Running standard optimization passes on a LLVM module

Question

Say I have a valid LLVM module:

std::unique_ptr<llvm::Module> module;

I want to run LLVM traditional optimization passes on it:

llvm::PassBuilder passBuilder;
llvm::ModulePassManager modulePassManager = passBuilder.buildPerModuleDefaultPipeline(llvm::PassBuilder::OptimizationLevel::O3);
llvm::ModuleAnalysisManager moduleAnalysisManager;
passBuilder.registerModuleAnalyses(moduleAnalysisManager);
modulePassManager.run(*module, moduleAnalysisManager);

Unfortunately, the call crashes and debugging shows that the moduleAnalysisManager has only the module passes, but not the function ones that are wrapped with the proxy class.

How should I setup modulePassManager to handle all (module) passes of a specific level? I don't have individual functions, so I can't run the function passes just on them.

Answer 1

The proper LLVM way is to create all analysisManagers and then link all of them together. Let's start by creating them:

llvm::PassBuilder passBuilder;
llvm::LoopAnalysisManager loopAnalysisManager(true); // true is just to output debug info
llvm::FunctionAnalysisManager functionAnalysisManager(true);
llvm::CGSCCAnalysisManager cGSCCAnalysisManager(true);
llvm::ModuleAnalysisManager moduleAnalysisManager(true);

Then we register each manager individually, and then we cross register them. This means that the number of managers here is fixed by design and if LLVM (7 at this time) changes the number of managers, this will need to be adapted:

passBuilder.registerModuleAnalyses(moduleAnalysisManager);
passBuilder.registerCGSCCAnalyses(cGSCCAnalysisManager);
passBuilder.registerFunctionAnalyses(functionAnalysisManager);
passBuilder.registerLoopAnalyses(loopAnalysisManager);
// This is the important line:
passBuilder.crossRegisterProxies(
    loopAnalysisManager, functionAnalysisManager, cGSCCAnalysisManager, moduleAnalysisManager);

Once the passBuilder is created, we can finally make the optimization passes for the module with a call to the moduleAnalysisManager.

llvm::ModulePassManager modulePassManager =
    passBuilder.buildPerModuleDefaultPipeline(llvm::PassBuilder::OptimizationLevel::O3);
modulePassManager.run(*module, moduleAnalysisManager);

This will run module level passes as well as all inner passes that LLVM can run on pieces of the module (function level, loop level...).