Step 04 · The new pass manager

LLVM has two pass managers: the legacy llvm::PassManager and the "new" one rooted at llvm::PassBuilder. New code targets the new PM.

llvm::LoopAnalysisManager     lam;
llvm::FunctionAnalysisManager fam;
llvm::CGSCCAnalysisManager    cam;
llvm::ModuleAnalysisManager   mam;

llvm::PassBuilder pb;
pb.registerModuleAnalyses(mam);
pb.registerCGSCCAnalyses(cam);
pb.registerFunctionAnalyses(fam);
pb.registerLoopAnalyses(lam);
pb.crossRegisterProxies(lam, fam, cam, mam);

auto mpm = pb.buildPerModuleDefaultPipeline(llvm::OptimizationLevel::O2);
mpm.run(mod, mam);

Anatomy

  • Analysis managers cache analyses (dominator tree, loop info, alias analysis, …) per IR unit. They must be registered with each other so a function pass can ask for a module-level analysis.
  • PassBuilder assembles a ModulePassManager corresponding to one of the standard -O0/-O1/-O2/-O3/-Os/-Oz pipelines.
  • mpm.run(mod, mam) mutates the module in place.

Custom pipelines

You can build your own with addPass(MyPass()). We rely on the canned O2 pipeline so we inherit decades of tuning.

Where this lives

llvm_codegen.cpp → optimise(Module&). mlcc calls it when given -O. Tests pass runOpt = true to exercise specific transformations.