Combine the implementations of the core part of the SSAUpdater and

MachineSSAUpdater to avoid duplicating all the code.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@103060 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 109 insertions, 437 deletions

diff --git a/lib/CodeGen/MachineSSAUpdater.cpp b/lib/CodeGen/MachineSSAUpdater.cpp
index b8996d46f9..99fb99ac3c 100644
--- a/lib/CodeGen/MachineSSAUpdater.cpp
+++ b/lib/CodeGen/MachineSSAUpdater.cpp
@@ -26,39 +26,17 @@
 #include "llvm/Support/Debug.h"
 #include "llvm/Support/ErrorHandling.h"
 #include "llvm/Support/raw_ostream.h"
+#include "llvm/Transforms/Utils/SSAUpdaterImpl.h"
 using namespace llvm;
 
-/// BBInfo - Per-basic block information used internally by MachineSSAUpdater.
-class MachineSSAUpdater::BBInfo {
-public:
-  MachineBasicBlock *BB; // Back-pointer to the corresponding block.
-  unsigned AvailableVal; // Value to use in this block.
-  BBInfo *DefBB;         // Block that defines the available value.
-  int BlkNum;            // Postorder number.
-  BBInfo *IDom;          // Immediate dominator.
-  unsigned NumPreds;     // Number of predecessor blocks.
-  BBInfo **Preds;        // Array[NumPreds] of predecessor blocks.
-  MachineInstr *PHITag;  // Marker for existing PHIs that match.
-
-  BBInfo(MachineBasicBlock *ThisBB, unsigned V)
-    : BB(ThisBB), AvailableVal(V), DefBB(V ? this : 0), BlkNum(0), IDom(0),
-      NumPreds(0), Preds(0), PHITag(0) { }
-};
-
-typedef DenseMap<MachineBasicBlock*, MachineSSAUpdater::BBInfo*> BBMapTy;
-
 typedef DenseMap<MachineBasicBlock*, unsigned> AvailableValsTy;
 static AvailableValsTy &getAvailableVals(void *AV) {
   return *static_cast<AvailableValsTy*>(AV);
 }
 
-static BBMapTy *getBBMap(void *BM) {
-  return static_cast<BBMapTy*>(BM);
-}
-
 MachineSSAUpdater::MachineSSAUpdater(MachineFunction &MF,
                                      SmallVectorImpl<MachineInstr*> *NewPHI)
-  : AV(0), BM(0), InsertedPHIs(NewPHI) {
+  : AV(0), InsertedPHIs(NewPHI) {
   TII = MF.getTarget().getInstrInfo();
   MRI = &MF.getRegInfo();
 }
@@ -134,7 +112,8 @@ static
 MachineInstr *InsertNewDef(unsigned Opcode,
                            MachineBasicBlock *BB, MachineBasicBlock::iterator I,
                            const TargetRegisterClass *RC,
-                           MachineRegisterInfo *MRI, const TargetInstrInfo *TII) {
+                           MachineRegisterInfo *MRI,
+                           const TargetInstrInfo *TII) {
   unsigned NewVR = MRI->createVirtualRegister(RC);
   return BuildMI(*BB, I, DebugLoc(), TII->get(Opcode), NewVR);
 }
@@ -263,438 +242,131 @@ void MachineSSAUpdater::ReplaceRegWith(unsigned OldReg, unsigned NewReg) {
       I->second = NewReg;
 }
 
