How to automatically register and load modern Pass in Clang?

Question

I'm trying to write a simple "modern" LLVM pass and use it with Clang. I want it to be able to be run with a command like: clang -Xclang -load -Xclang libMyPass.so file.cpp

There are a lot of manuals on how to integrate legacy pass into Clang. However, there is not so much info on the new pass manager. I came across a series of articles called 'Writing LLVM Pass in 2018'. But it only mentions a case when your pass code is placed inside the LLVM code tree. And I need the module to be built out-of-tree.

class MyPass : public llvm::PassInfoMixin<MyPass> {
public:
    llvm::PreservedAnalyses run(
        llvm::Function &F,
        llvm::FunctionAnalysisManager &FAM
    ) {
    // Pass code here
    }
};

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

At the moment, the pass is not being executed. I tried to look at -print-after-all option output and using std::cout to detect whether it has run. I cannot see my pass in logs. Neither I can see my debug output in the console window.

Answer 1

It's possible now.

Sample plugin registration code:

PassPluginLibraryInfo getPassPluginInfo() {
    const auto callback = [](PassBuilder &PB) {
        PB.registerPipelineEarlySimplificationEPCallback([&](ModulePassManager &MPM, auto) {
            MPM.addPass(MyPass());
            return true;
        });
    };

    return {LLVM_PLUGIN_API_VERSION, "name", "0.0.1", callback};
};

extern "C" LLVM_ATTRIBUTE_WEAK PassPluginLibraryInfo llvmGetPassPluginInfo() {
    return getPassPluginInfo();
}

Sample clang invocation code (assuming you built your plugin as a shared library):

clang++ -O0 -g -fpass-plugin=pass.so ...