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Writing an LLVM Pass

This repository contains a simple example of creating an LLVM pass. LLVM (Low-Level Virtual Machine) is a collection of modular and reusable compiler and toolchain technologies.

Downloading LLVM on MacOS

  1. Install Homebrew /bin/bash -c "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/HEAD/install.sh)"
  2. Install LLVM brew install llvm
  3. Follow the instructions as shown on your terminal
  4. Add llvm to path in .bash_profile by adding below line export PATH="/opt/homebrew/opt/llvm/bin:$PATH"
    • Note : Replace the LLVM path with yours if it is different.
  5. Run source ~/.bash_profile to apply the changes
  6. Now you can check versions of llvm, clang and clag++ by running llvm-config --version, clang --version and clang++ --version respectively
  7. Download thisrepository on your system and unzip it

Building the Hello World Pass (Similar for all Operating System)

  • Navigate to the project directory in uzipped foder named llvm cd llvm
  • Create a build directory and navigate into it mkdir build cd build
  • Generate the build files using CMake cmake ..
  • Note : You may encouter an error here No build type selected. You need to pass -DCMAKE_BUILD_TYPE=<type> in order to configure LLVM. In this case pass -DCMAKE_BUILD_TYPE=Debug .
  • Build the project cmake --build .
    • It may take some time, so you can enjoy a cup of coffee while waiting :)
  • Now come back to your project directory cd ..
  • Navigate to cd lib/Transforms/
  • Add the following into lib/Transforms/CMakeLists.txt 
    add_subdirectory(your_pass_name)
  • Make a new directory for your pass and navigate to it mkdir your_pass and cd your_pass
  • vim CMakeLists.txt and add the following lines
add_llvm_library( LLVMYourPass MODULE
  YourPass.cpp

  PLUGIN_TOOL
  opt
  )

Replace YourPass with your_own_pass_name

  • Make a .cpp file named YourPass.cpp and add the code from the provided .cpp file in this repository.
  • Navigate to build cd ../../../build/ and Build the updates using previous commands cmake .. and cmake --build .
  • This will register your new pass with LLVM

Running the Hello World Pass

  • Compile the source file into LLVM bitcode using Clang
clang -emit-llvm -c path/to/source.c

You may specify output file path as well -o path/to/output.bc. For simplicity, I suggest keeping the source file in the build directory.
You can use a most basic C file for your input, like :

#include <stdio.h>
int main(){
   printf("Hello, World!\n");
   return 0;
}
  • Run YourPass on the generated LLVM bitcode
./bin/opt -load path/to/HelloWorldPass.so -your_pass_name < path/to/output.bc > /dev/null

Replace the paths with your own.
Imp: For version greater than 12 add -enable-new-pm=0 in above command.

  • This will print Your pass output : function_name_in_input_file(main) in the terminal.

Understanding the Hello World Pass

  • This is a very simple LLVM pass that inserts a "Your pass output" string into the entry block of each function in the input code. You can find the pass implementation in the YourPass.cpp file.

  • Feel free to modify the pass to perform different transformations or analysis tasks according to your requirements.

References