llvm/lib/Target/PowerPC/PPCPreEmitPeephole.cpp

//===--------- PPCPreEmitPeephole.cpp - Late peephole optimizations -------===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// A pre-emit peephole for catching opportunities introduced by late passes such
// as MachineBlockPlacement.
//
//===----------------------------------------------------------------------===//

#include "PPC.h"
#include "PPCInstrInfo.h"
#include "PPCSubtarget.h"
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/CodeGen/LivePhysRegs.h"
#include "llvm/CodeGen/MachineBasicBlock.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/CodeGen/MachineInstrBuilder.h"
#include "llvm/CodeGen/MachineRegisterInfo.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Support/Debug.h"

using namespace llvm;

#define DEBUG_TYPE "ppc-pre-emit-peephole"

STATISTIC(NumRRConvertedInPreEmit,
          "Number of r+r instructions converted to r+i in pre-emit peephole");
STATISTIC(NumRemovedInPreEmit,
          "Number of instructions deleted in pre-emit peephole");

static cl::opt<bool>
RunPreEmitPeephole("ppc-late-peephole", cl::Hidden, cl::init(true),
                   cl::desc("Run pre-emit peephole optimizations."));

namespace {
  class PPCPreEmitPeephole : public MachineFunctionPass {
  public:
    static char ID;
    PPCPreEmitPeephole() : MachineFunctionPass(ID) {
      initializePPCPreEmitPeepholePass(*PassRegistry::getPassRegistry());
    }

    void getAnalysisUsage(AnalysisUsage &AU) const override {
      MachineFunctionPass::getAnalysisUsage(AU);
    }

    MachineFunctionProperties getRequiredProperties() const override {
      return MachineFunctionProperties().set(
          MachineFunctionProperties::Property::NoVRegs);
    }

    bool runOnMachineFunction(MachineFunction &MF) override {
      if (skipFunction(MF.getFunction()) || !RunPreEmitPeephole)
        return false;
      bool Changed = false;
      const PPCInstrInfo *TII = MF.getSubtarget<PPCSubtarget>().getInstrInfo();
      const TargetRegisterInfo *TRI = MF.getSubtarget().getRegisterInfo();
      SmallVector<MachineInstr *, 4> InstrsToErase;
      for (MachineBasicBlock &MBB : MF) {
        for (MachineInstr &MI : MBB) {
          MachineInstr *DefMIToErase = nullptr;
          if (TII->convertToImmediateForm(MI, &DefMIToErase)) {
            Changed = true;
            NumRRConvertedInPreEmit++;
            LLVM_DEBUG(dbgs() << "Converted instruction to imm form: ");
            LLVM_DEBUG(MI.dump());
            if (DefMIToErase) {
              InstrsToErase.push_back(DefMIToErase);
            }
          }
        }

        // Eliminate conditional branch based on a constant CR bit by
        // CRSET or CRUNSET. We eliminate the conditional branch or
        // convert it into an unconditional branch. Also, if the CR bit
        // is not used by other instructions, we eliminate CRSET as well.
        auto I = MBB.getFirstInstrTerminator();
        if (I == MBB.instr_end())
          continue;
        MachineInstr *Br = &*I;
        if (Br->getOpcode() != PPC::BC && Br->getOpcode() != PPC::BCn)
          continue;
        MachineInstr *CRSetMI = nullptr;
        unsigned CRBit = Br->getOperand(0).getReg();
        unsigned CRReg = getCRFromCRBit(CRBit);
        bool SeenUse = false;
        MachineBasicBlock::reverse_iterator It = Br, Er = MBB.rend();
        for (It++; It != Er; It++) {
          if (It->modifiesRegister(CRBit, TRI)) {
            if ((It->getOpcode() == PPC::CRUNSET ||
                 It->getOpcode() == PPC::CRSET) &&
                It->getOperand(0).getReg() == CRBit)
              CRSetMI = &*It;
            break;
          }
          if (It->readsRegister(CRBit, TRI))
            SeenUse = true;
        }
        if (!CRSetMI) continue;

        unsigned CRSetOp = CRSetMI->getOpcode();
        if ((Br->getOpcode() == PPC::BCn && CRSetOp == PPC::CRSET) ||
            (Br->getOpcode() == PPC::BC  && CRSetOp == PPC::CRUNSET)) {
          // Remove this branch since it cannot be taken.
          InstrsToErase.push_back(Br);
          MBB.removeSuccessor(Br->getOperand(1).getMBB());
        }
        else {
          // This conditional branch is always taken. So, remove all branches
          // and insert an unconditional branch to the destination of this.
          MachineBasicBlock::iterator It = Br, Er = MBB.end();
          for (; It != Er && !SeenUse; It++) {
            if (It->isDebugInstr()) continue;
            assert(It->isTerminator() && "Non-terminator after a terminator");
            InstrsToErase.push_back(&*It);
          }
          if (!MBB.isLayoutSuccessor(Br->getOperand(1).getMBB())) {
            ArrayRef<MachineOperand> NoCond;
            TII->insertBranch(MBB, Br->getOperand(1).getMBB(), nullptr,
                              NoCond, Br->getDebugLoc());
          }
          for (auto &Succ : MBB.successors())
            if (Succ != Br->getOperand(1).getMBB()) {
              MBB.removeSuccessor(Succ);
              break;
            }
        }

        // If the CRBit is not used by another instruction, we can eliminate
        // CRSET/CRUNSET instruction.
        if (!SeenUse) {
          // We need to check use of the CRBit in successors.
          for (auto &SuccMBB : MBB.successors())
            if (SuccMBB->isLiveIn(CRBit) || SuccMBB->isLiveIn(CRReg)) {
              SeenUse = true;
              break;
            }
          if (!SeenUse)
            InstrsToErase.push_back(CRSetMI);
        }
      }
      for (MachineInstr *MI : InstrsToErase) {
        LLVM_DEBUG(dbgs() << "PPC pre-emit peephole: erasing instruction: ");
        LLVM_DEBUG(MI->dump());
        MI->eraseFromParent();
        NumRemovedInPreEmit++;
      }
      return Changed;
    }
  };
}

INITIALIZE_PASS(PPCPreEmitPeephole, DEBUG_TYPE, "PowerPC Pre-Emit Peephole",
                false, false)
char PPCPreEmitPeephole::ID = 0;

FunctionPass *llvm::createPPCPreEmitPeepholePass() {
  return new PPCPreEmitPeephole();
}