xref: /external/llvm/lib/CodeGen/PrologEpilogInserter.cpp

//===-- PrologEpilogInserter.cpp - Insert Prolog/Epilog code in function --===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This pass is responsible for finalizing the functions frame layout, saving
// callee saved registers, and for emitting prolog & epilog code for the
// function.
//
// This pass must be run after register allocation.  After this pass is
// executed, it is illegal to construct MO_FrameIndex operands.
//
//===----------------------------------------------------------------------===//

#include "llvm/CodeGen/Passes.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/CodeGen/MachineInstr.h"
#include "llvm/CodeGen/MachineFrameInfo.h"
#include "llvm/CodeGen/MachineModuleInfo.h"
#include "llvm/CodeGen/MachineRegisterInfo.h"
#include "llvm/CodeGen/RegisterScavenging.h"
#include "llvm/Target/TargetMachine.h"
#include "llvm/Target/TargetRegisterInfo.h"
#include "llvm/Target/TargetFrameInfo.h"
#include "llvm/Target/TargetInstrInfo.h"
#include "llvm/Support/Compiler.h"
#include "llvm/ADT/STLExtras.h"
#include <climits>
using namespace llvm;

namespace {
  struct VISIBILITY_HIDDEN PEI : public MachineFunctionPass {
    static char ID;
    PEI() : MachineFunctionPass(&ID) {}

    const char *getPassName() const {
      return "Prolog/Epilog Insertion & Frame Finalization";
    }

    virtual void getAnalysisUsage(AnalysisUsage &AU) const {
      AU.addPreservedID(MachineLoopInfoID);
      AU.addPreservedID(MachineDominatorsID);
      MachineFunctionPass::getAnalysisUsage(AU);
    }

    /// runOnMachineFunction - Insert prolog/epilog code and replace abstract
    /// frame indexes with appropriate references.
    ///
    bool runOnMachineFunction(MachineFunction &Fn) {
      const TargetRegisterInfo *TRI = Fn.getTarget().getRegisterInfo();
      RS = TRI->requiresRegisterScavenging(Fn) ? new RegScavenger() : NULL;

      // Get MachineModuleInfo so that we can track the construction of the
      // frame.
      if (MachineModuleInfo *MMI = getAnalysisToUpdate<MachineModuleInfo>())
        Fn.getFrameInfo()->setMachineModuleInfo(MMI);

      // Allow the target machine to make some adjustments to the function
      // e.g. UsedPhysRegs before calculateCalleeSavedRegisters.
      TRI->processFunctionBeforeCalleeSavedScan(Fn, RS);

      // Scan the function for modified callee saved registers and insert spill
      // code for any callee saved registers that are modified.  Also calculate
      // the MaxCallFrameSize and HasCalls variables for the function's frame
      // information and eliminates call frame pseudo instructions.
      calculateCalleeSavedRegisters(Fn);

      // Add the code to save and restore the callee saved registers
      saveCalleeSavedRegisters(Fn);

      // Allow the target machine to make final modifications to the function
      // before the frame layout is finalized.
      TRI->processFunctionBeforeFrameFinalized(Fn);

      // Calculate actual frame offsets for all of the abstract stack objects...
      calculateFrameObjectOffsets(Fn);

      // Add prolog and epilog code to the function.  This function is required
      // to align the stack frame as necessary for any stack variables or
      // called functions.  Because of this, calculateCalleeSavedRegisters
      // must be called before this function in order to set the HasCalls
      // and MaxCallFrameSize variables.
      insertPrologEpilogCode(Fn);

      // Replace all MO_FrameIndex operands with physical register references
      // and actual offsets.
      //
      replaceFrameIndices(Fn);

      delete RS;
      return true;
    }

  private:
    RegScavenger *RS;

    // MinCSFrameIndex, MaxCSFrameIndex - Keeps the range of callee saved
    // stack frame indexes.
    unsigned MinCSFrameIndex, MaxCSFrameIndex;

    void calculateCalleeSavedRegisters(MachineFunction &Fn);
    void saveCalleeSavedRegisters(MachineFunction &Fn);
    void calculateFrameObjectOffsets(MachineFunction &Fn);
    void replaceFrameIndices(MachineFunction &Fn);
    void insertPrologEpilogCode(MachineFunction &Fn);
  };
  char PEI::ID = 0;
}


