include/llvm/Transforms/Utils/SSAUpdaterImpl.h - llvm - Git at Google

 //===- SSAUpdaterImpl.h - SSA Updater Implementation ------------*- C++ -*-===//
 //
 //                     The LLVM Compiler Infrastructure
 //
 // This file is distributed under the University of Illinois Open Source
 // License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
 //
 // This file provides a template that implements the core algorithm for the
 // SSAUpdater and MachineSSAUpdater.
 //
 //===----------------------------------------------------------------------===//

 #ifndef LLVM_TRANSFORMS_UTILS_SSAUPDATERIMPL_H
 #define LLVM_TRANSFORMS_UTILS_SSAUPDATERIMPL_H

 #include "llvm/ADT/DenseMap.h"
 #include "llvm/ADT/SmallVector.h"
 #include "llvm/Support/Allocator.h"
 #include "llvm/Support/Debug.h"
 #include "llvm/Support/raw_ostream.h"

 #define DEBUG_TYPE "ssaupdater"

 namespace llvm {

 template<typename T> class SSAUpdaterTraits;

 template<typename UpdaterT>
 class SSAUpdaterImpl {
 private:
   UpdaterT *Updater;

   using Traits = SSAUpdaterTraits<UpdaterT>;
   using BlkT = typename Traits::BlkT;
   using ValT = typename Traits::ValT;
   using PhiT = typename Traits::PhiT;

   /// BBInfo - Per-basic block information used internally by SSAUpdaterImpl.
   /// The predecessors of each block are cached here since pred_iterator is
   /// slow and we need to iterate over the blocks at least a few times.
   class BBInfo {
   public:
     // Back-pointer to the corresponding block.
     BlkT *BB;

     // Value to use in this block.
     ValT AvailableVal;

     // Block that defines the available value.
     BBInfo *DefBB;

     // Postorder number.
     int BlkNum = 0;

     // Immediate dominator.
     BBInfo *IDom = nullptr;

     // Number of predecessor blocks.
     unsigned NumPreds = 0;

     // Array[NumPreds] of predecessor blocks.
     BBInfo **Preds = nullptr;

     // Marker for existing PHIs that match.
     PhiT *PHITag = nullptr;

     BBInfo(BlkT *ThisBB, ValT V)
       : BB(ThisBB), AvailableVal(V), DefBB(V ? this : nullptr) {}
   };

   using AvailableValsTy = DenseMap<BlkT *, ValT>;

   AvailableValsTy *AvailableVals;

   SmallVectorImpl<PhiT *> *InsertedPHIs;

   using BlockListTy = SmallVectorImpl<BBInfo *>;
   using BBMapTy = DenseMap<BlkT *, BBInfo *>;

   BBMapTy BBMap;
   BumpPtrAllocator Allocator;

 public:
   explicit SSAUpdaterImpl(UpdaterT *U, AvailableValsTy *A,
                           SmallVectorImpl<PhiT *> *Ins) :
     Updater(U), AvailableVals(A), InsertedPHIs(Ins) {}

   /// GetValue - 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.
   ValT GetValue(BlkT *BB) {
     SmallVector<BBInfo *, 100> BlockList;
     BBInfo *PseudoEntry = BuildBlockList(BB, &BlockList);

     // Special case: bail out if BB is unreachable.
     if (BlockList.size() == 0) {
       ValT V = Traits::GetUndefVal(BB, Updater);
       (*AvailableVals)[BB] = V;
       return V;
     }

