Getting Started: Building and Running Clang

This page gives you the shortest path to checking out Clang and demos a few options. This should get you up and running with the minimum of muss and fuss. If you like what you see, please consider getting involved with the Clang community.

A Word of Warning

While this work aims to provide a fully functional C/C++/ObjC front-end, it is still relatively new and under heavy development. Currently we believe clang to be very usable as a C and Objective-C compiler, however there is no real C++ support yet (this is obviously a big project). Additionally, for C and Objective-C:

1. The semantic analyzer does not produce all of the warnings it should.
2. We don't consider the API to be stable yet, and reserve the right to change fundamental things.
3. Only the X86-32 and X86-64 targets have been well tested.

If you run into problems, please file bugs in LLVM Bugzilla or bring up the issue on the Clang development mailing list.

Building Clang and Working with the Code

If you would like to check out and build Clang, the current procedure is as follows:

1. Checkout and build LLVM from SVN head:
• svn co http://llvm.org/svn/llvm-project/llvm/trunk llvm
• cd llvm
• ./configure; make
2. Checkout Clang:
• From within the llvm directory (where you built llvm):
• cd tools
• svn co http://llvm.org/svn/llvm-project/cfe/trunk clang
3. If you intend to work on Clang C++ support, you may need to tell it how to find your C++ standard library headers. If Clang cannot find your system libstdc++ headers, please follow these instructions:
• 'touch empty.cpp; gcc -v empty.cpp -fsyntax-only' to get the path.
• Look for the comment "FIXME: temporary hack: hard-coded paths" in clang/lib/Frontend/InitHeaderSearch.cpp and change the lines below to include that path.
4. Build Clang:
• cd clang (assuming that you are in llvm/tools)
• make (this will give you a debug build)
5. Try it out (assuming you add llvm/Debug/bin to your path):
• clang-cc --help
• clang-cc file.c -fsyntax-only (check for correctness)
• clang-cc file.c -ast-dump (internal debug dump of ast)
• clang-cc file.c -ast-view (set up graphviz and rebuild llvm first)
• clang-cc file.c -emit-llvm (print out unoptimized llvm code)
• clang-cc file.c -emit-llvm -o - | llvm-as | opt -std-compile-opts | llvm-dis (print out optimized llvm code)
• clang-cc file.c -emit-llvm -o - | llvm-as | opt -std-compile-opts | llc > file.s (output native machine code)

Note: Here clang-cc is the "low-level" frontend executable that is similar in purpose to cc1. Clang also has a high-level compiler driver that acts as a drop-in replacement for gcc.

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.

Simultaneously Building Clang and LLVM:

Once you have checked out Clang into the llvm source tree it will build along with the rest of llvm. To build all of LLVM and Clang together all at once simply run make from the root LLVM directory.

Note: Observe that Clang is technically part of a separate Subversion repository. As mentioned above, the latest Clang sources are tied to the latest sources in the LLVM tree. You can update your toplevel LLVM project and all (possibly unrelated) projects inside it with make update. This will run svn update on all subdirectories related to subversion.

High-Level Compiler Driver (Drop-in Substitute for GCC)

While the clang-cc executable is a low-level frontend executable that can perform code generation, program analysis, and other actions, it is not designed to be a drop-in replacement for GCC's cc. For this purpose, use the high-level driver, aptly named clang. Here are some examples of how to use the high-level driver:

$ cat t.c
#include <stdio.h>
int main(int argc, char **argv) { printf("hello world\n"); }
$ clang t.c
$ ./a.out
hello world

Examples of using Clang

The high-level driver clang is designed to understand most of GCC's options, and the lower-level clang-cc executable also directly takes many of GCC's options. You can see which options clang-cc accepts with 'clang-cc --help'. Here are a few examples of using clang and clang-cc:

$ 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-cc ~/t.c -ast-print
typedef float V __attribute__(( vector_size(16) ));
V foo(V a, V b) {
   return a + b * a;
}

Code generation with LLVM:

$ clang-cc ~/t.c -emit-llvm -o - | 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
         %add = add <4 x float> %mul, %a
         ret <4 x float> %add
}
$ clang-cc ~/t.c -emit-llvm -o - | llvm-as | opt -std-compile-opts | llc -march=ppc32 -mcpu=g5
..
_foo:
         vmaddfp v2, v3, v2, v2
         blr
$ clang-cc ~/t.c -emit-llvm -o - | llvm-as | opt -std-compile-opts | llc -march=x86 -mcpu=yonah
..
_foo:
         mulps %xmm0, %xmm1
         addps %xmm0, %xmm1
         movaps %xmm1, %xmm0
         ret