/// createPrologEpilogCodeInserter - This function returns a pass that inserts
/// prolog and epilog code, and eliminates abstract frame references.
///
FunctionPass *llvm::createPrologEpilogCodeInserter() { return new PEI(); }


/// calculateCalleeSavedRegisters - Scan the function for modified callee saved
/// registers.  Also calculate the MaxCallFrameSize and HasCalls variables for
/// the function's frame information and eliminates call frame pseudo
/// instructions.
///
void PEI::calculateCalleeSavedRegisters(MachineFunction &Fn) {
  const TargetRegisterInfo *RegInfo = Fn.getTarget().getRegisterInfo();
  const TargetFrameInfo *TFI = Fn.getTarget().getFrameInfo();

  // Get the callee saved register list...
  const unsigned *CSRegs = RegInfo->getCalleeSavedRegs(&Fn);

  // Get the function call frame set-up and tear-down instruction opcode
  int FrameSetupOpcode   = RegInfo->getCallFrameSetupOpcode();
  int FrameDestroyOpcode = RegInfo->getCallFrameDestroyOpcode();

  // These are used to keep track the callee-save area. Initialize them.
  MinCSFrameIndex = INT_MAX;
  MaxCSFrameIndex = 0;

  // Early exit for targets which have no callee saved registers and no call
  // frame setup/destroy pseudo instructions.
  if ((CSRegs == 0 || CSRegs[0] == 0) &&
      FrameSetupOpcode == -1 && FrameDestroyOpcode == -1)
    return;

  unsigned MaxCallFrameSize = 0;
  bool HasCalls = false;

  std::vector<MachineBasicBlock::iterator> FrameSDOps;
  for (MachineFunction::iterator BB = Fn.begin(), E = Fn.end(); BB != E; ++BB)
    for (MachineBasicBlock::iterator I = BB->begin(); I != BB->end(); ++I)
      if (I->getOpcode() == FrameSetupOpcode ||
          I->getOpcode() == FrameDestroyOpcode) {
        assert(I->getNumOperands() >= 1 && "Call Frame Setup/Destroy Pseudo"
               " instructions should have a single immediate argument!");
        unsigned Size = I->getOperand(0).getImm();
        if (Size > MaxCallFrameSize) MaxCallFrameSize = Size;
        HasCalls = true;
        FrameSDOps.push_back(I);
      }

  MachineFrameInfo *FFI = Fn.getFrameInfo();
  FFI->setHasCalls(HasCalls);
  FFI->setMaxCallFrameSize(MaxCallFrameSize);

  for (unsigned i = 0, e = FrameSDOps.size(); i != e; ++i) {
    MachineBasicBlock::iterator I = FrameSDOps[i];
    // If call frames are not being included as part of the stack frame,
    // and there is no dynamic allocation (therefore referencing frame slots
    // off sp), leave the pseudo ops alone. We'll eliminate them later.
    if (RegInfo->hasReservedCallFrame(Fn) || RegInfo->hasFP(Fn))
      RegInfo->eliminateCallFramePseudoInstr(Fn, *I->getParent(), I);
  }

  // Now figure out which *callee saved* registers are modified by the current
  // function, thus needing to be saved and restored in the prolog/epilog.
  //
  const TargetRegisterClass* const *CSRegClasses =
    RegInfo->getCalleeSavedRegClasses(&Fn);
  std::vector<CalleeSavedInfo> CSI;
  for (unsigned i = 0; CSRegs[i]; ++i) {
    unsigned Reg = CSRegs[i];
    if (Fn.getRegInfo().isPhysRegUsed(Reg)) {
        // If the reg is modified, save it!
      CSI.push_back(CalleeSavedInfo(Reg, CSRegClasses[i]));
    } else {
      for (const unsigned *AliasSet = RegInfo->getAliasSet(Reg);
           *AliasSet; ++AliasSet) {  // Check alias registers too.
        if (Fn.getRegInfo().isPhysRegUsed(*AliasSet)) {
          CSI.push_back(CalleeSavedInfo(Reg, CSRegClasses[i]));
          break;
        }
      }
    }
  }

  if (CSI.empty())
    return;   // Early exit if no callee saved registers are modified!

