llvm/lib/Target/PowerPC/PPCVSXSwapRemoval.cpp

//===----------- PPCVSXSwapRemoval.cpp - Remove VSX LE Swaps -------------===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===---------------------------------------------------------------------===//
//
// This pass analyzes vector computations and removes unnecessary
// doubleword swaps (xxswapd instructions).  This pass is performed
// only for little-endian VSX code generation.
//
// For this specific case, loads and stores of v4i32, v4f32, v2i64,
// and v2f64 vectors are inefficient.  These are implemented using
// the lxvd2x and stxvd2x instructions, which invert the order of
// doublewords in a vector register.  Thus code generation inserts
// an xxswapd after each such load, and prior to each such store.
//
// The extra xxswapd instructions reduce performance.  The purpose
// of this pass is to reduce the number of xxswapd instructions
// required for correctness.
//
// The primary insight is that much code that operates on vectors
// does not care about the relative order of elements in a register,
// so long as the correct memory order is preserved.  If we have a
// computation where all input values are provided by lxvd2x/xxswapd,
// all outputs are stored using xxswapd/lxvd2x, and all intermediate
// computations are lane-insensitive (independent of element order),
// then all the xxswapd instructions associated with the loads and
// stores may be removed without changing observable semantics.
//
// This pass uses standard equivalence class infrastructure to create
// maximal webs of computations fitting the above description.  Each
// such web is then optimized by removing its unnecessary xxswapd
// instructions.
//
// There are some lane-sensitive operations for which we can still
// permit the optimization, provided we modify those operations
// accordingly.  Such operations are identified as using "special
// handling" within this module.
//
//===---------------------------------------------------------------------===//

#include "PPCInstrInfo.h"
#include "PPC.h"
#include "PPCInstrBuilder.h"
#include "PPCTargetMachine.h"
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/EquivalenceClasses.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/CodeGen/MachineInstrBuilder.h"
#include "llvm/CodeGen/MachineRegisterInfo.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/Format.h"
#include "llvm/Support/raw_ostream.h"

using namespace llvm;

#define DEBUG_TYPE "ppc-vsx-swaps"

namespace llvm {
  void initializePPCVSXSwapRemovalPass(PassRegistry&);
}

namespace {

// A PPCVSXSwapEntry is created for each machine instruction that
// is relevant to a vector computation.
struct PPCVSXSwapEntry {
  // Pointer to the instruction.
  MachineInstr *VSEMI;

  // Unique ID (position in the swap vector).
  int VSEId;

  // Attributes of this node.
  unsigned int IsLoad : 1;
  unsigned int IsStore : 1;
  unsigned int IsSwap : 1;
  unsigned int MentionsPhysVR : 1;
  unsigned int HasImplicitSubreg : 1;
  unsigned int IsSwappable : 1;
  unsigned int SpecialHandling : 3;
  unsigned int WebRejected : 1;
  unsigned int WillRemove : 1;
};

enum SHValues {
  SH_NONE = 0,
  SH_EXTRACT,
  SH_INSERT,
  SH_NOSWAP_LD,
  SH_NOSWAP_ST,
  SH_SPLAT
};

struct PPCVSXSwapRemoval : public MachineFunctionPass {

  static char ID;
  const PPCInstrInfo *TII;
  MachineFunction *MF;
  MachineRegisterInfo *MRI;

  // Swap entries are allocated in a vector for better performance.
  std::vector<PPCVSXSwapEntry> SwapVector;

  // A mapping is maintained between machine instructions and
  // their swap entries.  The key is the address of the MI.
  DenseMap<MachineInstr*, int> SwapMap;

  // Equivalence classes are used to gather webs of related computation.
  // Swap entries are represented by their VSEId fields.
  EquivalenceClasses<int> *EC;

  PPCVSXSwapRemoval() : MachineFunctionPass(ID) {
    initializePPCVSXSwapRemovalPass(*PassRegistry::getPassRegistry());
  }

private:
  // Initialize data structures.
  void initialize(MachineFunction &MFParm);