-/// GetValueAtEndOfBlockInternal - Check to see if AvailableVals has an entry
-/// for the specified BB and if so, return it.  If not, construct SSA form by
-/// first calculating the required placement of PHIs and then inserting new
-/// PHIs where needed.
-unsigned MachineSSAUpdater::GetValueAtEndOfBlockInternal(MachineBasicBlock *BB){
-  AvailableValsTy &AvailableVals = getAvailableVals(AV);
-  if (unsigned V = AvailableVals[BB])
-    return V;
+/// MachinePHIiter - Iterator for PHI operands.  This is used for the
+/// PHI_iterator in the SSAUpdaterImpl template.
+namespace {
+  class MachinePHIiter {
+  private:
+    MachineInstr *PHI;
+    unsigned idx;
+ 
+  public:
+    explicit MachinePHIiter(MachineInstr *P) // begin iterator
+      : PHI(P), idx(1) {}
+    MachinePHIiter(MachineInstr *P, bool) // end iterator
+      : PHI(P), idx(PHI->getNumOperands()) {}
+
+    MachinePHIiter &operator++() { idx += 2; return *this; } 
+    bool operator==(const MachinePHIiter& x) const { return idx == x.idx; }
+    bool operator!=(const MachinePHIiter& x) const { return !operator==(x); }
+    unsigned getIncomingValue() { return PHI->getOperand(idx).getReg(); }
+    MachineBasicBlock *getIncomingBlock() {
+      return PHI->getOperand(idx+1).getMBB();
+    }
+  };
+}
 
-  // Pool allocation used internally by GetValueAtEndOfBlock.
-  BumpPtrAllocator Allocator;
-  BBMapTy BBMapObj;
-  BM = &BBMapObj;
+/// SSAUpdaterTraits<MachineSSAUpdater> - Traits for the SSAUpdaterImpl
+/// template, specialized for MachineSSAUpdater.
+namespace llvm {
+template<>
+class SSAUpdaterTraits<MachineSSAUpdater> {
+public:
+  typedef MachineBasicBlock BlkT;
+  typedef unsigned ValT;
+  typedef MachineInstr PhiT;
+
+  typedef MachineBasicBlock::succ_iterator BlkSucc_iterator;
+  static BlkSucc_iterator BlkSucc_begin(BlkT *BB) { return BB->succ_begin(); }
+  static BlkSucc_iterator BlkSucc_end(BlkT *BB) { return BB->succ_end(); }
+
+  typedef MachinePHIiter PHI_iterator;
+  static inline PHI_iterator PHI_begin(PhiT *PHI) { return PHI_iterator(PHI); }
+  static inline PHI_iterator PHI_end(PhiT *PHI) {
+    return PHI_iterator(PHI, true);
+  }
 
-  SmallVector<BBInfo*, 100> BlockList;
-  BuildBlockList(BB, &BlockList, &Allocator);
+  /// FindPredecessorBlocks - Put the predecessors of BB into the Preds
+  /// vector.
+  static void FindPredecessorBlocks(MachineBasicBlock *BB,
+                                    SmallVectorImpl<MachineBasicBlock*> *Preds){
+    for (MachineBasicBlock::pred_iterator PI = BB->pred_begin(),
+           E = BB->pred_end(); PI != E; ++PI)
+      Preds->push_back(*PI);
+  }
 