  unsigned NumFixedSpillSlots;
  const std::pair<unsigned,int> *FixedSpillSlots =
    TFI->getCalleeSavedSpillSlots(NumFixedSpillSlots);

  // Now that we know which registers need to be saved and restored, allocate
  // stack slots for them.
  for (unsigned i = 0, e = CSI.size(); i != e; ++i) {
    unsigned Reg = CSI[i].getReg();
    const TargetRegisterClass *RC = CSI[i].getRegClass();

    // Check to see if this physreg must be spilled to a particular stack slot
    // on this target.
    const std::pair<unsigned,int> *FixedSlot = FixedSpillSlots;
    while (FixedSlot != FixedSpillSlots+NumFixedSpillSlots &&
           FixedSlot->first != Reg)
      ++FixedSlot;

    int FrameIdx;
    if (FixedSlot == FixedSpillSlots+NumFixedSpillSlots) {
      // Nope, just spill it anywhere convenient.
      unsigned Align = RC->getAlignment();
      unsigned StackAlign = TFI->getStackAlignment();
      // We may not be able to sastify the desired alignment specification of
      // the TargetRegisterClass if the stack alignment is smaller. Use the min.
      Align = std::min(Align, StackAlign);
      FrameIdx = FFI->CreateStackObject(RC->getSize(), Align);
      if ((unsigned)FrameIdx < MinCSFrameIndex) MinCSFrameIndex = FrameIdx;
      if ((unsigned)FrameIdx > MaxCSFrameIndex) MaxCSFrameIndex = FrameIdx;
    } else {
      // Spill it to the stack where we must.
      FrameIdx = FFI->CreateFixedObject(RC->getSize(), FixedSlot->second);
    }
    CSI[i].setFrameIdx(FrameIdx);
  }

  FFI->setCalleeSavedInfo(CSI);
}

/// saveCalleeSavedRegisters -  Insert spill code for any callee saved registers
/// that are modified in the function.
///
void PEI::saveCalleeSavedRegisters(MachineFunction &Fn) {
  // Get callee saved register information.
  MachineFrameInfo *FFI = Fn.getFrameInfo();
  const std::vector<CalleeSavedInfo> &CSI = FFI->getCalleeSavedInfo();

  // Early exit if no callee saved registers are modified!
  if (CSI.empty())
    return;

  const TargetInstrInfo &TII = *Fn.getTarget().getInstrInfo();

  // Now that we have a stack slot for each register to be saved, insert spill
  // code into the entry block.
  MachineBasicBlock *MBB = Fn.begin();
  MachineBasicBlock::iterator I = MBB->begin();

  if (!TII.spillCalleeSavedRegisters(*MBB, I, CSI)) {
    for (unsigned i = 0, e = CSI.size(); i != e; ++i) {
      // Add the callee-saved register as live-in. It's killed at the spill.
      MBB->addLiveIn(CSI[i].getReg());

      // Insert the spill to the stack frame.
      TII.storeRegToStackSlot(*MBB, I, CSI[i].getReg(), true,
                                   CSI[i].getFrameIdx(), CSI[i].getRegClass());
    }
  }

  // Add code to restore the callee-save registers in each exiting block.
  for (MachineFunction::iterator FI = Fn.begin(), E = Fn.end(); FI != E; ++FI)
    // If last instruction is a return instruction, add an epilogue.
    if (!FI->empty() && FI->back().getDesc().isReturn()) {
      MBB = FI;
      I = MBB->end(); --I;

      // Skip over all terminator instructions, which are part of the return
      // sequence.
      MachineBasicBlock::iterator I2 = I;
      while (I2 != MBB->begin() && (--I2)->getDesc().isTerminator())
        I = I2;

      bool AtStart = I == MBB->begin();
      MachineBasicBlock::iterator BeforeI = I;
      if (!AtStart)
        --BeforeI;

      // Restore all registers immediately before the return and any terminators
      // that preceed it.
      if (!TII.restoreCalleeSavedRegisters(*MBB, I, CSI)) {
        for (unsigned i = 0, e = CSI.size(); i != e; ++i) {
          TII.loadRegFromStackSlot(*MBB, I, CSI[i].getReg(),
                                        CSI[i].getFrameIdx(),
                                        CSI[i].getRegClass());
          assert(I != MBB->begin() &&
                 "loadRegFromStackSlot didn't insert any code!");
          // Insert in reverse order.  loadRegFromStackSlot can insert multiple
          // instructions.
          if (AtStart)
            I = MBB->begin();
          else {
            I = BeforeI;
            ++I;
          }
        }
      }
    }
}