     FindDominators(&BlockList, PseudoEntry);
     FindPHIPlacement(&BlockList);
     FindAvailableVals(&BlockList);

     return BBMap[BB]->DefBB->AvailableVal;
   }

   /// 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.
   BBInfo *BuildBlockList(BlkT *BB, BlockListTy *BlockList) {
     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<BlkT *, 10> Preds;
     while (!WorkList.empty()) {
       Info = WorkList.pop_back_val();
       Preds.clear();
       Traits::FindPredecessorBlocks(Info->BB, &Preds);
       Info->NumPreds = Preds.size();
       if (Info->NumPreds == 0)
         Info->Preds = nullptr;
       else
         Info->Preds = static_cast<BBInfo **>(Allocator.Allocate(
             Info->NumPreds * sizeof(BBInfo *), alignof(BBInfo *)));

       for (unsigned p = 0; p != Info->NumPreds; ++p) {
         BlkT *Pred = Preds[p];
         // Check if BBMap already has a BBInfo for the predecessor block.
         typename BBMapTy::value_type &BBMapBucket =
           BBMap.FindAndConstruct(Pred);
         if (BBMapBucket.second) {
           Info->Preds[p] = BBMapBucket.second;
           continue;
         }

         // Create a new BBInfo for the predecessor.
         ValT 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);
       }
     }

     // 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(nullptr, 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);
     }

     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 (typename Traits::BlkSucc_iterator SI =
              Traits::BlkSucc_begin(Info->BB),
              E = Traits::BlkSucc_end(Info->BB); SI != E; ++SI) {
         BBInfo *SuccInfo = BBMap[*SI];
         if (!SuccInfo || SuccInfo->BlkNum)
           continue;
         SuccInfo->BlkNum = -1;
         WorkList.push_back(SuccInfo);
       }
     }
     PseudoEntry->BlkNum = BlkNum;
     return PseudoEntry;
   }

   /// 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.
   BBInfo *IntersectDominators(BBInfo *Blk1, 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 FindDominators(BlockListTy *BlockList, BBInfo *PseudoEntry) {
     bool Changed;
     do {
       Changed = false;
       // Iterate over the list in reverse order, i.e., forward on CFG edges.
       for (typename BlockListTy::reverse_iterator I = BlockList->rbegin(),
              E = BlockList->rend(); I != E; ++I) {
         BBInfo *Info = *I;
         BBInfo *NewIDom = nullptr;

         // Iterate through the block's predecessors.
         for (unsigned p = 0; p != Info->NumPreds; ++p) {
           BBInfo *Pred = Info->Preds[p];

           // Treat an unreachable predecessor as a definition with 'undef'.
           if (Pred->BlkNum == 0) {
             Pred->AvailableVal = Traits::GetUndefVal(Pred->BB, Updater);
             (*AvailableVals)[Pred->BB] = Pred->AvailableVal;
             Pred->DefBB = Pred;
             Pred->BlkNum = PseudoEntry->BlkNum;
             PseudoEntry->BlkNum++;
           }

           if (!NewIDom)
             NewIDom = Pred;
           else
             NewIDom = IntersectDominators(NewIDom, Pred);
         }

         // Check if the IDom value has changed.
         if (NewIDom && 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.
   bool IsDefInDomFrontier(const BBInfo *Pred, const BBInfo *IDom) {
     for (; Pred != IDom; Pred = Pred->IDom) {
       if (Pred->DefBB == Pred)
         return true;
     }
     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 FindPHIPlacement(BlockListTy *BlockList) {
     bool Changed;
     do {
       Changed = false;
       // Iterate over the list in reverse order, i.e., forward on CFG edges.
       for (typename 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 FindAvailableVals(BlockListTy *BlockList) {
     // 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 (typename 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;

       ValT PHI = Traits::CreateEmptyPHI(Info->BB, Info->NumPreds, Updater);
       Info->AvailableVal = PHI;
       (*AvailableVals)[Info->BB] = PHI;
     }

     // Now go back through the worklist in reverse order to fill in the
     // arguments for any new PHIs added in the forward traversal.
     for (typename BlockListTy::reverse_iterator I = BlockList->rbegin(),
            E = BlockList->rend(); I != E; ++I) {
       BBInfo *Info = *I;

       if (Info->DefBB != Info) {
         // Record the available value to speed up subsequent uses of this
         // SSAUpdater for the same value.
         (*AvailableVals)[Info->BB] = Info->DefBB->AvailableVal;
         continue;
       }

