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. If you run into problems, please file bugs in LLVM Bugzilla.

Release Clang Versions

Clang has been released as part of regular LLVM releases since LLVM 2.6. You can download the release versions from http://llvm.org/releases/.

Building Clang and Working with the Code

On Unix-like Systems

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

  1. Get the required tools.
  2. Checkout LLVM:
    • Change directory to where you want the llvm directory placed.
    • svn co http://llvm.org/svn/llvm-project/llvm/trunk llvm
  3. Checkout Clang:
    • cd llvm/tools
    • svn co http://llvm.org/svn/llvm-project/cfe/trunk clang
  4. Checkout Compiler-RT:
    • cd ../.. (back to where you started)
    • cd llvm/projects
    • svn co http://llvm.org/svn/llvm-project/compiler-rt/trunk compiler-rt
  5. Build LLVM and Clang:
    • cd ../.. (back to where you started)
    • mkdir build (for building without polluting the source dir)
    • cd build
    • ../llvm/configure
    • make
    • This builds both LLVM and Clang for debug mode.
    • Note: For subsequent Clang development, you can just do make at the clang directory level.
    • It is also possible to use CMake instead of the makefiles. With CMake it is also possible to generate project files for several IDEs: Eclipse CDT4, CodeBlocks, Qt-Creator (use the CodeBlocks generator), KDevelop3.
  6. 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:
    • 'gcc -v -x c++ /dev/null -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.
  7. Try it out (assuming you add llvm/Debug+Asserts/bin to your path):
    • clang --help
    • clang file.c -fsyntax-only (check for correctness)
    • clang file.c -S -emit-llvm -o - (print out unoptimized llvm code)
    • clang file.c -S -emit-llvm -o - -O3
    • clang file.c -S -O3 -o - (output native machine code)

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.

Using Visual Studio

The following details setting up for and building Clang on Windows using Visual Studio:

  1. Get the required tools:
  2. Checkout LLVM:
    • svn co http://llvm.org/svn/llvm-project/llvm/trunk llvm
  3. Checkout Clang:
    • cd llvm\tools
    • svn co http://llvm.org/svn/llvm-project/cfe/trunk clang
  4. Run cmake to generate the Visual Studio solution and project files:
    • cd ..\.. (back to where you started)
    • mkdir build (for building without polluting the source dir)
    • cd build
    • If you are using Visual Studio 2008: cmake -G "Visual Studio 9 2008" ..\llvm
    • Or if you are using Visual Studio 2010: cmake -G "Visual Studio 10" ..\llvm
    • By default, cmake will target LLVM to X86. If you want all targets (needed if you want to run the LLVM tests), add the -DLLVM_TARGETS_TO_BUILD=all option to the cmake command line. Or specify a target from the LLVM_TARGETS_TO_BUILD definition in CMakeLists.txt.
    • See the LLVM CMake guide for more information on other configuration options for cmake.
    • The above, if successful, will have created an LLVM.sln file in the build directory.
  5. Build Clang:
    • Open LLVM.sln in Visual Studio.
    • Build the "clang" project for just the compiler driver and front end, or the "ALL_BUILD" project to build everything, including tools.
  6. Try it out (assuming you added llvm/debug/bin to your path). (See the running examples from above.)
  7. See Hacking on clang - Testing using Visual Studio on Windows for information on running regression tests on Windows.

Note that once you have checked out both llvm and clang, to synchronize to the latest code base, use the svn update command in both the llvm and llvm\tools\clang directories, as they are separate repositories.

Clang Compiler Driver (Drop-in Substitute for GCC)

The clang tool is the compiler driver and front-end, which is designed to be a drop-in replacement for the gcc command. 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

The 'clang' driver is designed to work as closely to GCC as possible to maximize portability. The only major difference between the two is that Clang defaults to gnu99 mode while GCC defaults to gnu89 mode. If you see weird link-time errors relating to inline functions, try passing -std=gnu89 to clang.

Examples of using Clang

$ 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:

Note, the -cc1 argument indicates the the compiler front-end, and not the driver, should be run. The compiler front-end has several additional Clang specific features which are not exposed through the GCC compatible driver interface.

$ clang -cc1 ~/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 ~/t.c -S -emit-llvm -o -
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 -fomit-frame-pointer -O3 -S -o - t.c # On x86_64
...
_foo:
Leh_func_begin1:
	mulps	%xmm0, %xmm1
	addps	%xmm1, %xmm0
	ret
Leh_func_end1: