//===- HexagonCFGOptimizer.cpp - CFG optimizations ------------------------===//

//

// 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

//

//===----------------------------------------------------------------------===//

#include "Hexagon.h"

#include "MCTargetDesc/HexagonMCTargetDesc.h"

#include "llvm/CodeGen/MachineBasicBlock.h"

#include "llvm/CodeGen/MachineBranchProbabilityInfo.h"

#include "llvm/CodeGen/MachineFunction.h"

#include "llvm/CodeGen/MachineFunctionPass.h"

#include "llvm/CodeGen/MachineInstr.h"

#include "llvm/CodeGen/MachineOperand.h"

#include "llvm/CodeGen/TargetInstrInfo.h"

#include "llvm/CodeGen/TargetSubtargetInfo.h"

#include "llvm/Pass.h"

#include "llvm/Support/ErrorHandling.h"

#include <cassert>

#include <vector>

using namespace llvm;

#define DEBUG_TYPE "hexagon_cfg"

namespace llvm {

FunctionPass *createHexagonCFGOptimizer();

void initializeHexagonCFGOptimizerPass(PassRegistry&);

} // end namespace llvm

namespace {

class HexagonCFGOptimizer : public MachineFunctionPass {

private:

  void InvertAndChangeJumpTarget(MachineInstr &, MachineBasicBlock *);

  bool isOnFallThroughPath(MachineBasicBlock *MBB);

public:

  static char ID;

  HexagonCFGOptimizer() : MachineFunctionPass(ID) {

    initializeHexagonCFGOptimizerPass(*PassRegistry::getPassRegistry());

  }

  StringRef getPassName() const override { return "Hexagon CFG Optimizer"; }

  bool runOnMachineFunction(MachineFunction &Fn) override;

  MachineFunctionProperties getRequiredProperties() const override {

    return MachineFunctionProperties().set(

        MachineFunctionProperties::Property::NoVRegs);

  }

};

} // end anonymous namespace

char HexagonCFGOptimizer::ID = 0;

static bool IsConditionalBranch(int Opc) {

  switch (Opc) {

    case Hexagon::J2_jumpt:

    case Hexagon::J2_jumptpt:

    case Hexagon::J2_jumpf:

    case Hexagon::J2_jumpfpt:

    case Hexagon::J2_jumptnew:

    case Hexagon::J2_jumpfnew:

    case Hexagon::J2_jumptnewpt:

    case Hexagon::J2_jumpfnewpt:

      return true;

  }

  return false;

}

static bool IsUnconditionalJump(int Opc) {

  return (Opc == Hexagon::J2_jump);

}

void HexagonCFGOptimizer::InvertAndChangeJumpTarget(

    MachineInstr &MI, MachineBasicBlock *NewTarget) {

  const TargetInstrInfo *TII =

      MI.getParent()->getParent()->getSubtarget().getInstrInfo();

  int NewOpcode = 0;

  switch (MI.getOpcode()) {

  case Hexagon::J2_jumpt:

    NewOpcode = Hexagon::J2_jumpf;

    break;

  case Hexagon::J2_jumpf:

    NewOpcode = Hexagon::J2_jumpt;

    break;

  case Hexagon::J2_jumptnewpt:

    NewOpcode = Hexagon::J2_jumpfnewpt;

    break;

  case Hexagon::J2_jumpfnewpt:

    NewOpcode = Hexagon::J2_jumptnewpt;

    break;

  default:

    llvm_unreachable("Cannot handle this case");

  }

  MI.setDesc(TII->get(NewOpcode));

  MI.getOperand(1).setMBB(NewTarget);

}

bool HexagonCFGOptimizer::isOnFallThroughPath(MachineBasicBlock *MBB) {

  if (MBB->canFallThrough())

    return true;

  for (MachineBasicBlock *PB : MBB->predecessors())

    if (PB->isLayoutSuccessor(MBB) && PB->canFallThrough())

      return true;

  return false;

}

bool HexagonCFGOptimizer::runOnMachineFunction(MachineFunction &Fn) {

  if (skipFunction(Fn.getFunction()))

    return false;

  // Loop over all of the basic blocks.

  for (MachineBasicBlock &MBB : Fn) {

    // Traverse the basic block.

    MachineBasicBlock::iterator MII = MBB.getFirstTerminator();

    if (MII != MBB.end()) {

      MachineInstr &MI = *MII;

      int Opc = MI.getOpcode();

      if (IsConditionalBranch(Opc)) {

        // (Case 1) Transform the code if the following condition occurs:

        //   BB1: if (p0) jump BB3

        //   ...falls-through to BB2 ...

        //   BB2: jump BB4

        //   ...next block in layout is BB3...

        //   BB3: ...

        //

        //  Transform this to:

        //  BB1: if (!p0) jump BB4

        //  Remove BB2

        //  BB3: ...

        //

        // (Case 2) A variation occurs when BB3 contains a JMP to BB4:

        //   BB1: if (p0) jump BB3

        //   ...falls-through to BB2 ...

        //   BB2: jump BB4

        //   ...other basic blocks ...

        //   BB4:

        //   ...not a fall-thru

        //   BB3: ...

        //     jump BB4

        //

        // Transform this to:

        //   BB1: if (!p0) jump BB4

        //   Remove BB2

        //   BB3: ...

        //   BB4: ...

        unsigned NumSuccs = MBB.succ_size();

        MachineBasicBlock::succ_iterator SI = MBB.succ_begin();

        MachineBasicBlock* FirstSucc = *SI;

        MachineBasicBlock* SecondSucc = *(++SI);

        MachineBasicBlock* LayoutSucc = nullptr;

        MachineBasicBlock* JumpAroundTarget = nullptr;

        if (MBB.isLayoutSuccessor(FirstSucc)) {

          LayoutSucc = FirstSucc;

          JumpAroundTarget = SecondSucc;

        } else if (MBB.isLayoutSuccessor(SecondSucc)) {

          LayoutSucc = SecondSucc;

          JumpAroundTarget = FirstSucc;

        } else {

          // Odd case...cannot handle.

        }

        // The target of the unconditional branch must be JumpAroundTarget.

        // TODO: If not, we should not invert the unconditional branch.

        MachineBasicBlock* CondBranchTarget = nullptr;

        if (MI.getOpcode() == Hexagon::J2_jumpt ||

            MI.getOpcode() == Hexagon::J2_jumpf) {

          CondBranchTarget = MI.getOperand(1).getMBB();

        }

        if (!LayoutSucc || (CondBranchTarget != JumpAroundTarget)) {

          continue;

        }

        if ((NumSuccs == 2) && LayoutSucc && (LayoutSucc->pred_size() == 1)) {

          // Ensure that BB2 has one instruction -- an unconditional jump.

          if ((LayoutSucc->size() == 1) &&

              IsUnconditionalJump(LayoutSucc->front().getOpcode())) {

            assert(JumpAroundTarget && "jump target is needed to process second basic block");

            MachineBasicBlock* UncondTarget =

              LayoutSucc->front().getOperand(0).getMBB();

            // Check if the layout successor of BB2 is BB3.

            bool case1 = LayoutSucc->isLayoutSuccessor(JumpAroundTarget);

            bool case2 = JumpAroundTarget->isSuccessor(UncondTarget) &&

              !JumpAroundTarget->empty() &&

              IsUnconditionalJump(JumpAroundTarget->back().getOpcode()) &&

              JumpAroundTarget->pred_size() == 1 &&

              JumpAroundTarget->succ_size() == 1;

            if (case1 || case2) {

              InvertAndChangeJumpTarget(MI, UncondTarget);

              MBB.replaceSuccessor(JumpAroundTarget, UncondTarget);

              // Remove the unconditional branch in LayoutSucc.

              LayoutSucc->erase(LayoutSucc->begin());

              LayoutSucc->replaceSuccessor(UncondTarget, JumpAroundTarget);

              // This code performs the conversion for case 2, which moves

              // the block to the fall-thru case (BB3 in the code above).

              if (case2 && !case1) {

                JumpAroundTarget->moveAfter(LayoutSucc);

                // only move a block if it doesn't have a fall-thru. otherwise

                // the CFG will be incorrect.

                if (!isOnFallThroughPath(UncondTarget))

                  UncondTarget->moveAfter(JumpAroundTarget);

              }

              // Correct live-in information. Is used by post-RA scheduler

              // The live-in to LayoutSucc is now all values live-in to

              // JumpAroundTarget.

              std::vector<MachineBasicBlock::RegisterMaskPair> OrigLiveIn(

                  LayoutSucc->livein_begin(), LayoutSucc->livein_end());

              std::vector<MachineBasicBlock::RegisterMaskPair> NewLiveIn(

                  JumpAroundTarget->livein_begin(),

                  JumpAroundTarget->livein_end());

              for (const auto &OrigLI : OrigLiveIn)

                LayoutSucc->removeLiveIn(OrigLI.PhysReg);

              for (const auto &NewLI : NewLiveIn)

                LayoutSucc->addLiveIn(NewLI);

            }

          }

        }

      }

    }

  }

  return true;

}

//===----------------------------------------------------------------------===//

//                         Public Constructor Functions

//===----------------------------------------------------------------------===//

INITIALIZE_PASS(HexagonCFGOptimizer, "hexagon-cfg", "Hexagon CFG Optimizer",

                false, false)

FunctionPass *llvm::createHexagonCFGOptimizer() {

  return new HexagonCFGOptimizer();

}