VirtRegRewriter.cpp

      // Check to see if this stack slot is available.
      unsigned PhysReg = Spills.getSpillSlotOrReMatPhysReg(SSorRMId);

      // If this is a sub-register use, make sure the reuse register is in the
      // right register class. For example, for x86 not all of the 32-bit
      // registers have accessible sub-registers.
      // Similarly so for EXTRACT_SUBREG. Consider this:
      // EDI = op
      // MOV32_mr fi#1, EDI
      // ...
      //       = EXTRACT_SUBREG fi#1
      // fi#1 is available in EDI, but it cannot be reused because it's not in
      // the right register file.
      if (PhysReg && !AvoidReload && SubIdx) {
        const TargetRegisterClass* RC = MRI->getRegClass(VirtReg);
        if (!RC->contains(PhysReg))
          PhysReg = 0;
      }

      if (PhysReg && !AvoidReload) {
        // This spilled operand might be part of a two-address operand.  If this
        // is the case, then changing it will necessarily require changing the
        // def part of the instruction as well.  However, in some cases, we
        // aren't allowed to modify the reused register.  If none of these cases
        // apply, reuse it.
        bool CanReuse = true;
        bool isTied = MI.isRegTiedToDefOperand(i);
        if (isTied) {
          // Okay, we have a two address operand.  We can reuse this physreg as
          // long as we are allowed to clobber the value and there isn't an
          // earlier def that has already clobbered the physreg.
          CanReuse = !ReusedOperands.isClobbered(PhysReg) &&
            Spills.canClobberPhysReg(PhysReg);
        }
        // If this is an asm, and PhysReg is used elsewhere as an earlyclobber
        // operand, we can't also use it as an input.  (Outputs always come
        // before inputs, so we can stop looking at i.)
        if (MI.isInlineAsm()) {
          for (unsigned k=0; k<i; ++k) {
            MachineOperand &MOk = MI.getOperand(k);
            if (MOk.isReg() && MOk.getReg()==PhysReg && MOk.isEarlyClobber()) {
              CanReuse = false;
              break;
            }
          }
        }

        if (CanReuse) {
          // If this stack slot value is already available, reuse it!
          if (ReuseSlot > VirtRegMap::MAX_STACK_SLOT)
            DEBUG(dbgs() << "Reusing RM#"
                  << ReuseSlot-VirtRegMap::MAX_STACK_SLOT-1);
          else
            DEBUG(dbgs() << "Reusing SS#" << ReuseSlot);
          DEBUG(dbgs() << " from physreg "
                << TRI->getName(PhysReg) << " for vreg"
                << VirtReg <<" instead of reloading into physreg "
                << TRI->getName(VRM->getPhys(VirtReg)) << '\n');
          unsigned RReg = SubIdx ? TRI->getSubReg(PhysReg, SubIdx) : PhysReg;
          MI.getOperand(i).setReg(RReg);
          MI.getOperand(i).setSubReg(0);

          // The only technical detail we have is that we don't know that
          // PhysReg won't be clobbered by a reloaded stack slot that occurs
          // later in the instruction.  In particular, consider 'op V1, V2'.
          // If V1 is available in physreg R0, we would choose to reuse it
          // here, instead of reloading it into the register the allocator
          // indicated (say R1).  However, V2 might have to be reloaded
          // later, and it might indicate that it needs to live in R0.  When
          // this occurs, we need to have information available that
          // indicates it is safe to use R1 for the reload instead of R0.
          //
          // To further complicate matters, we might conflict with an alias,
          // or R0 and R1 might not be compatible with each other.  In this
          // case, we actually insert a reload for V1 in R1, ensuring that
          // we can get at R0 or its alias.
          ReusedOperands.addReuse(i, ReuseSlot, PhysReg,
                                  VRM->getPhys(VirtReg), VirtReg);
          if (isTied)
            // Only mark it clobbered if this is a use&def operand.
            ReusedOperands.markClobbered(PhysReg);
          ++NumReused;

          if (MI.getOperand(i).isKill() &&
              ReuseSlot <= VirtRegMap::MAX_STACK_SLOT) {

            // The store of this spilled value is potentially dead, but we
            // won't know for certain until we've confirmed that the re-use
            // above is valid, which means waiting until the other operands
            // are processed. For now we just track the spill slot, we'll
            // remove it after the other operands are processed if valid.