  // Walk the machine instructions to gather vector usage information.
  // Return true iff vector mentions are present.
  bool gatherVectorInstructions();

  // Add an entry to the swap vector and swap map.
  int addSwapEntry(MachineInstr *MI, PPCVSXSwapEntry &SwapEntry);

  // Hunt backwards through COPY and SUBREG_TO_REG chains for a
  // source register.  VecIdx indicates the swap vector entry to
  // mark as mentioning a physical register if the search leads
  // to one.
  unsigned lookThruCopyLike(unsigned SrcReg, unsigned VecIdx);

  // Generate equivalence classes for related computations (webs).
  void formWebs();

  // Analyze webs and determine those that cannot be optimized.
  void recordUnoptimizableWebs();

  // Record which swap instructions can be safely removed.
  void markSwapsForRemoval();

  // Remove swaps and update other instructions requiring special
  // handling.  Return true iff any changes are made.
  bool removeSwaps();

  // Update instructions requiring special handling.
  void handleSpecialSwappables(int EntryIdx);

  // Dump a description of the entries in the swap vector.
  void dumpSwapVector();

  // Return true iff the given register is in the given class.
  bool isRegInClass(unsigned Reg, const TargetRegisterClass *RC) {
    if (TargetRegisterInfo::isVirtualRegister(Reg))
      return RC->hasSubClassEq(MRI->getRegClass(Reg));
    if (RC->contains(Reg))
      return true;
    return false;
  }

  // Return true iff the given register is a full vector register.
  bool isVecReg(unsigned Reg) {
    return (isRegInClass(Reg, &PPC::VSRCRegClass) ||
            isRegInClass(Reg, &PPC::VRRCRegClass));
  }

public:
  // Main entry point for this pass.
  bool runOnMachineFunction(MachineFunction &MF) override {
    // If we don't have VSX on the subtarget, don't do anything.
    const PPCSubtarget &STI = MF.getSubtarget<PPCSubtarget>();
    if (!STI.hasVSX())
      return false;

    bool Changed = false;
    initialize(MF);

    if (gatherVectorInstructions()) {
      formWebs();
      recordUnoptimizableWebs();
      markSwapsForRemoval();
      Changed = removeSwaps();
    }

    // FIXME: See the allocation of EC in initialize().
    delete EC;
    return Changed;
  }
};

// Initialize data structures for this pass.  In particular, clear the
// swap vector and allocate the equivalence class mapping before
// processing each function.
void PPCVSXSwapRemoval::initialize(MachineFunction &MFParm) {
  MF = &MFParm;
  MRI = &MF->getRegInfo();
  TII = static_cast<const PPCInstrInfo*>(MF->getSubtarget().getInstrInfo());

  // An initial vector size of 256 appears to work well in practice.
  // Small/medium functions with vector content tend not to incur a
  // reallocation at this size.  Three of the vector tests in
  // projects/test-suite reallocate, which seems like a reasonable rate.
  const int InitialVectorSize(256);
  SwapVector.clear();
  SwapVector.reserve(InitialVectorSize);

  // FIXME: Currently we allocate EC each time because we don't have
  // access to the set representation on which to call clear().  Should
  // consider adding a clear() method to the EquivalenceClasses class.
  EC = new EquivalenceClasses<int>;
}

// Create an entry in the swap vector for each instruction that mentions
// a full vector register, recording various characteristics of the
// instructions there.
bool PPCVSXSwapRemoval::gatherVectorInstructions() {
  bool RelevantFunction = false;

  for (MachineBasicBlock &MBB : *MF) {
    for (MachineInstr &MI : MBB) {

      bool RelevantInstr = false;
      bool ImplicitSubreg = false;

      for (const MachineOperand &MO : MI.operands()) {
        if (!MO.isReg())
          continue;
        unsigned Reg = MO.getReg();
        if (isVecReg(Reg)) {
          RelevantInstr = true;
          if (MO.getSubReg() != 0)
            ImplicitSubreg = true;
          break;
        }
      }

      if (!RelevantInstr)
        continue;

