# Tutorial - Using LLVMC

Contents

- Introduction
- Compiling with LLVMC
- Using LLVMC to generate toolchain drivers

Written by Mikhail Glushenkov

# Introduction

LLVMC is a generic compiler driver, which plays the same role for LLVM as the `gcc` program does for GCC - the difference being that LLVMC is designed to be more adaptable and easier to customize. Most of LLVMC functionality is implemented via plugins, which can be loaded dynamically or compiled in. This tutorial describes the basic usage and configuration of LLVMC.

# Compiling with LLVMC

In general, LLVMC tries to be command-line compatible with `gcc` as much as possible, so most of the familiar options work:

    $ llvmc -O3 -Wall hello.cpp
    $ ./a.out
    hello

This will invoke `llvm-g++` under the hood (you can see which commands are executed by using the `-v` option). For further help on command-line LLVMC usage, refer to the `llvmc --help` output.

# Using LLVMC to generate toolchain drivers

LLVMC plugins are written mostly using TableGen, so you need to be familiar with it to get anything done.

Start by compiling `example/Simple`, which is a primitive wrapper for `gcc`:

    $ cd $LLVM_DIR/tools/llvmc
    $ cp -r example/Simple plugins/Simple

      # NB: A less verbose way to compile standalone LLVMC-based drivers is
      # described in the reference manual.

    $ make LLVMC_BASED_DRIVER_NAME=mygcc LLVMC_BUILTIN_PLUGINS=Simple
    $ cat > hello.c
    [...]
    $ mygcc hello.c
    $ ./hello.out
    Hello

Here we link our plugin with the LLVMC core statically to form an executable file called `mygcc`. It is also possible to build our plugin as a dynamic library to be loaded by the `llvmc` executable (or any other LLVMC-based standalone driver); this is described in the reference manual.

Contents of the file `Simple.td` look like this:

    // Include common definitions
    include "llvm/CompilerDriver/Common.td"

    // Tool descriptions
    def gcc : Tool<
    [(in_language "c"),
     (out_language "executable"),
     (output_suffix "out"),
     (cmd_line "gcc $INFILE -o $OUTFILE"),
     (sink)
    ]>;

    // Language map
    def LanguageMap : LanguageMap<[LangToSuffixes<"c", ["c"]>]>;

    // Compilation graph
    def CompilationGraph : CompilationGraph<[Edge<"root", "gcc">]>;

As you can see, this file consists of three parts: tool descriptions, language map, and the compilation graph definition.

At the heart of LLVMC is the idea of a compilation graph: vertices in this graph are tools, and edges represent a transformation path between two tools (for example, assembly source produced by the compiler can be transformed into executable code by an assembler). The compilation graph is basically a list of edges; a special node named `root` is used to mark graph entry points.

Tool descriptions are represented as property lists: most properties in the example above should be self-explanatory; the `sink` property means that all options lacking an explicit description should be forwarded to this tool.

The `LanguageMap` associates a language name with a list of suffixes and is used for deciding which toolchain corresponds to a given input file.

To learn more about LLVMC customization, refer to the reference manual and plugin source code in the `plugins` directory.