Variables - This chapter talks about adding user-defined local variables along with an assignment operator. The interesting part about this is how easy and trivial it is to construct SSA form in LLVM: no, LLVM does not require your front-end to construct SSA form!
  • Chapter #8: Conclusion and other useful LLVM tidbits - This chapter wraps up the series by talking about potential ways to extend the language, but also includes a bunch of pointers to info about “special topics” like adding garbage collection support, exceptions, debugging, support for “spaghetti stacks”, and a bunch of other tips and tricks.

By the end of the tutorial, we’ll have written a bit less than 700 lines of non-comment, non-blank, lines of code. With this small amount of code, we’ll have built up a very reasonable compiler for a non-trivial language including a hand-written lexer, parser, AST, as well as code generation support with a JIT compiler. While other systems may have interesting “hello world” tutorials, I think the breadth of this tutorial is a great testament to the strengths of LLVM and why you should consider it if you’re interested in language or compiler design.

A note about this tutorial: we expect you to extend the language and play with it on your own. Take the code and go crazy hacking away at it, compilers don’t need to be scary creatures - it can be a lot of fun to play with languages!