/// calculateFrameObjectOffsets - Calculate actual frame offsets for all of the
/// abstract stack objects.
///
void PEI::calculateFrameObjectOffsets(MachineFunction &Fn) {
  const TargetFrameInfo &TFI = *Fn.getTarget().getFrameInfo();

  bool StackGrowsDown =
    TFI.getStackGrowthDirection() == TargetFrameInfo::StackGrowsDown;

  // Loop over all of the stack objects, assigning sequential addresses...
  MachineFrameInfo *FFI = Fn.getFrameInfo();

  unsigned MaxAlign = FFI->getMaxAlignment();

  // Start at the beginning of the local area.
  // The Offset is the distance from the stack top in the direction
  // of stack growth -- so it's always nonnegative.
  int64_t Offset = TFI.getOffsetOfLocalArea();
  if (StackGrowsDown)
    Offset = -Offset;
  assert(Offset >= 0
         && "Local area offset should be in direction of stack growth");

  // If there are fixed sized objects that are preallocated in the local area,
  // non-fixed objects can't be allocated right at the start of local area.
  // We currently don't support filling in holes in between fixed sized objects,
  // so we adjust 'Offset' to point to the end of last fixed sized
  // preallocated object.
  for (int i = FFI->getObjectIndexBegin(); i != 0; ++i) {
    int64_t FixedOff;
    if (StackGrowsDown) {
      // The maximum distance from the stack pointer is at lower address of
      // the object -- which is given by offset. For down growing stack
      // the offset is negative, so we negate the offset to get the distance.
      FixedOff = -FFI->getObjectOffset(i);
    } else {
      // The maximum distance from the start pointer is at the upper
      // address of the object.
      FixedOff = FFI->getObjectOffset(i) + FFI->getObjectSize(i);
    }
    if (FixedOff > Offset) Offset = FixedOff;
  }

  // First assign frame offsets to stack objects that are used to spill
  // callee saved registers.
  if (StackGrowsDown) {
    for (unsigned i = MinCSFrameIndex; i <= MaxCSFrameIndex; ++i) {
      // If stack grows down, we need to add size of find the lowest
      // address of the object.
      Offset += FFI->getObjectSize(i);

      unsigned Align = FFI->getObjectAlignment(i);
      // If the alignment of this object is greater than that of the stack, then
      // increase the stack alignment to match.
      MaxAlign = std::max(MaxAlign, Align);
      // Adjust to alignment boundary
      Offset = (Offset+Align-1)/Align*Align;

      FFI->setObjectOffset(i, -Offset);        // Set the computed offset
    }
  } else {
    int MaxCSFI = MaxCSFrameIndex, MinCSFI = MinCSFrameIndex;
    for (int i = MaxCSFI; i >= MinCSFI ; --i) {
      unsigned Align = FFI->getObjectAlignment(i);
      // If the alignment of this object is greater than that of the stack, then
      // increase the stack alignment to match.
      MaxAlign = std::max(MaxAlign, Align);
      // Adjust to alignment boundary
      Offset = (Offset+Align-1)/Align*Align;

      FFI->setObjectOffset(i, Offset);
      Offset += FFI->getObjectSize(i);
    }
  }

  // Make sure the special register scavenging spill slot is closest to the
  // frame pointer if a frame pointer is required.
  const TargetRegisterInfo *RegInfo = Fn.getTarget().getRegisterInfo();
  if (RS && RegInfo->hasFP(Fn)) {
    int SFI = RS->getScavengingFrameIndex();
    if (SFI >= 0) {
      // If stack grows down, we need to add size of the lowest
      // address of the object.
      if (StackGrowsDown)
        Offset += FFI->getObjectSize(SFI);

      unsigned Align = FFI->getObjectAlignment(SFI);
      // Adjust to alignment boundary
      Offset = (Offset+Align-1)/Align*Align;

      if (StackGrowsDown) {
        FFI->setObjectOffset(SFI, -Offset);        // Set the computed offset
      } else {
        FFI->setObjectOffset(SFI, Offset);
        Offset += FFI->getObjectSize(SFI);
      }
    }
  }