-  // Special case: bail out if BB is unreachable.
-  if (BlockList.size() == 0) {
-    BM = 0;
+  /// GetUndefVal - Create an IMPLICIT_DEF instruction with a new register.
+  /// Add it into the specified block and return the register.
+  static unsigned GetUndefVal(MachineBasicBlock *BB,
+                              MachineSSAUpdater *Updater) {
     // Insert an implicit_def to represent an undef value.
     MachineInstr *NewDef = InsertNewDef(TargetOpcode::IMPLICIT_DEF,
                                         BB, BB->getFirstTerminator(),
-                                        VRC, MRI, TII);
-    unsigned V = NewDef->getOperand(0).getReg();
-    AvailableVals[BB] = V;
-    return V;
-  }
-
-  FindDominators(&BlockList);
-  FindPHIPlacement(&BlockList);
-  FindAvailableVals(&BlockList);
-
-  BM = 0;
-  return BBMapObj[BB]->DefBB->AvailableVal;
-}
-
-/// FindPredecessorBlocks - Put the predecessors of Info->BB into the Preds
-/// vector, set Info->NumPreds, and allocate space in Info->Preds.
-static void FindPredecessorBlocks(MachineSSAUpdater::BBInfo *Info,
-                                  SmallVectorImpl<MachineBasicBlock*> *Preds,
-                                  BumpPtrAllocator *Allocator) {
-  MachineBasicBlock *BB = Info->BB;
-  for (MachineBasicBlock::pred_iterator PI = BB->pred_begin(),
-         E = BB->pred_end(); PI != E; ++PI)
-    Preds->push_back(*PI);
-
-  Info->NumPreds = Preds->size();
-  Info->Preds = static_cast<MachineSSAUpdater::BBInfo**>
-    (Allocator->Allocate(Info->NumPreds * sizeof(MachineSSAUpdater::BBInfo*),
-                         AlignOf<MachineSSAUpdater::BBInfo*>::Alignment));
-}
-
-/// BuildBlockList - Starting from the specified basic block, traverse back
-/// through its predecessors until reaching blocks with known values.  Create
-/// BBInfo structures for the blocks and append them to the block list.
-void MachineSSAUpdater::BuildBlockList(MachineBasicBlock *BB,
-                                       BlockListTy *BlockList,
-                                       BumpPtrAllocator *Allocator) {
-  AvailableValsTy &AvailableVals = getAvailableVals(AV);
-  BBMapTy *BBMap = getBBMap(BM);
-  SmallVector<BBInfo*, 10> RootList;
-  SmallVector<BBInfo*, 64> WorkList;
-
-  BBInfo *Info = new (*Allocator) BBInfo(BB, 0);
-  (*BBMap)[BB] = Info;
-  WorkList.push_back(Info);
-
-  // Search backward from BB, creating BBInfos along the way and stopping when
-  // reaching blocks that define the value.  Record those defining blocks on
-  // the RootList.
-  SmallVector<MachineBasicBlock*, 10> Preds;
-  while (!WorkList.empty()) {
-    Info = WorkList.pop_back_val();
-    Preds.clear();
-    FindPredecessorBlocks(Info, &Preds, Allocator);
-
-    // Treat an unreachable predecessor as a definition with 'undef'.
-    if (Info->NumPreds == 0) {
-      // Insert an implicit_def to represent an undef value.
-      MachineInstr *NewDef = InsertNewDef(TargetOpcode::IMPLICIT_DEF,
-                                          Info->BB,
-                                          Info->BB->getFirstTerminator(),
-                                          VRC, MRI, TII);
-      Info->AvailableVal = NewDef->getOperand(0).getReg();
-      Info->DefBB = Info;
-      RootList.push_back(Info);
-      continue;
-    }
-
-    for (unsigned p = 0; p != Info->NumPreds; ++p) {
-      MachineBasicBlock *Pred = Preds[p];
-      // Check if BBMap already has a BBInfo for the predecessor block.
-      BBMapTy::value_type &BBMapBucket = BBMap->FindAndConstruct(Pred);
-      if (BBMapBucket.second) {
-        Info->Preds[p] = BBMapBucket.second;
-        continue;
-      }
-
-      // Create a new BBInfo for the predecessor.
-      unsigned PredVal = AvailableVals.lookup(Pred);
-      BBInfo *PredInfo = new (*Allocator) BBInfo(Pred, PredVal);
-      BBMapBucket.second = PredInfo;
-      Info->Preds[p] = PredInfo;
-
-      if (PredInfo->AvailableVal) {
-        RootList.push_back(PredInfo);
-        continue;
-      }
-      WorkList.push_back(PredInfo);
-    }
+                                        Updater->VRC, Updater->MRI,
+                                        Updater->TII);
+    return NewDef->getOperand(0).getReg();
   }
 
