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A step-by-step tutorial for building an out-of-source LLVM pass based on Adrian Sampson's "LLVM for Grad Students"

LLVM is an umbrella project for building compilers and code transformation tools. It consists of several sub-projects like Clang, LLD and, confusingly enough, the LLVM sub-project. We consider in this tutorial:

• Building the LLVM sub-project from source
• Building a trivial out-of-source LLVM pass.

We will be building LLVM v10.0.0 which is the latest as of this writing. We assume that you have a working compiler toolchain (GCC or LLVM) and that CMake is installed (minimum version 3.4).

Compiling LLVM from source is mandatory if you are developing an in-source pass (within LLVM source tree). It can also be convenient in the case of developing out-of-source passes as it gives you full control over the compilation options. For example, a debug build of LLVM is much more pleasant to work with compared to an optimized one. To compile LLVM, please follow the following steps:

1. Download LLVM source and unpack it in a directory of your choice which will refer to as [LLVM_SRC]

2. Create a separate build directory

   $ mkdir llvm-build
   $ cd llvm-build

3. Instruct CMake to detect and configure your build environment:

   $ cmake -DCMAKE_BUILD_TYPE=Debug -DLLVM_TARGETS_TO_BUILD=X86 [LLVM_SRC]

   Note that we instructed cmake to only build X86 backend. You can choose a different backend if needed. If you do not specify LLVM_TARGETS_TO_BUILD, then all supported backends will be built by default which requires more time.

4. Now start the actual compilation within your build directory

   $ cmake --build .

   The --build option is a portable why to tell cmake to invoke the underlying build tool (make, ninja, xcodebuild, msbuild, etc.)

5. Building takes some time to finish. After that you can install LLVM in its default directory which is /usr/local

   $ cmake --build . --target install

   Alternatively, it's possible to set a different install directory [LLVM_HOME]. Since we will need [LLVM_HOME] in the next stage, we assume that you have defined it as an environment variable $LLVM_HOME. Now you can issue the following command

   $ cmake -DCMAKE_INSTALL_PREFIX=$LLVM_HOME -P cmake_install.cmake

   Note that $LLVM_HOME must not contain ~ (tilde) to refer to your home directory as it won't be expanded. Use $HOME or an absolute path instead.

To build the skeleton LLVM pass found in skeleton folder:

$ cd llvm-pass-tutorial
$ mkdir build
$ cd build
$ cmake ..
$ make

cmake needs to find its LLVM configurations in [LLVM_DIR]. We automatically setup [LLVM_DIR] based on $LLVM_HOME for you. Now the easiest way to run the skeleton pass is to use Clang:

$ clang-7.0 -Xclang -load -Xclang build/skeleton/libSkeletonPass.* something.c$

Note that Clang is the compiler front-end of the LLVM project. It can be installed separately in binary form.

This tutorial is based on the following resources

• Adrian Sampson's blog entry "LLVM for Grad Students" (link)
• LLVM documentation: Writing an LLVM pass (link)
• LLVM documentation: Building LLVM with CMake (link)