llvm_pass

An LLVM pass is a modular unit of code that analyzes or transforms LLVM intermediate representation (IR) during compilation. It enables optimizations, instrumentation, or code transformations, forming the core of LLVM’s flexible compiler pipeline.

🔧 Real-Life Examples of LLVM Passes

LLVM passes are essential for optimizing and transforming code during compilation. Here are some practical examples used in real-world systems:

🚀 Common Optimization Passes

• Dead Code Elimination (DCE):
  Removes unused instructions or functions to reduce binary size and improve execution speed.

• Loop Unrolling:
  Expands loops into multiple repeated statements to reduce branching and enable vectorization.

• Function Inlining:
  Replaces function calls with the function's body to eliminate call overhead and expose further optimization opportunities.

🧪 Analysis and Debugging Passes

• AddressSanitizer (ASan):
  Instruments code to detect memory errors such as buffer overflows, use-after-free, and stack corruption at runtime.

• GCOV Profiling Pass:
  Injects profiling counters into the IR to measure code coverage during test runs.

🛡️ Custom & Security-Oriented Passes

• Security Hardening Pass:
  Inserts stack canaries or bounds checks to mitigate buffer overflow and other common attack vectors.

• Custom Logging/Tracing Pass:
  Adds logging statements to monitor function entry/exit or variable usage for profiling and debugging.

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These passes illustrate the flexibility and power of LLVM's modular pass infrastructure, which enables fine-grained control over code transformation and analysis workflows.

🔧 Build LLVM project only to run Passes

Steps:

• Install required prerequisites

sudo apt update
sudo apt install cmake ninja-build git clang libedit-dev python3 zlib1g-dev libxml2-dev

• Checkout to stable version

git checkout llvmorg-14.0.0

• Run cmake command

mkdir build && cd build

cmake -G Ninja ../llvm \
  -DLLVM_ENABLE_PROJECTS="llvm" \
  -DLLVM_TARGETS_TO_BUILD="X86" \
  -DLLVM_BUILD_LLVM_DYLIB=ON \
  -DLLVM_LINK_LLVM_DYLIB=ON \
  -DCMAKE_BUILD_TYPE=Release \
  -DLLVM_ENABLE_ASSERTIONS=ON \
  -DLLVM_INCLUDE_TESTS=OFF \
  -DLLVM_INCLUDE_EXAMPLES=OFF \
  -DLLVM_INCLUDE_DOCS=OFF

• Build

ninja opt llvm-config -j 2

If you're rebuilding frequently:(Optional)

sudo apt install ccache
export CC="ccache clang"
export CXX="ccache clang++"

This can cache and reuse compiled objects.

• Add to PATH

export PATH=$(pwd)/bin:$PATH
export LLVM_DIR=$(pwd)/lib/cmake/llvm

• Now build your pass & run

clang++ -fPIC -shared HelloPass.cpp -o libHelloPass.so `llvm-config --cxxflags --ldflags --system-libs --libs core passes` 

• Run pass

opt -load-pass-plugin ./libHelloPass.so -passes=yourpass < input.ll -disable-output