-  // Now that we know what blocks are backwards-reachable from the starting
-  // block, do a forward depth-first traversal to assign postorder numbers
-  // to those blocks.
-  BBInfo *PseudoEntry = new (*Allocator) BBInfo(0, 0);
-  unsigned BlkNum = 1;
-
-  // Initialize the worklist with the roots from the backward traversal.
-  while (!RootList.empty()) {
-    Info = RootList.pop_back_val();
-    Info->IDom = PseudoEntry;
-    Info->BlkNum = -1;
-    WorkList.push_back(Info);
+  /// CreateEmptyPHI - Create a PHI instruction that defines a new register.
+  /// Add it into the specified block and return the register.
+  static unsigned CreateEmptyPHI(MachineBasicBlock *BB, unsigned NumPreds,
+                                 MachineSSAUpdater *Updater) {
+    MachineBasicBlock::iterator Loc = BB->empty() ? BB->end() : BB->front();
+    MachineInstr *PHI = InsertNewDef(TargetOpcode::PHI, BB, Loc,
+                                     Updater->VRC, Updater->MRI,
+                                     Updater->TII);
+    return PHI->getOperand(0).getReg();
   }
 
-  while (!WorkList.empty()) {
-    Info = WorkList.back();
-
-    if (Info->BlkNum == -2) {
-      // All the successors have been handled; assign the postorder number.
-      Info->BlkNum = BlkNum++;
-      // If not a root, put it on the BlockList.
-      if (!Info->AvailableVal)
-        BlockList->push_back(Info);
-      WorkList.pop_back();
-      continue;
-    }
-
-    // Leave this entry on the worklist, but set its BlkNum to mark that its
-    // successors have been put on the worklist.  When it returns to the top
-    // the list, after handling its successors, it will be assigned a number.
-    Info->BlkNum = -2;
-
-    // Add unvisited successors to the work list.
-    for (MachineBasicBlock::succ_iterator SI = Info->BB->succ_begin(),
-           E = Info->BB->succ_end(); SI != E; ++SI) {
-      BBInfo *SuccInfo = (*BBMap)[*SI];
-      if (!SuccInfo || SuccInfo->BlkNum)
-        continue;
-      SuccInfo->BlkNum = -1;
-      WorkList.push_back(SuccInfo);
-    }
+  /// AddPHIOperand - Add the specified value as an operand of the PHI for
+  /// the specified predecessor block.
+  static void AddPHIOperand(MachineInstr *PHI, unsigned Val,
+                            MachineBasicBlock *Pred) {
+    PHI->addOperand(MachineOperand::CreateReg(Val, false));
+    PHI->addOperand(MachineOperand::CreateMBB(Pred));
   }
-  PseudoEntry->BlkNum = BlkNum;
-}
 