      RelevantFunction = true;

      // Create a SwapEntry initialized to zeros, then fill in the
      // instruction and ID fields before pushing it to the back
      // of the swap vector.
      PPCVSXSwapEntry SwapEntry{};
      int VecIdx = addSwapEntry(&MI, SwapEntry);

      if (ImplicitSubreg)
        SwapVector[VecIdx].HasImplicitSubreg = 1;

      switch(MI.getOpcode()) {
      default:
        // Unless noted otherwise, an instruction is considered
        // safe for the optimization.  There are a large number of
        // such true-SIMD instructions (all vector math, logical,
        // select, compare, etc.).
        SwapVector[VecIdx].IsSwappable = 1;
        break;
      case PPC::XXPERMDI:
        // This is a swap if it is of the form XXPERMDI t, s, s, 2.
        // Unfortunately, MachineCSE ignores COPY and SUBREG_TO_REG, so we
        // can also see XXPERMDI t, SUBREG_TO_REG(s), SUBREG_TO_REG(s), 2,
        // for example.  We have to look through chains of COPY and
        // SUBREG_TO_REG to find the real source value for comparison.
        // If the real source value is a physical register, then mark the
        // XXPERMDI as mentioning a physical register.
        // Any other form of XXPERMDI is lane-sensitive and unsafe
        // for the optimization.
        if (MI.getOperand(3).getImm() == 2) {
          unsigned trueReg1 = lookThruCopyLike(MI.getOperand(1).getReg(),
                                               VecIdx);
          unsigned trueReg2 = lookThruCopyLike(MI.getOperand(2).getReg(),
                                               VecIdx);
          if (trueReg1 == trueReg2)
            SwapVector[VecIdx].IsSwap = 1;
        }
        break;
      case PPC::LVX:
        // Non-permuting loads are currently unsafe.  We can use special
        // handling for this in the future.  By not marking these as
        // IsSwap, we ensure computations containing them will be rejected
        // for now.
        SwapVector[VecIdx].IsLoad = 1;
        break;
      case PPC::LXVD2X:
      case PPC::LXVW4X:
        // Permuting loads are marked as both load and swap, and are
        // safe for optimization.
        SwapVector[VecIdx].IsLoad = 1;
        SwapVector[VecIdx].IsSwap = 1;
        break;
      case PPC::STVX:
        // Non-permuting stores are currently unsafe.  We can use special
        // handling for this in the future.  By not marking these as
        // IsSwap, we ensure computations containing them will be rejected
        // for now.
        SwapVector[VecIdx].IsStore = 1;
        break;
      case PPC::STXVD2X:
      case PPC::STXVW4X:
        // Permuting stores are marked as both store and swap, and are
        // safe for optimization.
        SwapVector[VecIdx].IsStore = 1;
        SwapVector[VecIdx].IsSwap = 1;
        break;
      case PPC::SUBREG_TO_REG:
        // These are fine provided they are moving between full vector
        // register classes.  For example, the VRs are a subset of the
        // VSRs, but each VR and each VSR is a full 128-bit register.
        if (isVecReg(MI.getOperand(0).getReg()) &&
            isVecReg(MI.getOperand(2).getReg()))
          SwapVector[VecIdx].IsSwappable = 1;
        break;
      case PPC::COPY:
        // These are fine provided they are moving between full vector
        // register classes.
        if (isVecReg(MI.getOperand(0).getReg()) &&
            isVecReg(MI.getOperand(1).getReg()))
          SwapVector[VecIdx].IsSwappable = 1;
        break;
      case PPC::VSPLTB:
      case PPC::VSPLTH:
