At a glance

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

Setup

LLVM is an umbrella project for building compilers and code transformation tools. We consider in this tutorial:

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

We will be building LLVM v7.0.0 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 3.4.3).

Compiling LLVM

Compiling LLVM from source is mandatory if you are developing an in-source pass (within LLVM source tree). It can also be convenient even in the case of developing out-of-source passes as it gives you full control over the compilation options.

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 different backends if needed. Without specifying LLVM_TARGETS_TO_BUILD 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)

   $ 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 absolute paths instead.

Building a trivial LLVM pass

To build skeleton LLVM pass found in skeleton folder:

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

cmake needs to find its LLVM configurations which we provide using $LLVM_DIR. Now the easiest way to run the skeleton pass is using Clang which is the compiler front-end of the LLVM framework:

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

Note that Clang needs to be installed separately.

Further resources

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)