-/// IntersectDominators - This is the dataflow lattice "meet" operation for
-/// finding dominators.  Given two basic blocks, it walks up the dominator
-/// tree until it finds a common dominator of both.  It uses the postorder
-/// number of the blocks to determine how to do that.
-static MachineSSAUpdater::BBInfo *
-IntersectDominators(MachineSSAUpdater::BBInfo *Blk1,
-                    MachineSSAUpdater::BBInfo *Blk2) {
-  while (Blk1 != Blk2) {
-    while (Blk1->BlkNum < Blk2->BlkNum) {
-      Blk1 = Blk1->IDom;
-      if (!Blk1)
-        return Blk2;
-    }
-    while (Blk2->BlkNum < Blk1->BlkNum) {
-      Blk2 = Blk2->IDom;
-      if (!Blk2)
-        return Blk1;
-    }
-  }
-  return Blk1;
-}
-
-/// FindDominators - Calculate the dominator tree for the subset of the CFG
-/// corresponding to the basic blocks on the BlockList.  This uses the
-/// algorithm from: "A Simple, Fast Dominance Algorithm" by Cooper, Harvey and
-/// Kennedy, published in Software--Practice and Experience, 2001, 4:1-10.
-/// Because the CFG subset does not include any edges leading into blocks that
-/// define the value, the results are not the usual dominator tree.  The CFG
-/// subset has a single pseudo-entry node with edges to a set of root nodes
-/// for blocks that define the value.  The dominators for this subset CFG are
-/// not the standard dominators but they are adequate for placing PHIs within
-/// the subset CFG.
-void MachineSSAUpdater::FindDominators(BlockListTy *BlockList) {
-  bool Changed;
-  do {
-    Changed = false;
-    // Iterate over the list in reverse order, i.e., forward on CFG edges.
-    for (BlockListTy::reverse_iterator I = BlockList->rbegin(),
-           E = BlockList->rend(); I != E; ++I) {
-      BBInfo *Info = *I;
-
-      // Start with the first predecessor.
-      assert(Info->NumPreds > 0 && "unreachable block");
-      BBInfo *NewIDom = Info->Preds[0];
-
-      // Iterate through the block's other predecessors.
-      for (unsigned p = 1; p != Info->NumPreds; ++p) {
-        BBInfo *Pred = Info->Preds[p];
-        NewIDom = IntersectDominators(NewIDom, Pred);
-      }
-
-      // Check if the IDom value has changed.
-      if (NewIDom != Info->IDom) {
-        Info->IDom = NewIDom;
-        Changed = true;
-      }
-    }
-  } while (Changed);
-}
-
-/// IsDefInDomFrontier - Search up the dominator tree from Pred to IDom for
-/// any blocks containing definitions of the value.  If one is found, then the
-/// successor of Pred is in the dominance frontier for the definition, and
-/// this function returns true.
-static bool IsDefInDomFrontier(const MachineSSAUpdater::BBInfo *Pred,
-                               const MachineSSAUpdater::BBInfo *IDom) {
-  for (; Pred != IDom; Pred = Pred->IDom) {
-    if (Pred->DefBB == Pred)
-      return true;
+  /// InstrIsPHI - Check if an instruction is a PHI.
+  ///
+  static MachineInstr *InstrIsPHI(MachineInstr *I) {
+    if (I->isPHI())
+      return I;
+    return 0;
   }
-  return false;
-}
-
-/// FindPHIPlacement - PHIs are needed in the iterated dominance frontiers of
-/// the known definitions.  Iteratively add PHIs in the dom frontiers until
-/// nothing changes.  Along the way, keep track of the nearest dominating
-/// definitions for non-PHI blocks.
-void MachineSSAUpdater::FindPHIPlacement(BlockListTy *BlockList) {
-  bool Changed;
-  do {
-    Changed = false;
-    // Iterate over the list in reverse order, i.e., forward on CFG edges.
-    for (BlockListTy::reverse_iterator I = BlockList->rbegin(),
-           E = BlockList->rend(); I != E; ++I) {
-      BBInfo *Info = *I;
-
-      // If this block already needs a PHI, there is nothing to do here.
-      if (Info->DefBB == Info)
-        continue;
-
-      // Default to use the same def as the immediate dominator.
-      BBInfo *NewDefBB = Info->IDom->DefBB;
-      for (unsigned p = 0; p != Info->NumPreds; ++p) {
-        if (IsDefInDomFrontier(Info->Preds[p], Info->IDom)) {
-          // Need a PHI here.
-          NewDefBB = Info;
-          break;
-        }
-      }
 
-      // Check if anything changed.
-      if (NewDefBB != Info->DefBB) {
-        Info->DefBB = NewDefBB;
-        Changed = true;
-      }
-    }
-  } while (Changed);
-}
-
-/// FindAvailableVal - If this block requires a PHI, first check if an existing
-/// PHI matches the PHI placement and reaching definitions computed earlier,
-/// and if not, create a new PHI.  Visit all the block's predecessors to
-/// calculate the available value for each one and fill in the incoming values
-/// for a new PHI.
-void MachineSSAUpdater::FindAvailableVals(BlockListTy *BlockList) {
-  AvailableValsTy &AvailableVals = getAvailableVals(AV);
-
-  // Go through the worklist in forward order (i.e., backward through the CFG)
-  // and check if existing PHIs can be used.  If not, create empty PHIs where
-  // they are needed.
-  for (BlockListTy::iterator I = BlockList->begin(), E = BlockList->end();
-       I != E; ++I) {
-    BBInfo *Info = *I;
-    // Check if there needs to be a PHI in BB.
-    if (Info->DefBB != Info)
-      continue;
-
-    // Look for an existing PHI.
-    FindExistingPHI(Info->BB, BlockList);
-    if (Info->AvailableVal)
-      continue;
-
-    MachineBasicBlock::iterator Loc =
-      Info->BB->empty() ? Info->BB->end() : Info->BB->front();
-    MachineInstr *InsertedPHI = InsertNewDef(TargetOpcode::PHI, Info->BB, Loc,
-                                             VRC, MRI, TII);
-    unsigned PHI = InsertedPHI->getOperand(0).getReg();
-    Info->AvailableVal = PHI;
-    AvailableVals[Info->BB] = PHI;
+  /// ValueIsPHI - Check if the instruction that defines the specified register
+  /// is a PHI instruction.
+  static MachineInstr *ValueIsPHI(unsigned Val, MachineSSAUpdater *Updater) {
+    return InstrIsPHI(Updater->MRI->getVRegDef(Val));
   }
 