      case PPC::VSPLTW:
        // Splats are lane-sensitive, but we can use special handling
        // to adjust the source lane for the splat.  This is not yet
        // implemented.  When it is, we need to uncomment the following:
        SwapVector[VecIdx].IsSwappable = 1;
        SwapVector[VecIdx].SpecialHandling = SHValues::SH_SPLAT;
        break;
      // The presence of the following lane-sensitive operations in a
      // web will kill the optimization, at least for now.  For these
      // we do nothing, causing the optimization to fail.
      // FIXME: Some of these could be permitted with special handling,
      // and will be phased in as time permits.
      // FIXME: There is no simple and maintainable way to express a set
      // of opcodes having a common attribute in TableGen.  Should this
      // change, this is a prime candidate to use such a mechanism.
      case PPC::INLINEASM:
      case PPC::EXTRACT_SUBREG:
      case PPC::INSERT_SUBREG:
      case PPC::COPY_TO_REGCLASS:
      case PPC::LVEBX:
      case PPC::LVEHX:
      case PPC::LVEWX:
      case PPC::LVSL:
      case PPC::LVSR:
      case PPC::LVXL:
      case PPC::LXVDSX:
      case PPC::STVEBX:
      case PPC::STVEHX:
      case PPC::STVEWX:
      case PPC::STVXL:
      case PPC::STXSDX:
      case PPC::VCIPHER:
      case PPC::VCIPHERLAST:
      case PPC::VMRGHB:
      case PPC::VMRGHH:
      case PPC::VMRGHW:
      case PPC::VMRGLB:
      case PPC::VMRGLH:
      case PPC::VMRGLW:
      case PPC::VMULESB:
      case PPC::VMULESH:
      case PPC::VMULESW:
      case PPC::VMULEUB:
      case PPC::VMULEUH:
      case PPC::VMULEUW:
      case PPC::VMULOSB:
      case PPC::VMULOSH:
      case PPC::VMULOSW:
      case PPC::VMULOUB:
      case PPC::VMULOUH:
      case PPC::VMULOUW:
      case PPC::VNCIPHER:
      case PPC::VNCIPHERLAST:
      case PPC::VPERM:
      case PPC::VPERMXOR:
      case PPC::VPKPX:
      case PPC::VPKSHSS:
      case PPC::VPKSHUS:
      case PPC::VPKSDSS:
      case PPC::VPKSDUS:
      case PPC::VPKSWSS:
      case PPC::VPKSWUS:
      case PPC::VPKUDUM:
      case PPC::VPKUDUS:
      case PPC::VPKUHUM:
      case PPC::VPKUHUS:
      case PPC::VPKUWUM:
      case PPC::VPKUWUS:
      case PPC::VPMSUMB:
      case PPC::VPMSUMD:
      case PPC::VPMSUMH:
      case PPC::VPMSUMW:
      case PPC::VRLB:
      case PPC::VRLD:
      case PPC::VRLH:
      case PPC::VRLW:
      case PPC::VSBOX:
      case PPC::VSHASIGMAD:
      case PPC::VSHASIGMAW:
      case PPC::VSL:
      case PPC::VSLDOI:
      case PPC::VSLO:
      case PPC::VSR:
      case PPC::VSRO:
      case PPC::VSUM2SWS:
      case PPC::VSUM4SBS:
      case PPC::VSUM4SHS:
      case PPC::VSUM4UBS:
      case PPC::VSUMSWS:
      case PPC::VUPKHPX:
      case PPC::VUPKHSB:
      case PPC::VUPKHSH:
      case PPC::VUPKHSW:
      case PPC::VUPKLPX:
      case PPC::VUPKLSB:
      case PPC::VUPKLSH:
      case PPC::VUPKLSW:
      case PPC::XXMRGHW:
      case PPC::XXMRGLW:
      case PPC::XXSPLTW:
        break;
      }
    }
  }

  if (RelevantFunction) {
    DEBUG(dbgs() << "Swap vector when first built\n\n");
    dumpSwapVector();
  }

  return RelevantFunction;
}

// Add an entry to the swap vector and swap map, and make a
// singleton equivalence class for the entry.
int PPCVSXSwapRemoval::addSwapEntry(MachineInstr *MI,
                                  PPCVSXSwapEntry& SwapEntry) {
  SwapEntry.VSEMI = MI;
  SwapEntry.VSEId = SwapVector.size();
  SwapVector.push_back(SwapEntry);
  EC->insert(SwapEntry.VSEId);
  SwapMap[MI] = SwapEntry.VSEId;
  return SwapEntry.VSEId;
}

