Getting Involved

There are many tasks that are open to new developers who want to get involved with the Clang project. Below, you will find details on how to get started with Clang, plus a few tasks that we need help with.

Please note that the information provided here is not completely thorough. This is intentional. If you plan to work on Clang, we would like you to get involved with the other developers. This will allow us to work together better and will give you a better feel for how things are done. You can talk with other developers at the following mailing list: cfe-dev mailing list. The clang mailing list is a very friendly place. You can see the archives for records of past discussion. Note that a significant amount of design discussion takes place on the cfe-commits mailing list.

Getting Started

A word of warning

While this work aims to provide a fully functional C/C++/ObjC front-end, it is still very early work and is under heavy development. In particular, there is no real C++ support yet (this is obviously a big project), and C/ObjC support is still missing some features. Some of the more notable missing pieces of C support are:
  1. The semantic analyzer does not produce all of the warnings and errors it should.
  2. The LLVM code generator is still very early on. It does not support many important things, like any support for structs and unions. That said, it does handle scalar operations and vectors. clang is not ready to be used as a general purpose C code generator yet.
  3. We don't consider the API to be stable yet, and reserve the right to change fundamental things :)
Our plan is to continue chipping away at these issues until C works really well, but we'd love help from other interested contributors.

Follow what's going on

Clang is a subproject of the LLVM Project, but has its own mailing lists because the communities have people with different interests. If you are interested in clang only, these two lists should be all you need. If you are interested in the LLVM optimizer and code generator, please consider signing up for llvmdev and llvm-commits as well.

Building clang / working with the code 

If you would like to check out and build the project, the current scheme is:

  1. Checkout and build LLVM (latest instructions for SVN access)
  2. Checkout clang
  3. Non-mac users: Paths to system header files are currently hard coded into clang; as a result, if clang can't find your system headers, please follow these instructions:
    • 'touch empty.c; gcc -v empty.c -fsyntax-only' to get the path.
    • Look for the comment "FIXME: temporary hack: hard-coded paths" in clang/Driver/clang.cpp and change the lines below to include that path.
  4. Build clang

Note that the C front-end uses LLVM, but does not depend on llvm-gcc. If you encounter problems with building clang, make sure you have the latest SVN version of LLVM. LLVM contains support libraries for clang that will be updated as well as development on clang progresses.


We will eventually integrate this better as a sub-project, but for now it builds a single tool named 'clang'.
Once llvm is built in this way, you can compile C code.

Examples of using clang

The clang driver takes a lot of GCC compatible options, which you can see with 'clang --help'. Here are a few examples: 
$ cat ~/t.c

typedef float V __attribute__((vector_size(16)));
V foo(V a, V b) { return a+b*a; }

Preprocessing:
$ clang ~/t.c -E
# 1 "/Users/sabre/t.c" 1

typedef float V __attribute__((vector_size(16)));

V foo(V a, V b) { return a+b*a; }


Type checking:
$ clang -fsyntax-only ~/t.c


GCC options:
$ clang -fsyntax-only ~/t.c -pedantic
/Users/sabre/t.c:2:17: warning: extension used
typedef float V __attribute__((vector_size(16)));
                ^
1 diagnostic generated.



Pretty printing from the AST:
$ clang ~/t.c -ast-print
typedef float V __attribute__(( vector_size(16) ));

V foo(V a, V b) {
   return a + b * a;
}


LLVM code generation:
$ clang ~/t.c -emit-llvm | llvm-as | opt -std-compile-opts | llvm-dis
define <4 x float> @foo(<4 x float> %a, <4 x float> %b) {
entry:
         %mul = mul <4 x float> %b, %a           ; <<4 x float>>  
[#uses=1]
         %add = add <4 x float> %mul, %a         ; <<4 x float>>  
[#uses=1]
         ret <4 x float> %add
}
$ clang ~/t.c -emit-llvm | llvm-as | opt -std-compile-opts | llc - 
march=ppc32 -mcpu=g5
..
_foo:
         vmaddfp v2, v3, v2, v2
         blr
$ clang ~/t.c -emit-llvm | llvm-as | opt -std-compile-opts | llc - 
march=x86 -mcpu=yonah
..
_foo:
         mulps %xmm0, %xmm1
         addps %xmm0, %xmm1
         movaps %xmm1, %xmm0
         ret

Available tasks

Here are a few tasks that are currently available for newcomers to work on: