How to Implement & Run LLVM Passes

---

Based on LLVM 20.1.4

Abstract

• Legacy Version of Pass
  • C++ class that inherits the Pass class in LLVM
    • Implement functionality by overriding virtual methods
    • e.g., runOnModule or runOnFunction
• New Version of Pass
  • C++ class that inherits the PassInfoMixIn struct in llvm/IR/PassManager.h
  • Implement PreservedAnalyses run (Module &M, ModuleAnalysisManager &AM);

Skeleton Code

1. Define the Module Pass in a Header File

#include "llvm/IR/Module.h"  
#include "llvm/IR/PassManager.h"  

struct HelloModulePass : llvm::PassInfoMixin<HelloModulePass> {  
    static llvm::PreservedAnalyses run(
	    const llvm::Module &M, 
	    llvm::ModuleAnalysisManager &MAM);  
};  

2. Write the source for the module pass

llvm::PreservedAnalyses HelloModulePass::run(const llvm::Module &M, llvm::ModuleAnalysisManager &MAM) {  
    /** Some Code */
    return llvm::PreservedAnalyses::all();  /**If Editing IR, return PreservedAnalyses::None() */
}  

3. Register Pass

extern "C" llvm::PassPluginLibraryInfo llvmGetPassPluginInfo() {  
    return {  
        LLVM_PLUGIN_API_VERSION,  
        "HelloPass",  
        LLVM_VERSION_STRING,  
        [](llvm::PassBuilder &PB) {  
            PB.registerPipelineParsingCallback(  
                [](const llvm::StringRef name, llvm::ModulePassManager &MPM,  
                   llvm::ArrayRef<llvm::PassBuilder::PipelineElement>) {  
                    if (name == "HelloModulePass") {  
                        MPM.addPass(HelloModulePass());  
                        return true;  
                    }  
                    return false;  
                }  
            );  
        }  
    };  
}

Compile and Run Pass

1. Compile a pass and make it a shared-library

clang++ -fPIC -fno-rtti -shared \                                   
  HelloModule.cpp \
  `llvm-config --cxxflags --ldflags --libs core support passes` \
  -o [OUTPUT.so]

2. Run the LLVM pass using opt

opt -load-pass-plugin [OUTPUT.so] -passes=HelloModulePass [GENERATED_BITCODE] -o [OUTPUT]

References

• Writing An LLVM New PM Pass: LLVM.org