Build Custom Passes as a Part of the LLVM Build Environment

There is no dynamic linking on Windows (this is OS weirdness) so we cannot use the plugins at all, unfortunately. In this post, I would like to share a how-to about building your own LLVM pass as a part of the LLVM build tree (LLVMExperimentPasses and the demo pass FunctionArgumentCount can be found on GitHub).

To build the pass, do the following:

1. Copy the LLVMExperimentPasses pass directory in the lib/Transforms one. Here and throughout, all paths are given at the root of the LLVM source directory.

2. Add the add_subdirectory(LLVMExperimentPasses) line into lib/Transforms/CMakeLists.txt

3. For each implemented pass, add a function named initialize${THE_NAME_OF_THE_PASS}Pass(PassRegistry &); into the include/llvm/InitializePasses.h header file. Also add the funtion void initializeLLVMExperimentPasses(PassRegistry &); there. For example, for the FunctionArgumentCount pass add the following lines:

// my experiment passes
void initializeLLVMExperimentPasses(PassRegistry &);
void initializeFunctionArgumentCountPass(PassRegistry &);
} // end namespace llvm

(the functions must be defined inside the llvm namespace).

4. Add the LLVMExperimentPasses library to the LLVM_LINK_COMPONENTS list in the tools/opt/CMakeLists.txt file:

set(LLVM_LINK_COMPONENTS
    ${LLVM_TARGETS_TO_BUILD}
    AggressiveInstCombine
    ...
    ExperimentPasses
)

Note: The form of ExperimentPasses, not LLVMExperimentPasses is used here.

5. Register the passes into the opt tool by adding an invocation of the initializeLLVMExperimentPasses function to the main method of the tool (file tools/opt/opt.cpp):

// Initialize passes
PassRegistry &Registry = *PassRegistry::getPassRegistry();
initializeCore(Registry);
...
initializeLLVMExperimentPasses(Registry);
// For codegen passes, only passes that do IR to IR transformation are
// supported.

6. Rebuild LLVM (YOUR_LLVM_BUILDTREE is the directory where you build LLVM) and install the new output files:

cd YOUR_LLVM_BUILDTREE

cmake -DCMAKE_CXX_COMPILER=YOUR_FAVOURITE_COMPILER \
-DCMAKE_C_COMPILER=YOUR_FAVOURITE_COMPILER \
-DCMAKE_LINKER=YOUR_FAVOURITE_LINKER .. -G"Ninja"

cmake --build .

cmake --build . --target install

The passes are ready. For instance, the FunctionArgumentCount pass is registered as fnargcnt in the opt tool and can be invoked using the following command line:

opt.exe -fnargcnt < sum.bc > sum-out.bc

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