-  // Now go back through the worklist in reverse order to fill in the arguments
-  // for any new PHIs added in the forward traversal.
-  for (BlockListTy::reverse_iterator I = BlockList->rbegin(),
-         E = BlockList->rend(); I != E; ++I) {
-    BBInfo *Info = *I;
-
-    if (Info->DefBB != Info) {
-      // Record the available value at join nodes to speed up subsequent
-      // uses of this SSAUpdater for the same value.
-      if (Info->NumPreds > 1)
-        AvailableVals[Info->BB] = Info->DefBB->AvailableVal;
-      continue;
-    }
-
-    // Check if this block contains a newly added PHI.
-    unsigned PHI = Info->AvailableVal;
-    MachineInstr *InsertedPHI = MRI->getVRegDef(PHI);
-    if (!InsertedPHI->isPHI() || InsertedPHI->getNumOperands() > 1)
-      continue;
-
-    // Iterate through the block's predecessors.
-    MachineInstrBuilder MIB(InsertedPHI);
-    for (unsigned p = 0; p != Info->NumPreds; ++p) {
-      BBInfo *PredInfo = Info->Preds[p];
-      MachineBasicBlock *Pred = PredInfo->BB;
-      // Skip to the nearest preceding definition.
-      if (PredInfo->DefBB != PredInfo)
-        PredInfo = PredInfo->DefBB;
-      MIB.addReg(PredInfo->AvailableVal).addMBB(Pred);
-    }
-
-    DEBUG(dbgs() << "  Inserted PHI: " << *InsertedPHI << "\n");
-
-    // If the client wants to know about all new instructions, tell it.
-    if (InsertedPHIs) InsertedPHIs->push_back(InsertedPHI);
+  /// ValueIsNewPHI - Like ValueIsPHI but also check if the PHI has no source
+  /// operands, i.e., it was just added.
+  static MachineInstr *ValueIsNewPHI(unsigned Val, MachineSSAUpdater *Updater) {
+    MachineInstr *PHI = ValueIsPHI(Val, Updater);
+    if (PHI && PHI->getNumOperands() <= 1)
+      return PHI;
+    return 0;
   }
-}
 
