MachineFunctionPass

/// MachineFunctionPass - This class adapts the FunctionPass interface to
/// allow convenient creation of passes that operate on the MachineFunction
/// representation. Instead of overriding runOnFunction, subclasses
/// override runOnMachineFunction.
class MachineFunctionPass : public FunctionPass {
public:
  bool doInitialization(Module&) override {
    // Cache the properties info at module-init time so we don't have to
    // construct them for every function.
    RequiredProperties = getRequiredProperties();
    SetProperties = getSetProperties();
    ClearedProperties = getClearedProperties();
    return false;
  }
  ...
protected:
  /// runOnMachineFunction - This method must be overloaded to perform the
  /// desired machine code transformation or analysis.
  ///
  virtual bool runOnMachineFunction(MachineFunction &MF) = 0;

private:

  /// createPrinterPass - Get a machine function printer pass.
  Pass *createPrinterPass(raw_ostream &O,
                          const std::string &Banner) const override;

  bool runOnFunction(Function &F) override;

};

bool MachineFunctionPass::runOnFunction(Function &F) {
  // Do not codegen any 'available_externally' functions at all, they have
  // definitions outside the translation unit.
  if (F.hasAvailableExternallyLinkage())
    return false;

  MachineModuleInfo &MMI = getAnalysis<MachineModuleInfoWrapperPass>().getMMI();
  MachineFunction &MF = MMI.getOrCreateMachineFunction(F);

  MachineFunctionProperties &MFProps = MF.getProperties();

#ifndef NDEBUG
  if (!MFProps.verifyRequiredProperties(RequiredProperties)) {
    errs() << "MachineFunctionProperties required by " << getPassName()
           << " pass are not met by function " << F.getName() << ".\n"
           << "Required properties: ";
    RequiredProperties.print(errs());
    errs() << "\nCurrent properties: ";
    MFProps.print(errs());
    errs() << "\n";
    llvm_unreachable("MachineFunctionProperties check failed");
  }
#endif
  // Collect the MI count of the function before the pass.
  unsigned CountBefore, CountAfter;

  // Check if the user asked for size remarks.
  bool ShouldEmitSizeRemarks =
      F.getParent()->shouldEmitInstrCountChangedRemark();

  // If we want size remarks, collect the number of MachineInstrs in our
  // MachineFunction before the pass runs.
  if (ShouldEmitSizeRemarks)
    CountBefore = MF.getInstructionCount();

  bool RV = runOnMachineFunction(MF);

  if (ShouldEmitSizeRemarks) {
    // We wanted size remarks. Check if there was a change to the number of
    // MachineInstrs in the module. Emit a remark if there was a change.
    CountAfter = MF.getInstructionCount();
    if (CountBefore != CountAfter) {
      MachineOptimizationRemarkEmitter MORE(MF, nullptr);
      MORE.emit([&]() {
        int64_t Delta = static_cast<int64_t>(CountAfter) -
                        static_cast<int64_t>(CountBefore);
        MachineOptimizationRemarkAnalysis R("size-info", "FunctionMISizeChange",
                                            MF.getFunction().getSubprogram(),
                                            &MF.front());
        R << NV("Pass", getPassName())
          << ": Function: " << NV("Function", F.getName()) << ": "
          << "MI Instruction count changed from "
          << NV("MIInstrsBefore", CountBefore) << " to "
          << NV("MIInstrsAfter", CountAfter)
          << "; Delta: " << NV("Delta", Delta);
        return R;
      });
    }
  }

  MFProps.set(SetProperties);
  MFProps.reset(ClearedProperties);
  return RV;
}