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 [1], so you need to be familiar with it to get anything done.

Start by compiling plugins/Simple/Simple.td, which is a primitive wrapper for gcc:

$ cd $LLVM_DIR/tools/llvmc
$ make DRIVER_NAME=mygcc 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 standalone dynamic library; 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.

References

[1]	TableGen Fundamentals http://llvm.cs.uiuc.edu/docs/TableGenFundamentals.html

---

Mikhail Glushenkov
LLVM Compiler Infrastructure
Last modified: $Date: 2008-12-11 11:34:48 -0600 (Thu, 11 Dec 2008) $