// This is used to find the "true" source register for an
// XXPERMDI instruction, since MachineCSE does not handle the
// "copy-like" operations (Copy and SubregToReg).  Returns
// the original SrcReg unless it is the target of a copy-like
// operation, in which case we chain backwards through all
// such operations to the ultimate source register.  If a
// physical register is encountered, we stop the search and
// flag the swap entry indicated by VecIdx (the original
// XXPERMDI) as mentioning a physical register.  Similarly
// for implicit subregister mentions (which should never
// happen).
unsigned PPCVSXSwapRemoval::lookThruCopyLike(unsigned SrcReg,
                                             unsigned VecIdx) {
  MachineInstr *MI = MRI->getVRegDef(SrcReg);
  if (!MI->isCopyLike())
    return SrcReg;

  unsigned CopySrcReg, CopySrcSubreg;
  if (MI->isCopy()) {
    CopySrcReg = MI->getOperand(1).getReg();
    CopySrcSubreg = MI->getOperand(1).getSubReg();
  } else {
    assert(MI->isSubregToReg() && "bad opcode for lookThruCopyLike");
    CopySrcReg = MI->getOperand(2).getReg();
    CopySrcSubreg = MI->getOperand(2).getSubReg();
  }

  if (!TargetRegisterInfo::isVirtualRegister(CopySrcReg)) {
    SwapVector[VecIdx].MentionsPhysVR = 1;
    return CopySrcReg;
  }

  if (CopySrcSubreg != 0) {
    SwapVector[VecIdx].HasImplicitSubreg = 1;
    return CopySrcReg;
  }

  return lookThruCopyLike(CopySrcReg, VecIdx);
}

// Generate equivalence classes for related computations (webs) by
// def-use relationships of virtual registers.  Mention of a physical
// register terminates the generation of equivalence classes as this
// indicates a use of a parameter, definition of a return value, use
// of a value returned from a call, or definition of a parameter to a
// call.  Computations with physical register mentions are flagged
// as such so their containing webs will not be optimized.
void PPCVSXSwapRemoval::formWebs() {

  DEBUG(dbgs() << "\n*** Forming webs for swap removal ***\n\n");

  for (unsigned EntryIdx = 0; EntryIdx < SwapVector.size(); ++EntryIdx) {

    MachineInstr *MI = SwapVector[EntryIdx].VSEMI;

    DEBUG(dbgs() << "\n" << SwapVector[EntryIdx].VSEId << " ");
    DEBUG(MI->dump());

    // It's sufficient to walk vector uses and join them to their unique
    // definitions.  In addition, check *all* vector register operands
    // for physical regs.
    for (const MachineOperand &MO : MI->operands()) {
      if (!MO.isReg())
        continue;

      unsigned Reg = MO.getReg();
      if (!isVecReg(Reg))
        continue;

      if (!TargetRegisterInfo::isVirtualRegister(Reg)) {
        SwapVector[EntryIdx].MentionsPhysVR = 1;
        continue;
      }

      if (!MO.isUse())
        continue;

      MachineInstr* DefMI = MRI->getVRegDef(Reg);
      assert(SwapMap.find(DefMI) != SwapMap.end() &&
             "Inconsistency: def of vector reg not found in swap map!");
      int DefIdx = SwapMap[DefMI];
      (void)EC->unionSets(SwapVector[DefIdx].VSEId,
                          SwapVector[EntryIdx].VSEId);