-/// FindExistingPHI - Look through the PHI nodes in a block to see if any of
-/// them match what is needed.
-void MachineSSAUpdater::FindExistingPHI(MachineBasicBlock *BB,
-                                        BlockListTy *BlockList) {
-  for (MachineBasicBlock::iterator BBI = BB->begin(), BBE = BB->end();
-       BBI != BBE && BBI->isPHI(); ++BBI) {
-    if (CheckIfPHIMatches(BBI)) {
-      RecordMatchingPHI(BBI);
-      break;
-    }
-    // Match failed: clear all the PHITag values.
-    for (BlockListTy::iterator I = BlockList->begin(), E = BlockList->end();
-         I != E; ++I)
-      (*I)->PHITag = 0;
+  /// GetPHIValue - For the specified PHI instruction, return the register
+  /// that it defines.
+  static unsigned GetPHIValue(MachineInstr *PHI) {
+    return PHI->getOperand(0).getReg();
   }
-}
-
-/// CheckIfPHIMatches - Check if a PHI node matches the placement and values
-/// in the BBMap.
-bool MachineSSAUpdater::CheckIfPHIMatches(MachineInstr *PHI) {
-  BBMapTy *BBMap = getBBMap(BM);
-  SmallVector<MachineInstr*, 20> WorkList;
-  WorkList.push_back(PHI);
-
-  // Mark that the block containing this PHI has been visited.
-  (*BBMap)[PHI->getParent()]->PHITag = PHI;
-
-  while (!WorkList.empty()) {
-    PHI = WorkList.pop_back_val();
-
-    // Iterate through the PHI's incoming values.
-    for (unsigned i = 1, e = PHI->getNumOperands(); i != e; i += 2) {
-      unsigned IncomingVal = PHI->getOperand(i).getReg();
-      BBInfo *PredInfo = (*BBMap)[PHI->getOperand(i+1).getMBB()];
-      // Skip to the nearest preceding definition.
-      if (PredInfo->DefBB != PredInfo)
-        PredInfo = PredInfo->DefBB;
-
-      // Check if it matches the expected value.
-      if (PredInfo->AvailableVal) {
-        if (IncomingVal == PredInfo->AvailableVal)
-          continue;
-        return false;
-      }
-
-      // Check if the value is a PHI in the correct block.
-      MachineInstr *IncomingPHIVal = MRI->getVRegDef(IncomingVal);
-      if (!IncomingPHIVal->isPHI() ||
-          IncomingPHIVal->getParent() != PredInfo->BB)
-        return false;
-
-      // If this block has already been visited, check if this PHI matches.
-      if (PredInfo->PHITag) {
-        if (IncomingPHIVal == PredInfo->PHITag)
-          continue;
-        return false;
-      }
-      PredInfo->PHITag = IncomingPHIVal;
+};
 
-      WorkList.push_back(IncomingPHIVal);
-    }
-  }
-  return true;
-}
+} // End llvm namespace
 
-/// RecordMatchingPHI - For a PHI node that matches, record it and its input
-/// PHIs in both the BBMap and the AvailableVals mapping.
-void MachineSSAUpdater::RecordMatchingPHI(MachineInstr *PHI) {
-  BBMapTy *BBMap = getBBMap(BM);
+/// GetValueAtEndOfBlockInternal - Check to see if AvailableVals has an entry
+/// for the specified BB and if so, return it.  If not, construct SSA form by
+/// first calculating the required placement of PHIs and then inserting new
+/// PHIs where needed.
+unsigned MachineSSAUpdater::GetValueAtEndOfBlockInternal(MachineBasicBlock *BB){
   AvailableValsTy &AvailableVals = getAvailableVals(AV);
-  SmallVector<MachineInstr*, 20> WorkList;
-  WorkList.push_back(PHI);
-
-  // Record this PHI.
-  MachineBasicBlock *BB = PHI->getParent();
-  AvailableVals[BB] = PHI->getOperand(0).getReg();
-  (*BBMap)[BB]->AvailableVal = PHI->getOperand(0).getReg();
-
-  while (!WorkList.empty()) {
-    PHI = WorkList.pop_back_val();
-
-    // Iterate through the PHI's incoming values.
-    for (unsigned i = 1, e = PHI->getNumOperands(); i != e; i += 2) {
-      unsigned IncomingVal = PHI->getOperand(i).getReg();
-      MachineInstr *IncomingPHIVal = MRI->getVRegDef(IncomingVal);
-      if (!IncomingPHIVal->isPHI()) continue;
-      BB = IncomingPHIVal->getParent();
-      BBInfo *Info = (*BBMap)[BB];
-      if (!Info || Info->AvailableVal)
-        continue;
-
-      // Record the PHI and add it to the worklist.
-      AvailableVals[BB] = IncomingVal;
-      Info->AvailableVal = IncomingVal;
-      WorkList.push_back(IncomingPHIVal);
-    }
-  }
+  if (unsigned V = AvailableVals[BB])
+    return V;
+
+  SSAUpdaterImpl<MachineSSAUpdater> Impl(this, &AvailableVals, InsertedPHIs);
+  return Impl.GetValue(BB);
 }