            PotentialDeadStoreSlots.push_back(ReuseSlot);
          }

          // Mark is isKill if it's there no other uses of the same virtual
          // register and it's not a two-address operand. IsKill will be
          // unset if reg is reused.
          if (!isTied && KilledMIRegs.count(VirtReg) == 0) {
            MI.getOperand(i).setIsKill();
            KilledMIRegs.insert(VirtReg);
          }

          continue;
        }  // CanReuse

        // Otherwise we have a situation where we have a two-address instruction
        // whose mod/ref operand needs to be reloaded.  This reload is already
        // available in some register "PhysReg", but if we used PhysReg as the
        // operand to our 2-addr instruction, the instruction would modify
        // PhysReg.  This isn't cool if something later uses PhysReg and expects
        // to get its initial value.
        //
        // To avoid this problem, and to avoid doing a load right after a store,
        // we emit a copy from PhysReg into the designated register for this
        // operand.
        //
        // This case also applies to an earlyclobber'd PhysReg.
        unsigned DesignatedReg = VRM->getPhys(VirtReg);
        assert(DesignatedReg && "Must map virtreg to physreg!");

        // Note that, if we reused a register for a previous operand, the
        // register we want to reload into might not actually be
        // available.  If this occurs, use the register indicated by the
        // reuser.
        if (ReusedOperands.hasReuses())
          DesignatedReg = ReusedOperands.
            GetRegForReload(VirtReg, DesignatedReg, &MI, Spills,
                            MaybeDeadStores, RegKills, KillOps, *VRM);

        // If the mapped designated register is actually the physreg we have
        // incoming, we don't need to inserted a dead copy.
        if (DesignatedReg == PhysReg) {
          // If this stack slot value is already available, reuse it!
          if (ReuseSlot > VirtRegMap::MAX_STACK_SLOT)
            DEBUG(dbgs() << "Reusing RM#"
                  << ReuseSlot-VirtRegMap::MAX_STACK_SLOT-1);
          else
            DEBUG(dbgs() << "Reusing SS#" << ReuseSlot);
          DEBUG(dbgs() << " from physreg " << TRI->getName(PhysReg)
                << " for vreg" << VirtReg
                << " instead of reloading into same physreg.\n");
          unsigned RReg = SubIdx ? TRI->getSubReg(PhysReg, SubIdx) : PhysReg;
          MI.getOperand(i).setReg(RReg);
          MI.getOperand(i).setSubReg(0);
          ReusedOperands.markClobbered(RReg);
          ++NumReused;
          continue;
        }

        const TargetRegisterClass* RC = MRI->getRegClass(VirtReg);
        MRI->setPhysRegUsed(DesignatedReg);
        ReusedOperands.markClobbered(DesignatedReg);

        // Back-schedule reloads and remats.
        MachineBasicBlock::iterator InsertLoc =
          ComputeReloadLoc(&MI, MBB->begin(), PhysReg, TRI, DoReMat,
                           SSorRMId, TII, MF);

        TII->copyRegToReg(*MBB, InsertLoc, DesignatedReg, PhysReg, RC, RC,
                          MI.getDebugLoc());

        MachineInstr *CopyMI = prior(InsertLoc);
        CopyMI->setAsmPrinterFlag(MachineInstr::ReloadReuse);
        UpdateKills(*CopyMI, TRI, RegKills, KillOps);

        // This invalidates DesignatedReg.
        Spills.ClobberPhysReg(DesignatedReg);

        Spills.addAvailable(ReuseSlot, DesignatedReg);
        unsigned RReg =
          SubIdx ? TRI->getSubReg(DesignatedReg, SubIdx) : DesignatedReg;
        MI.getOperand(i).setReg(RReg);
        MI.getOperand(i).setSubReg(0);
        DEBUG(dbgs() << '\t' << *prior(MII));
        ++NumReused;
        continue;
      } // if (PhysReg)

        // Otherwise, reload it and remember that we have it.
      PhysReg = VRM->getPhys(VirtReg);
      assert(PhysReg && "Must map virtreg to physreg!");

      // Note that, if we reused a register for a previous operand, the
      // register we want to reload into might not actually be
      // available.  If this occurs, use the register indicated by the
      // reuser.
      if (ReusedOperands.hasReuses())
        PhysReg = ReusedOperands.GetRegForReload(VirtReg, PhysReg, &MI,
                    Spills, MaybeDeadStores, RegKills, KillOps, *VRM);

      MRI->setPhysRegUsed(PhysReg);
      ReusedOperands.markClobbered(PhysReg);
      if (AvoidReload)
        ++NumAvoided;
      else {
        // Back-schedule reloads and remats.
        MachineBasicBlock::iterator InsertLoc =
          ComputeReloadLoc(MII, MBB->begin(), PhysReg, TRI, DoReMat,
                           SSorRMId, TII, MF);

        if (DoReMat) {
          ReMaterialize(*MBB, InsertLoc, PhysReg, VirtReg, TII, TRI, *VRM);
        } else {
          const TargetRegisterClass* RC = MRI->getRegClass(VirtReg);
          TII->loadRegFromStackSlot(*MBB, InsertLoc, PhysReg, SSorRMId, RC,TRI);
          MachineInstr *LoadMI = prior(InsertLoc);
          VRM->addSpillSlotUse(SSorRMId, LoadMI);
          ++NumLoads;
          DistanceMap.insert(std::make_pair(LoadMI, DistanceMap.size()));
        }
        // This invalidates PhysReg.
        Spills.ClobberPhysReg(PhysReg);

        // Any stores to this stack slot are not dead anymore.
        if (!DoReMat)
          MaybeDeadStores[SSorRMId] = NULL;
        Spills.addAvailable(SSorRMId, PhysReg);
        // Assumes this is the last use. IsKill will be unset if reg is reused
        // unless it's a two-address operand.
        if (!MI.isRegTiedToDefOperand(i) &&
            KilledMIRegs.count(VirtReg) == 0) {
          MI.getOperand(i).setIsKill();
          KilledMIRegs.insert(VirtReg);
        }

        UpdateKills(*prior(InsertLoc), TRI, RegKills, KillOps);
        DEBUG(dbgs() << '\t' << *prior(InsertLoc));
      }
      unsigned RReg = SubIdx ? TRI->getSubReg(PhysReg, SubIdx) : PhysReg;
      MI.getOperand(i).setReg(RReg);
      MI.getOperand(i).setSubReg(0);
    }

    // Ok - now we can remove stores that have been confirmed dead.
    for (unsigned j = 0, e = PotentialDeadStoreSlots.size(); j != e; ++j) {
      // This was the last use and the spilled value is still available
      // for reuse. That means the spill was unnecessary!
      int PDSSlot = PotentialDeadStoreSlots[j];
      MachineInstr* DeadStore = MaybeDeadStores[PDSSlot];
      if (DeadStore) {
        DEBUG(dbgs() << "Removed dead store:\t" << *DeadStore);
        InvalidateKills(*DeadStore, TRI, RegKills, KillOps);
        VRM->RemoveMachineInstrFromMaps(DeadStore);
        MBB->erase(DeadStore);
        MaybeDeadStores[PDSSlot] = NULL;
        ++NumDSE;
      }
    }


    DEBUG(dbgs() << '\t' << MI);


    // If we have folded references to memory operands, make sure we clear all
    // physical registers that may contain the value of the spilled virtual
    // register
    SmallSet<int, 2> FoldedSS;
    for (tie(I, End) = VRM->getFoldedVirts(&MI); I != End; ) {
      unsigned VirtReg = I->second.first;
      VirtRegMap::ModRef MR = I->second.second;
      DEBUG(dbgs() << "Folded vreg: " << VirtReg << "  MR: " << MR);

      // MI2VirtMap be can updated which invalidate the iterator.
      // Increment the iterator first.
      ++I;
      int SS = VRM->getStackSlot(VirtReg);
      if (SS == VirtRegMap::NO_STACK_SLOT)
        continue;
      FoldedSS.insert(SS);
      DEBUG(dbgs() << " - StackSlot: " << SS << "\n");

      // If this folded instruction is just a use, check to see if it's a
      // straight load from the virt reg slot.
      if ((MR & VirtRegMap::isRef) && !(MR & VirtRegMap::isMod)) {
        int FrameIdx;
        unsigned DestReg = TII->isLoadFromStackSlot(&MI, FrameIdx);
        if (DestReg && FrameIdx == SS) {
          // If this spill slot is available, turn it into a copy (or nothing)
          // instead of leaving it as a load!
          if (unsigned InReg = Spills.getSpillSlotOrReMatPhysReg(SS)) {
            DEBUG(dbgs() << "Promoted Load To Copy: " << MI);
            if (DestReg != InReg) {
              const TargetRegisterClass *RC = MRI->getRegClass(VirtReg);
              TII->copyRegToReg(*MBB, &MI, DestReg, InReg, RC, RC,
                                MI.getDebugLoc());
              MachineOperand *DefMO = MI.findRegisterDefOperand(DestReg);
              unsigned SubIdx = DefMO->getSubReg();
              // Revisit the copy so we make sure to notice the effects of the
              // operation on the destreg (either needing to RA it if it's
              // virtual or needing to clobber any values if it's physical).
              NextMII = &MI;
              --NextMII;  // backtrack to the copy.
              NextMII->setAsmPrinterFlag(MachineInstr::ReloadReuse);
              // Propagate the sub-register index over.
              if (SubIdx) {
                DefMO = NextMII->findRegisterDefOperand(DestReg);
                DefMO->setSubReg(SubIdx);
              }

              // Mark is killed.
              MachineOperand *KillOpnd = NextMII->findRegisterUseOperand(InReg);
              KillOpnd->setIsKill();

              BackTracked = true;
            } else {
              DEBUG(dbgs() << "Removing now-noop copy: " << MI);
              // Unset last kill since it's being reused.
              InvalidateKill(InReg, TRI, RegKills, KillOps);
              Spills.disallowClobberPhysReg(InReg);
            }

            InvalidateKills(MI, TRI, RegKills, KillOps);
            VRM->RemoveMachineInstrFromMaps(&MI);
            MBB->erase(&MI);
            Erased = true;
            goto ProcessNextInst;
          }
        } else {
          unsigned PhysReg = Spills.getSpillSlotOrReMatPhysReg(SS);
          SmallVector<MachineInstr*, 4> NewMIs;
          if (PhysReg &&
              TII->unfoldMemoryOperand(MF, &MI, PhysReg, false, false, NewMIs)) {
            MBB->insert(MII, NewMIs[0]);
            InvalidateKills(MI, TRI, RegKills, KillOps);
            VRM->RemoveMachineInstrFromMaps(&MI);
            MBB->erase(&MI);
            Erased = true;
            --NextMII;  // backtrack to the unfolded instruction.
            BackTracked = true;
            goto ProcessNextInst;
          }
        }
      }

      // If this reference is not a use, any previous store is now dead.
      // Otherwise, the store to this stack slot is not dead anymore.
      MachineInstr* DeadStore = MaybeDeadStores[SS];
      if (DeadStore) {
        bool isDead = !(MR & VirtRegMap::isRef);
        MachineInstr *NewStore = NULL;
        if (MR & VirtRegMap::isModRef) {
          unsigned PhysReg = Spills.getSpillSlotOrReMatPhysReg(SS);
          SmallVector<MachineInstr*, 4> NewMIs;
          // We can reuse this physreg as long as we are allowed to clobber
          // the value and there isn't an earlier def that has already clobbered
          // the physreg.
          if (PhysReg &&
              !ReusedOperands.isClobbered(PhysReg) &&
              Spills.canClobberPhysReg(PhysReg) &&
              !TII->isStoreToStackSlot(&MI, SS)) { // Not profitable!
            MachineOperand *KillOpnd =
              DeadStore->findRegisterUseOperand(PhysReg, true);
            // Note, if the store is storing a sub-register, it's possible the
            // super-register is needed below.
            if (KillOpnd && !KillOpnd->getSubReg() &&
                TII->unfoldMemoryOperand(MF, &MI, PhysReg, false, true,NewMIs)){
              MBB->insert(MII, NewMIs[0]);
              NewStore = NewMIs[1];
              MBB->insert(MII, NewStore);
              VRM->addSpillSlotUse(SS, NewStore);
              InvalidateKills(MI, TRI, RegKills, KillOps);
              VRM->RemoveMachineInstrFromMaps(&MI);
              MBB->erase(&MI);
              Erased = true;
              --NextMII;
              --NextMII;  // backtrack to the unfolded instruction.
              BackTracked = true;
              isDead = true;
              ++NumSUnfold;
            }
          }
        }

        if (isDead) {  // Previous store is dead.
          // If we get here, the store is dead, nuke it now.
          DEBUG(dbgs() << "Removed dead store:\t" << *DeadStore);
          InvalidateKills(*DeadStore, TRI, RegKills, KillOps);
          VRM->RemoveMachineInstrFromMaps(DeadStore);
          MBB->erase(DeadStore);
          if (!NewStore)
            ++NumDSE;
        }

