llvm-passes

LLVM Passes

Purpose

Guide agents through writing LLVM optimization passes with the New Pass Manager: FunctionPass and ModulePass structure, PassPluginLibraryInfo registration, running via opt -load-pass-plugin, common analysis utilities (DominatorTree, LoopInfo, AliasAnalysis), IR modification patterns, llvm-lit testing, and debugging with opt -print-after-all.

When to Use

• Adding a custom optimization to an LLVM-based compiler
• Writing an IR transformation pass (inlining, DCE, custom lowering)
• Analyzing control flow with dominator trees or loop info
• Testing passes with FileCheck and llvm-lit
• Debugging pass ordering and IR corruption
• Integrating passes into Clang via plugin

Workflow

1. New Pass Manager architecture

opt / clang
├── ModulePassManager
│   └── FunctionPassManager (per function)
│       └── FunctionPass instances
└── AnalysisManager (cached analyses)

Passes declare analysis usage; analyses are invalidated on IR mutation.

2. Minimal FunctionPass (C++ plugin)

// MyPass.cpp
#include "llvm/IR/PassManager.h"
#include "llvm/Passes/PassBuilder.h"
#include "llvm/Passes/PassPlugin.h"
#include "llvm/Support/raw_ostream.h"

using namespace llvm;

namespace {

struct MyPass : PassInfoMixin<MyPass> {
    PreservedAnalyses run(Function &F, FunctionAnalysisManager &AM) {
        bool changed = false;
        for (BasicBlock &BB : F) {
            for (Instruction &I : BB) {
                if (auto *Call = dyn_cast<CallInst>(&I)) {
                    if (Call->getCalledFunction() &&
                        Call->getCalledFunction()->getName() == "dead_func") {
                        Call->eraseFromParent();
                        changed = true;
                    }
                }
            }
        }
        return changed ? PreservedAnalyses::none() : PreservedAnalyses::all();
    }
};

} // namespace

extern "C" LLVM_ATTRIBUTE_WEAK PassPluginLibraryInfo llvmGetPassPluginInfo() {
    return {
        LLVM_PLUGIN_API_VERSION, "MyPass", "v0.1",
        [](PassBuilder &PB) {
            PB.registerPipelineParsingCallback(
                [](StringRef Name, FunctionPassManager &FPM,
                   ArrayRef<PassBuilder::PipelineElement>) {
                    if (Name == "my-pass") {
                        FPM.addPass(MyPass());
                        return true;
                    }
                    return false;
                });
        }
    };
}