  // Make sure that the stack protector comes before the local variables on the
  // stack.
  if (FFI->getStackProtectorIndex() >= 0) {
    int FI = FFI->getStackProtectorIndex();

    // If stack grows down, we need to add size of find the lowest
    // address of the object.
    if (StackGrowsDown)
      Offset += FFI->getObjectSize(FI);

    unsigned Align = FFI->getObjectAlignment(FI);

    // If the alignment of this object is greater than that of the stack, then
    // increase the stack alignment to match.
    MaxAlign = std::max(MaxAlign, Align);

    // Adjust to alignment boundary.
    Offset = (Offset + Align - 1) / Align * Align;

    if (StackGrowsDown) {
      FFI->setObjectOffset(FI, -Offset); // Set the computed offset
    } else {
      FFI->setObjectOffset(FI, Offset);
      Offset += FFI->getObjectSize(FI);
    }
  }

  // Then assign frame offsets to stack objects that are not used to spill
  // callee saved registers.
  for (unsigned i = 0, e = FFI->getObjectIndexEnd(); i != e; ++i) {
    if (i >= MinCSFrameIndex && i <= MaxCSFrameIndex)
      continue;
    if (RS && (int)i == RS->getScavengingFrameIndex())
      continue;
    if (FFI->isDeadObjectIndex(i))
      continue;
    if (FFI->getStackProtectorIndex() == (int)i)
      continue;

    // If stack grows down, we need to add size of find the lowest
    // address of the object.
    if (StackGrowsDown)
      Offset += FFI->getObjectSize(i);

    unsigned Align = FFI->getObjectAlignment(i);
    // If the alignment of this object is greater than that of the stack, then
    // increase the stack alignment to match.
    MaxAlign = std::max(MaxAlign, Align);
    // Adjust to alignment boundary
    Offset = (Offset+Align-1)/Align*Align;

    if (StackGrowsDown) {
      FFI->setObjectOffset(i, -Offset);        // Set the computed offset
    } else {
      FFI->setObjectOffset(i, Offset);
      Offset += FFI->getObjectSize(i);
    }
  }

  // Make sure the special register scavenging spill slot is closest to the
  // stack pointer.
  if (RS && !RegInfo->hasFP(Fn)) {
    int SFI = RS->getScavengingFrameIndex();
    if (SFI >= 0) {
      // If stack grows down, we need to add size of find the lowest
      // address of the object.
      if (StackGrowsDown)
        Offset += FFI->getObjectSize(SFI);

      unsigned Align = FFI->getObjectAlignment(SFI);
      // If the alignment of this object is greater than that of the
      // stack, then increase the stack alignment to match.
      MaxAlign = std::max(MaxAlign, Align);
      // Adjust to alignment boundary
      Offset = (Offset+Align-1)/Align*Align;

      if (StackGrowsDown) {
        FFI->setObjectOffset(SFI, -Offset);        // Set the computed offset
      } else {
        FFI->setObjectOffset(SFI, Offset);
        Offset += FFI->getObjectSize(SFI);
      }
    }
  }

  // Round up the size to a multiple of the alignment, but only if there are
  // calls or alloca's in the function.  This ensures that any calls to
  // subroutines have their stack frames suitable aligned.
  // Also do this if we need runtime alignment of the stack.  In this case
  // offsets will be relative to SP not FP; round up the stack size so this
  // works.
  if (!RegInfo->targetHandlesStackFrameRounding() &&
      (FFI->hasCalls() || FFI->hasVarSizedObjects() ||
       (RegInfo->needsStackRealignment(Fn) &&
        FFI->getObjectIndexEnd() != 0))) {
    // If we have reserved argument space for call sites in the function
    // immediately on entry to the current function, count it as part of the
    // overall stack size.
    if (RegInfo->hasReservedCallFrame(Fn))
      Offset += FFI->getMaxCallFrameSize();

    unsigned AlignMask = std::max(TFI.getStackAlignment(),MaxAlign) - 1;
    Offset = (Offset + AlignMask) & ~uint64_t(AlignMask);
  }