        MaybeDeadStores[SS] = NULL;
        if (NewStore) {
          // Treat this store as a spill merged into a copy. That makes the
          // stack slot value available.
          VRM->virtFolded(VirtReg, NewStore, VirtRegMap::isMod);
          goto ProcessNextInst;
        }
      }

      // If the spill slot value is available, and this is a new definition of
      // the value, the value is not available anymore.
      if (MR & VirtRegMap::isMod) {
        // Notice that the value in this stack slot has been modified.
        Spills.ModifyStackSlotOrReMat(SS);

        // If this is *just* a mod of the value, check to see if this is just a
        // store to the spill slot (i.e. the spill got merged into the copy). If
        // so, realize that the vreg is available now, and add the store to the
        // MaybeDeadStore info.
        int StackSlot;
        if (!(MR & VirtRegMap::isRef)) {
          if (unsigned SrcReg = TII->isStoreToStackSlot(&MI, StackSlot)) {
            assert(TargetRegisterInfo::isPhysicalRegister(SrcReg) &&
                   "Src hasn't been allocated yet?");

            if (CommuteToFoldReload(MII, VirtReg, SrcReg, StackSlot,
                                    Spills, RegKills, KillOps, TRI)) {
              NextMII = llvm::next(MII);
              BackTracked = true;
              goto ProcessNextInst;
            }

            // Okay, this is certainly a store of SrcReg to [StackSlot].  Mark
            // this as a potentially dead store in case there is a subsequent
            // store into the stack slot without a read from it.
            MaybeDeadStores[StackSlot] = &MI;

            // If the stack slot value was previously available in some other
            // register, change it now.  Otherwise, make the register
            // available in PhysReg.
            Spills.addAvailable(StackSlot, SrcReg, MI.killsRegister(SrcReg));
          }
        }
      }
    }

    // Process all of the spilled defs.
    SpilledMIRegs.clear();
    for (unsigned i = 0, e = MI.getNumOperands(); i != e; ++i) {
      MachineOperand &MO = MI.getOperand(i);
      if (!(MO.isReg() && MO.getReg() && MO.isDef()))
        continue;

      unsigned VirtReg = MO.getReg();
      if (!TargetRegisterInfo::isVirtualRegister(VirtReg)) {
        // Check to see if this is a noop copy.  If so, eliminate the
        // instruction before considering the dest reg to be changed.
        // Also check if it's copying from an "undef", if so, we can't
        // eliminate this or else the undef marker is lost and it will
        // confuses the scavenger. This is extremely rare.
        unsigned Src, Dst, SrcSR, DstSR;
        if (TII->isMoveInstr(MI, Src, Dst, SrcSR, DstSR) &&
            Src == Dst && SrcSR == DstSR &&
            !MI.findRegisterUseOperand(Src)->isUndef()) {
          ++NumDCE;
          DEBUG(dbgs() << "Removing now-noop copy: " << MI);
          SmallVector<unsigned, 2> KillRegs;
          InvalidateKills(MI, TRI, RegKills, KillOps, &KillRegs);
          if (MO.isDead() && !KillRegs.empty()) {
            // Source register or an implicit super/sub-register use is killed.
            assert(KillRegs[0] == Dst ||
                   TRI->isSubRegister(KillRegs[0], Dst) ||
                   TRI->isSuperRegister(KillRegs[0], Dst));
            // Last def is now dead.
            TransferDeadness(Src, RegKills, KillOps);
          }
          VRM->RemoveMachineInstrFromMaps(&MI);
          MBB->erase(&MI);
          Erased = true;
          Spills.disallowClobberPhysReg(VirtReg);
          goto ProcessNextInst;
        }

        // If it's not a no-op copy, it clobbers the value in the destreg.
        Spills.ClobberPhysReg(VirtReg);
        ReusedOperands.markClobbered(VirtReg);

        // Check to see if this instruction is a load from a stack slot into
        // a register.  If so, this provides the stack slot value in the reg.
        int FrameIdx;
        if (unsigned DestReg = TII->isLoadFromStackSlot(&MI, FrameIdx)) {
          assert(DestReg == VirtReg && "Unknown load situation!");

