docs/tutorial/LangImpl1.rst

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Kaleidoscope: Tutorial Introduction and the Lexer
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.. contents::
   :local:

Written by `Chris Lattner <mailto:sabre@nondot.org>`_

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 <http://en.wikipedia.org/wiki/Visitor_pattern>`_, 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 <#language>`_: 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 <LangImpl2.html>`_: 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 <LangImpl3.html>`_: Code generation to LLVM IR - With
   the AST ready, we can show off how easy generation of LLVM IR really
   is.
-  `Chapter #4 <LangImpl4.html>`_: 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 shows off its power. :)
-  `Chapter #5 <LangImpl5.html>`_: 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 <LangImpl6.html>`_: 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 <LangImpl7.html>`_: 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 <LangImpl8.html>`_: 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!

The Basic Language
==================

This tutorial will be illustrated with a toy language that we'll call
"`Kaleidoscope <http://en.wikipedia.org/wiki/Kaleidoscope>`_" (derived
from "meaning beautiful, form, and view"). Kaleidoscope is a procedural
language that allows you to define functions, use conditionals, math,
etc. Over the course of the tutorial, we'll extend Kaleidoscope to
support the if/then/else construct, a for loop, user defined operators,
JIT compilation with a simple command line interface, etc.

Because we want to keep things simple, the only datatype in Kaleidoscope
is a 64-bit floating point type (aka 'double' in C parlance). As such,
all values are implicitly double precision and the language doesn't
require type declarations. This gives the language a very nice and
simple syntax. For example, the following simple example computes
`Fibonacci numbers: <http://en.wikipedia.org/wiki/Fibonacci_number>`_

::

    # Compute the x'th fibonacci number.
    def fib(x)
      if x < 3 then
        1
      else
        fib(x-1)+fib(x-2)

    # This expression will compute the 40th number.
    fib(40)

We also allow Kaleidoscope to call into standard library functions (the
LLVM JIT makes this completely trivial). This means that you can use the
'extern' keyword to define a function before you use it (this is also
useful for mutually recursive functions). For example:

::

    extern sin(arg);
    extern cos(arg);
    extern atan2(arg1 arg2);

    atan2(sin(.4), cos(42))

A more interesting example is included in Chapter 6 where we write a
little Kaleidoscope application that `displays a Mandelbrot
Set <LangImpl6.html#example>`_ at various levels of magnification.

Lets dive into the implementation of this language!

The Lexer
=========

When it comes to implementing a language, the first thing needed is the
ability to process a text file and recognize what it says. The
traditional way to do this is to use a
"`lexer <http://en.wikipedia.org/wiki/Lexical_analysis>`_" (aka
'scanner') to break the input up into "tokens". Each token returned by
the lexer includes a token code and potentially some metadata (e.g. the
numeric value of a number). First, we define the possibilities:

.. code-block:: c++

    // The lexer returns tokens [0-255] if it is an unknown character, otherwise one
    // of these for known things.
    enum Token {
      tok_eof = -1,

      // commands
      tok_def = -2, tok_extern = -3,

      // primary
      tok_identifier = -4, tok_number = -5,
    };

    static std::string IdentifierStr;  // Filled in if tok_identifier
    static double NumVal;              // Filled in if tok_number

Each token returned by our lexer will either be one of the Token enum
values or it will be an 'unknown' character like '+', which is returned
as its ASCII value. If the current token is an identifier, the
``IdentifierStr`` global variable holds the name of the identifier. If
the current token is a numeric literal (like 1.0), ``NumVal`` holds its
value. Note that we use global variables for simplicity, this is not the
best choice for a real language implementation :).

The actual implementation of the lexer is a single function named
``gettok``. The ``gettok`` function is called to return the next token
from standard input. Its definition starts as:

.. code-block:: c++

    /// gettok - Return the next token from standard input.
    static int gettok() {
      static int LastChar = ' ';

      // Skip any whitespace.
      while (isspace(LastChar))
        LastChar = getchar();

``gettok`` works by calling the C ``getchar()`` function to read
characters one at a time from standard input. It eats them as it
recognizes them and stores the last character read, but not processed,
in LastChar. The first thing that it has to do is ignore whitespace
between tokens. This is accomplished with the loop above.

The next thing ``gettok`` needs to do is recognize identifiers and
specific keywords like "def". Kaleidoscope does this with this simple
loop:

.. code-block:: c++

      if (isalpha(LastChar)) { // identifier: [a-zA-Z][a-zA-Z0-9]*
        IdentifierStr = LastChar;
        while (isalnum((LastChar = getchar())))
          IdentifierStr += LastChar;

        if (IdentifierStr == "def") return tok_def;
        if (IdentifierStr == "extern") return tok_extern;
        return tok_identifier;
      }

Note that this code sets the '``IdentifierStr``' global whenever it
lexes an identifier. Also, since language keywords are matched by the
same loop, we handle them here inline. Numeric values are similar:

.. code-block:: c++

      if (isdigit(LastChar) || LastChar == '.') {   // Number: [0-9.]+
        std::string NumStr;
        do {
          NumStr += LastChar;
          LastChar = getchar();
        } while (isdigit(LastChar) || LastChar == '.');

        NumVal = strtod(NumStr.c_str(), 0);
        return tok_number;
      }

This is all pretty straight-forward code for processing input. When
reading a numeric value from input, we use the C ``strtod`` function to
convert it to a numeric value that we store in ``NumVal``. Note that
this isn't doing sufficient error checking: it will incorrectly read
"1.23.45.67" and handle it as if you typed in "1.23". Feel free to
extend it :). Next we handle comments:

.. code-block:: c++

      if (LastChar == '#') {
        // Comment until end of line.
        do LastChar = getchar();
        while (LastChar != EOF && LastChar != '\n' && LastChar != '\r');

        if (LastChar != EOF)
          return gettok();
      }

We handle comments by skipping to the end of the line and then return
the next token. Finally, if the input doesn't match one of the above
cases, it is either an operator character like '+' or the end of the
file. These are handled with this code:

.. code-block:: c++

      // Check for end of file.  Don't eat the EOF.
      if (LastChar == EOF)
        return tok_eof;

      // Otherwise, just return the character as its ascii value.
      int ThisChar = LastChar;
      LastChar = getchar();
      return ThisChar;
    }

With this, we have the complete lexer for the basic Kaleidoscope
language (the `full code listing <LangImpl2.html#code>`_ for the Lexer
is available in the `next chapter <LangImpl2.html>`_ of the tutorial).
Next we'll `build a simple parser that uses this to build an Abstract
Syntax Tree <LangImpl2.html>`_. When we have that, we'll include a
driver so that you can use the lexer and parser together.

`Next: Implementing a Parser and AST <LangImpl2.html>`_