  // Update frame info to pretend that this is part of the stack...
  FFI->setStackSize(Offset+TFI.getOffsetOfLocalArea());

  // Remember the required stack alignment in case targets need it to perform
  // dynamic stack alignment.
  FFI->setMaxAlignment(MaxAlign);
}


/// insertPrologEpilogCode - Scan the function for modified callee saved
/// registers, insert spill code for these callee saved registers, then add
/// prolog and epilog code to the function.
///
void PEI::insertPrologEpilogCode(MachineFunction &Fn) {
  const TargetRegisterInfo *TRI = Fn.getTarget().getRegisterInfo();

  // Add prologue to the function...
  TRI->emitPrologue(Fn);

  // Add epilogue to restore the callee-save registers in each exiting block
  for (MachineFunction::iterator I = Fn.begin(), E = Fn.end(); I != E; ++I) {
    // If last instruction is a return instruction, add an epilogue
    if (!I->empty() && I->back().getDesc().isReturn())
      TRI->emitEpilogue(Fn, *I);
  }
}


/// replaceFrameIndices - Replace all MO_FrameIndex operands with physical
/// register references and actual offsets.
///
void PEI::replaceFrameIndices(MachineFunction &Fn) {
  if (!Fn.getFrameInfo()->hasStackObjects()) return; // Nothing to do?

  const TargetMachine &TM = Fn.getTarget();
  assert(TM.getRegisterInfo() && "TM::getRegisterInfo() must be implemented!");
  const TargetRegisterInfo &TRI = *TM.getRegisterInfo();
  const TargetFrameInfo *TFI = TM.getFrameInfo();
  bool StackGrowsDown =
    TFI->getStackGrowthDirection() == TargetFrameInfo::StackGrowsDown;
  int FrameSetupOpcode   = TRI.getCallFrameSetupOpcode();
  int FrameDestroyOpcode = TRI.getCallFrameDestroyOpcode();

  for (MachineFunction::iterator BB = Fn.begin(), E = Fn.end(); BB != E; ++BB) {
    int SPAdj = 0;  // SP offset due to call frame setup / destroy.
    if (RS) RS->enterBasicBlock(BB);
    for (MachineBasicBlock::iterator I = BB->begin(); I != BB->end(); ) {
      MachineInstr *MI = I;

      if (I->getOpcode() == TargetInstrInfo::DECLARE) {
        // Ignore it.
        ++I;
        continue;
      }

      if (I->getOpcode() == FrameSetupOpcode ||
          I->getOpcode() == FrameDestroyOpcode) {
        // Remember how much SP has been adjusted to create the call
        // frame.
        int Size = I->getOperand(0).getImm();

        if ((!StackGrowsDown && I->getOpcode() == FrameSetupOpcode) ||
            (StackGrowsDown && I->getOpcode() == FrameDestroyOpcode))
          Size = -Size;

        SPAdj += Size;

        MachineBasicBlock::iterator PrevI = prior(I);
        TRI.eliminateCallFramePseudoInstr(Fn, *BB, I);

        // Visit the instructions created by eliminateCallFramePseudoInstr().
        I = next(PrevI);
        continue;
      }

      bool DoIncr = true;

      for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i)
        if (MI->getOperand(i).isFI()) {
          // Some instructions (e.g. inline asm instructions) can have
          // multiple frame indices and/or cause eliminateFrameIndex
          // to insert more than one instruction. We need the register
          // scavenger to go through all of these instructions so that
          // it can update its register information. We keep the
          // iterator at the point before insertion so that we can
          // revisit them in full.
          bool AtBeginning = (I == BB->begin());
          if (!AtBeginning) --I;

          // If this instruction has a FrameIndex operand, we need to
          // use that target machine register info object to eliminate
          // it.
          TRI.eliminateFrameIndex(MI, SPAdj, RS);

          // Reset the iterator if we were at the beginning of the BB.
          if (AtBeginning) {
            I = BB->begin();
            DoIncr = false;
          }

          MI = 0;
          break;
        }

      if (DoIncr) ++I;

      // Update register states.
      if (RS && MI) RS->forward(MI);
    }

    assert(SPAdj == 0 && "Unbalanced call frame setup / destroy pairs?");
  }
}