          // If it is a folded reference, then it's not safe to clobber.
          bool Folded = FoldedSS.count(FrameIdx);
          // Otherwise, if it wasn't available, remember that it is now!
          Spills.addAvailable(FrameIdx, DestReg, !Folded);
          goto ProcessNextInst;
        }

        continue;
      }

      unsigned SubIdx = MO.getSubReg();
      bool DoReMat = VRM->isReMaterialized(VirtReg);
      if (DoReMat)
        ReMatDefs.insert(&MI);

      // The only vregs left are stack slot definitions.
      int StackSlot = VRM->getStackSlot(VirtReg);
      const TargetRegisterClass *RC = MRI->getRegClass(VirtReg);

      // If this def is part of a two-address operand, make sure to execute
      // the store from the correct physical register.
      unsigned PhysReg;
      unsigned TiedOp;
      if (MI.isRegTiedToUseOperand(i, &TiedOp)) {
        PhysReg = MI.getOperand(TiedOp).getReg();
        if (SubIdx) {
          unsigned SuperReg = findSuperReg(RC, PhysReg, SubIdx, TRI);
          assert(SuperReg && TRI->getSubReg(SuperReg, SubIdx) == PhysReg &&
                 "Can't find corresponding super-register!");
          PhysReg = SuperReg;
        }
      } else {
        PhysReg = VRM->getPhys(VirtReg);
        if (ReusedOperands.isClobbered(PhysReg)) {
          // Another def has taken the assigned physreg. It must have been a
          // use&def which got it due to reuse. Undo the reuse!
          PhysReg = ReusedOperands.GetRegForReload(VirtReg, PhysReg, &MI,
                      Spills, MaybeDeadStores, RegKills, KillOps, *VRM);
        }
      }

      assert(PhysReg && "VR not assigned a physical register?");
      MRI->setPhysRegUsed(PhysReg);
      unsigned RReg = SubIdx ? TRI->getSubReg(PhysReg, SubIdx) : PhysReg;
      ReusedOperands.markClobbered(RReg);
      MI.getOperand(i).setReg(RReg);
      MI.getOperand(i).setSubReg(0);

      if (!MO.isDead() && SpilledMIRegs.insert(VirtReg)) {
        MachineInstr *&LastStore = MaybeDeadStores[StackSlot];
        SpillRegToStackSlot(MII, -1, PhysReg, StackSlot, RC, true,
          LastStore, Spills, ReMatDefs, RegKills, KillOps);
        NextMII = llvm::next(MII);

        // Check to see if this is a noop copy.  If so, eliminate the
        // instruction before considering the dest reg to be changed.
        {
          unsigned Src, Dst, SrcSR, DstSR;
          if (TII->isMoveInstr(MI, Src, Dst, SrcSR, DstSR) &&
              Src == Dst && SrcSR == DstSR) {
            ++NumDCE;
            DEBUG(dbgs() << "Removing now-noop copy: " << MI);
            InvalidateKills(MI, TRI, RegKills, KillOps);
            VRM->RemoveMachineInstrFromMaps(&MI);
            MBB->erase(&MI);
            Erased = true;
            UpdateKills(*LastStore, TRI, RegKills, KillOps);
            goto ProcessNextInst;
          }
        }
      }
    }
    ProcessNextInst:
    // Delete dead instructions without side effects.
    if (!Erased && !BackTracked && isSafeToDelete(MI)) {
      InvalidateKills(MI, TRI, RegKills, KillOps);
      VRM->RemoveMachineInstrFromMaps(&MI);
      MBB->erase(&MI);
      Erased = true;
    }
    if (!Erased)
      DistanceMap.insert(std::make_pair(&MI, DistanceMap.size()));
    if (!Erased && !BackTracked) {
      for (MachineBasicBlock::iterator II = &MI; II != NextMII; ++II)
        UpdateKills(*II, TRI, RegKills, KillOps);
    }
    MII = NextMII;
  }

}

llvm::VirtRegRewriter* llvm::createVirtRegRewriter() {
  switch (RewriterOpt) {
  default: llvm_unreachable("Unreachable!");
  case local:
    return new LocalRewriter();
  case trivial:
    return new TrivialRewriter();
  }
}