DeadMachineInstructionElim.cpp

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//===- DeadMachineInstructionElim.cpp - Remove dead machine instructions --===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
//
// This is an extremely simple MachineInstr-level dead-code-elimination pass.
//
//===----------------------------------------------------------------------===//
 
#include "llvm/ADT/PostOrderIterator.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/CodeGen/LiveRegUnits.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/CodeGen/MachineRegisterInfo.h"
#include "llvm/CodeGen/TargetSubtargetInfo.h"
#include "llvm/InitializePasses.h"
#include "llvm/Pass.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
 
using namespace llvm;
 
#define DEBUG_TYPE "dead-mi-elimination"
 
STATISTIC(NumDeletes,          "Number of dead instructions deleted");
 
namespace {
  class DeadMachineInstructionElim : public MachineFunctionPass {
    bool runOnMachineFunction(MachineFunction &MF) override;
 
    const MachineRegisterInfo *MRI = nullptr;
    const TargetInstrInfo *TII = nullptr;
    LiveRegUnits LivePhysRegs;
 
  public:
    static char ID; // Pass identification, replacement for typeid
    DeadMachineInstructionElim() : MachineFunctionPass(ID) {
     initializeDeadMachineInstructionElimPass(*PassRegistry::getPassRegistry());
    }
 
    void getAnalysisUsage(AnalysisUsage &AU) const override {
      AU.setPreservesCFG();
      MachineFunctionPass::getAnalysisUsage(AU);
    }
 
  private:
    bool isDead(const MachineInstr *MI) const;
 
    bool eliminateDeadMI(MachineFunction &MF);
  };
}
char DeadMachineInstructionElim::ID = 0;
char &llvm::DeadMachineInstructionElimID = DeadMachineInstructionElim::ID;
 
INITIALIZE_PASS(DeadMachineInstructionElim, DEBUG_TYPE,
                "Remove dead machine instructions", false, false)
 
bool DeadMachineInstructionElim::isDead(const MachineInstr *MI) const {
  // Technically speaking inline asm without side effects and no defs can still
  // be deleted. But there is so much bad inline asm code out there, we should
  // let them be.
  if (MI->isInlineAsm())
    return false;
 
  // Don't delete frame allocation labels.
  if (MI->getOpcode() == TargetOpcode::LOCAL_ESCAPE)
    return false;
 
  // Don't delete instructions with side effects.
  bool SawStore = false;
  if (!MI->isSafeToMove(nullptr, SawStore) && !MI->isPHI())
    return false;
 
  // Examine each operand.
  for (const MachineOperand &MO : MI->all_defs()) {
    Register Reg = MO.getReg();
    if (Reg.isPhysical()) {
      // Don't delete live physreg defs, or any reserved register defs.
      if (!LivePhysRegs.available(Reg) || MRI->isReserved(Reg))
        return false;
    } else {
      if (MO.isDead()) {
#ifndef NDEBUG
        // Basic check on the register. All of them should be 'undef'.
        for (auto &U : MRI->use_nodbg_operands(Reg))
          assert(U.isUndef() && "'Undef' use on a 'dead' register is found!");
#endif
        continue;
      }
      for (const MachineInstr &Use : MRI->use_nodbg_instructions(Reg)) {
        if (&Use != MI)
          // This def has a non-debug use. Don't delete the instruction!
          return false;
      }
    }
  }
 
  // If there are no defs with uses, the instruction is dead.
  return true;
}
 
bool DeadMachineInstructionElim::runOnMachineFunction(MachineFunction &MF) {
  if (skipFunction(MF.getFunction()))
    return false;
 
  MRI = &MF.getRegInfo();
 
  const TargetSubtargetInfo &ST = MF.getSubtarget();
  TII = ST.getInstrInfo();
  LivePhysRegs.init(*ST.getRegisterInfo());
 
  bool AnyChanges = eliminateDeadMI(MF);
  while (AnyChanges && eliminateDeadMI(MF))
    ;
  return AnyChanges;
}
 
bool DeadMachineInstructionElim::eliminateDeadMI(MachineFunction &MF) {
  bool AnyChanges = false;
 
  // Loop over all instructions in all blocks, from bottom to top, so that it's
  // more likely that chains of dependent but ultimately dead instructions will
  // be cleaned up.
  for (MachineBasicBlock *MBB : post_order(&MF)) {
    LivePhysRegs.addLiveOuts(*MBB);
 
    // Now scan the instructions and delete dead ones, tracking physreg
    // liveness as we go.
    for (MachineInstr &MI : make_early_inc_range(reverse(*MBB))) {
      // If the instruction is dead, delete it!
      if (isDead(&MI)) {
        LLVM_DEBUG(dbgs() << "DeadMachineInstructionElim: DELETING: " << MI);
        // It is possible that some DBG_VALUE instructions refer to this
        // instruction. They will be deleted in the live debug variable
        // analysis.
        MI.eraseFromParent();
        AnyChanges = true;
        ++NumDeletes;
        continue;
      }
 
      LivePhysRegs.stepBackward(MI);
    }
  }
 
  LivePhysRegs.clear();
  return AnyChanges;
}