      DEBUG(dbgs() << format("Unioning %d with %d\n", SwapVector[DefIdx].VSEId,
                             SwapVector[EntryIdx].VSEId));
      DEBUG(dbgs() << "  Def: ");
      DEBUG(DefMI->dump());
    }
  }
}

// Walk the swap vector entries looking for conditions that prevent their
// containing computations from being optimized.  When such conditions are
// found, mark the representative of the computation's equivalence class
// as rejected.
void PPCVSXSwapRemoval::recordUnoptimizableWebs() {

  DEBUG(dbgs() << "\n*** Rejecting webs for swap removal ***\n\n");

  for (unsigned EntryIdx = 0; EntryIdx < SwapVector.size(); ++EntryIdx) {
    int Repr = EC->getLeaderValue(SwapVector[EntryIdx].VSEId);

    // Reject webs containing mentions of physical registers or implicit
    // subregs, or containing operations that we don't know how to handle
    // in a lane-permuted region.
    if (SwapVector[EntryIdx].MentionsPhysVR ||
        SwapVector[EntryIdx].HasImplicitSubreg ||
        !(SwapVector[EntryIdx].IsSwappable || SwapVector[EntryIdx].IsSwap)) {

      SwapVector[Repr].WebRejected = 1;

      DEBUG(dbgs() <<
            format("Web %d rejected for physreg, subreg, or not swap[pable]\n",
                   Repr));
      DEBUG(dbgs() << "  in " << EntryIdx << ": ");
      DEBUG(SwapVector[EntryIdx].VSEMI->dump());
      DEBUG(dbgs() << "\n");
    }

    // Reject webs than contain swapping loads that feed something other
    // than a swap instruction.
    else if (SwapVector[EntryIdx].IsLoad && SwapVector[EntryIdx].IsSwap) {
      MachineInstr *MI = SwapVector[EntryIdx].VSEMI;
      unsigned DefReg = MI->getOperand(0).getReg();

      // We skip debug instructions in the analysis.  (Note that debug
      // location information is still maintained by this optimization
      // because it remains on the LXVD2X and STXVD2X instructions after
      // the XXPERMDIs are removed.)
      for (MachineInstr &UseMI : MRI->use_nodbg_instructions(DefReg)) {
        int UseIdx = SwapMap[&UseMI];

        if (!SwapVector[UseIdx].IsSwap || SwapVector[UseIdx].IsLoad ||
            SwapVector[UseIdx].IsStore) {

          SwapVector[Repr].WebRejected = 1;

          DEBUG(dbgs() <<
                format("Web %d rejected for load not feeding swap\n", Repr));
          DEBUG(dbgs() << "  def " << EntryIdx << ": ");
          DEBUG(MI->dump());
          DEBUG(dbgs() << "  use " << UseIdx << ": ");
          DEBUG(UseMI.dump());
          DEBUG(dbgs() << "\n");
        }
      }

    // Reject webs than contain swapping stores that are fed by something
    // other than a swap instruction.
    } else if (SwapVector[EntryIdx].IsStore && SwapVector[EntryIdx].IsSwap) {
      MachineInstr *MI = SwapVector[EntryIdx].VSEMI;
      unsigned UseReg = MI->getOperand(0).getReg();
      MachineInstr *DefMI = MRI->getVRegDef(UseReg);
      int DefIdx = SwapMap[DefMI];

      if (!SwapVector[DefIdx].IsSwap || SwapVector[DefIdx].IsLoad ||
          SwapVector[DefIdx].IsStore) {

        SwapVector[Repr].WebRejected = 1;

        DEBUG(dbgs() <<
              format("Web %d rejected for store not fed by swap\n", Repr));
        DEBUG(dbgs() << "  def " << DefIdx << ": ");
        DEBUG(DefMI->dump());
        DEBUG(dbgs() << "  use " << EntryIdx << ": ");
        DEBUG(MI->dump());
        DEBUG(dbgs() << "\n");
      }
    }
  }

  DEBUG(dbgs() << "Swap vector after web analysis:\n\n");
  dumpSwapVector();
}

// Walk the swap vector entries looking for swaps fed by permuting loads
// and swaps that feed permuting stores.  If the containing computation
// has not been marked rejected, mark each such swap for removal.
// (Removal is delayed in case optimization has disturbed the pattern,
// such that multiple loads feed the same swap, etc.)
void PPCVSXSwapRemoval::markSwapsForRemoval() {

  DEBUG(dbgs() << "\n*** Marking swaps for removal ***\n\n");

  for (unsigned EntryIdx = 0; EntryIdx < SwapVector.size(); ++EntryIdx) {

    if (SwapVector[EntryIdx].IsLoad && SwapVector[EntryIdx].IsSwap) {
      int Repr = EC->getLeaderValue(SwapVector[EntryIdx].VSEId);

      if (!SwapVector[Repr].WebRejected) {
        MachineInstr *MI = SwapVector[EntryIdx].VSEMI;
        unsigned DefReg = MI->getOperand(0).getReg();

        for (MachineInstr &UseMI : MRI->use_nodbg_instructions(DefReg)) {
          int UseIdx = SwapMap[&UseMI];
          SwapVector[UseIdx].WillRemove = 1;

          DEBUG(dbgs() << "Marking swap fed by load for removal: ");
          DEBUG(UseMI.dump());
        }
      }

    } else if (SwapVector[EntryIdx].IsStore && SwapVector[EntryIdx].IsSwap) {
      int Repr = EC->getLeaderValue(SwapVector[EntryIdx].VSEId);

      if (!SwapVector[Repr].WebRejected) {
        MachineInstr *MI = SwapVector[EntryIdx].VSEMI;
        unsigned UseReg = MI->getOperand(0).getReg();
        MachineInstr *DefMI = MRI->getVRegDef(UseReg);
        int DefIdx = SwapMap[DefMI];
        SwapVector[DefIdx].WillRemove = 1;

        DEBUG(dbgs() << "Marking swap feeding store for removal: ");
        DEBUG(DefMI->dump());
      }

    } else if (SwapVector[EntryIdx].IsSwappable &&
               SwapVector[EntryIdx].SpecialHandling != 0) {
      int Repr = EC->getLeaderValue(SwapVector[EntryIdx].VSEId);

      if (!SwapVector[Repr].WebRejected)
        handleSpecialSwappables(EntryIdx);
    }
  }
}

// The identified swap entry requires special handling to allow its
// containing computation to be optimized.  Perform that handling
// here.
// FIXME: This code is to be phased in with subsequent patches.
void PPCVSXSwapRemoval::handleSpecialSwappables(int EntryIdx) {
  switch (SwapVector[EntryIdx].SpecialHandling) {

  default:
    assert(false && "Unexpected special handling type");
    break;

  // For splats based on an index into a vector, add N/2 modulo N
  // to the index, where N is the number of vector elements.
  case SHValues::SH_SPLAT: {
    MachineInstr *MI = SwapVector[EntryIdx].VSEMI;
    unsigned NElts;

    DEBUG(dbgs() << "Changing splat: ");
    DEBUG(MI->dump());

    switch (MI->getOpcode()) {
    default:
      assert(false && "Unexpected splat opcode");
    case PPC::VSPLTB: NElts = 16; break;
    case PPC::VSPLTH: NElts = 8;  break;
    case PPC::VSPLTW: NElts = 4;  break;
    }

    unsigned EltNo = MI->getOperand(1).getImm();
    EltNo = (EltNo + NElts / 2) % NElts;
    MI->getOperand(1).setImm(EltNo);

    DEBUG(dbgs() << "  Into: ");
    DEBUG(MI->dump());
    break;
  }

  }
}

// Walk the swap vector and replace each entry marked for removal with
// a copy operation.
bool PPCVSXSwapRemoval::removeSwaps() {

  DEBUG(dbgs() << "\n*** Removing swaps ***\n\n");

  bool Changed = false;

  for (unsigned EntryIdx = 0; EntryIdx < SwapVector.size(); ++EntryIdx) {
    if (SwapVector[EntryIdx].WillRemove) {
      Changed = true;
      MachineInstr *MI = SwapVector[EntryIdx].VSEMI;
      MachineBasicBlock *MBB = MI->getParent();
      BuildMI(*MBB, MI, MI->getDebugLoc(),
              TII->get(TargetOpcode::COPY), MI->getOperand(0).getReg())
        .addOperand(MI->getOperand(1));

      DEBUG(dbgs() << format("Replaced %d with copy: ",
                             SwapVector[EntryIdx].VSEId));
      DEBUG(MI->dump());

      MI->eraseFromParent();
    }
  }

  return Changed;
}

// For debug purposes, dump the contents of the swap vector.
void PPCVSXSwapRemoval::dumpSwapVector() {

  for (unsigned EntryIdx = 0; EntryIdx < SwapVector.size(); ++EntryIdx) {

    MachineInstr *MI = SwapVector[EntryIdx].VSEMI;
    int ID = SwapVector[EntryIdx].VSEId;

    DEBUG(dbgs() << format("%6d", ID));
    DEBUG(dbgs() << format("%6d", EC->getLeaderValue(ID)));
    DEBUG(dbgs() << format(" BB#%3d", MI->getParent()->getNumber()));
    DEBUG(dbgs() << format("  %14s  ", TII->getName(MI->getOpcode())));

    if (SwapVector[EntryIdx].IsLoad)
      DEBUG(dbgs() << "load ");
    if (SwapVector[EntryIdx].IsStore)
      DEBUG(dbgs() << "store ");
    if (SwapVector[EntryIdx].IsSwap)
      DEBUG(dbgs() << "swap ");
    if (SwapVector[EntryIdx].MentionsPhysVR)
      DEBUG(dbgs() << "physreg ");
    if (SwapVector[EntryIdx].HasImplicitSubreg)
      DEBUG(dbgs() << "implsubreg ");

    if (SwapVector[EntryIdx].IsSwappable) {
      DEBUG(dbgs() << "swappable ");
      switch(SwapVector[EntryIdx].SpecialHandling) {
      default:
        DEBUG(dbgs() << "special:**unknown**");
        break;
      case SH_NONE:
        break;
      case SH_EXTRACT:
        DEBUG(dbgs() << "special:extract ");
        break;
      case SH_INSERT:
        DEBUG(dbgs() << "special:insert ");
        break;
      case SH_NOSWAP_LD:
        DEBUG(dbgs() << "special:load ");
        break;
      case SH_NOSWAP_ST:
        DEBUG(dbgs() << "special:store ");
        break;
      case SH_SPLAT:
        DEBUG(dbgs() << "special:splat ");
        break;
      }
    }

    if (SwapVector[EntryIdx].WebRejected)
      DEBUG(dbgs() << "rejected ");
    if (SwapVector[EntryIdx].WillRemove)
      DEBUG(dbgs() << "remove ");

    DEBUG(dbgs() << "\n");

    // For no-asserts builds.
    (void)MI;
    (void)ID;
  }

  DEBUG(dbgs() << "\n");
}

} // end default namespace

INITIALIZE_PASS_BEGIN(PPCVSXSwapRemoval, DEBUG_TYPE,
                      "PowerPC VSX Swap Removal", false, false)
INITIALIZE_PASS_END(PPCVSXSwapRemoval, DEBUG_TYPE,
                    "PowerPC VSX Swap Removal", false, false)

char PPCVSXSwapRemoval::ID = 0;
FunctionPass*
llvm::createPPCVSXSwapRemovalPass() { return new PPCVSXSwapRemoval(); }