       // Check if this block contains a newly added PHI.
       PhiT *PHI = Traits::ValueIsNewPHI(Info->AvailableVal, Updater);
       if (!PHI)
         continue;

       // Iterate through the block's predecessors.
       for (unsigned p = 0; p != Info->NumPreds; ++p) {
         BBInfo *PredInfo = Info->Preds[p];
         BlkT *Pred = PredInfo->BB;
         // Skip to the nearest preceding definition.
         if (PredInfo->DefBB != PredInfo)
           PredInfo = PredInfo->DefBB;
         Traits::AddPHIOperand(PHI, PredInfo->AvailableVal, Pred);
       }

       LLVM_DEBUG(dbgs() << "  Inserted PHI: " << *PHI << "\n");

       // If the client wants to know about all new instructions, tell it.
       if (InsertedPHIs) InsertedPHIs->push_back(PHI);
     }
   }

   /// FindExistingPHI - Look through the PHI nodes in a block to see if any of
   /// them match what is needed.
   void FindExistingPHI(BlkT *BB, BlockListTy *BlockList) {
     for (auto &SomePHI : BB->phis()) {
       if (CheckIfPHIMatches(&SomePHI)) {
         RecordMatchingPHIs(BlockList);
         break;
       }
       // Match failed: clear all the PHITag values.
       for (typename BlockListTy::iterator I = BlockList->begin(),
              E = BlockList->end(); I != E; ++I)
         (*I)->PHITag = nullptr;
     }
   }

   /// CheckIfPHIMatches - Check if a PHI node matches the placement and values
   /// in the BBMap.
   bool CheckIfPHIMatches(PhiT *PHI) {
     SmallVector<PhiT *, 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 (typename Traits::PHI_iterator I = Traits::PHI_begin(PHI),
              E = Traits::PHI_end(PHI); I != E; ++I) {
         ValT IncomingVal = I.getIncomingValue();
         BBInfo *PredInfo = BBMap[I.getIncomingBlock()];
         // 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.
         PhiT *IncomingPHIVal = Traits::ValueIsPHI(IncomingVal, Updater);
         if (!IncomingPHIVal || 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;
   }

   /// RecordMatchingPHIs - For each PHI node that matches, record it in both
   /// the BBMap and the AvailableVals mapping.
   void RecordMatchingPHIs(BlockListTy *BlockList) {
     for (typename BlockListTy::iterator I = BlockList->begin(),
            E = BlockList->end(); I != E; ++I)
       if (PhiT *PHI = (*I)->PHITag) {
         BlkT *BB = PHI->getParent();
         ValT PHIVal = Traits::GetPHIValue(PHI);
         (*AvailableVals)[BB] = PHIVal;
         BBMap[BB]->AvailableVal = PHIVal;
       }
   }
 };

 } // end namespace llvm

 #undef DEBUG_TYPE // "ssaupdater"

 #endif // LLVM_TRANSFORMS_UTILS_SSAUPDATERIMPL_H
	//===- SSAUpdaterImpl.h - SSA Updater Implementation ------------- C++ --===//
	//
	// The LLVM Compiler Infrastructure
	//
	// This file is distributed under the University of Illinois Open Source
	// License. See LICENSE.TXT for details.
	//
	//===----------------------------------------------------------------------===//
	//
	// This file provides a template that implements the core algorithm for the
	// SSAUpdater and MachineSSAUpdater.
	//
	//===----------------------------------------------------------------------===//

	#ifndef LLVM_TRANSFORMS_UTILS_SSAUPDATERIMPL_H
	#define LLVM_TRANSFORMS_UTILS_SSAUPDATERIMPL_H

	#include "llvm/ADT/DenseMap.h"
	#include "llvm/ADT/SmallVector.h"
	#include "llvm/Support/Allocator.h"
	#include "llvm/Support/Debug.h"
	#include "llvm/Support/raw_ostream.h"

	#define DEBUG_TYPE "ssaupdater"

	namespace llvm {

	template<typename T> class SSAUpdaterTraits;

	template<typename UpdaterT>
	class SSAUpdaterImpl {
	private:
	UpdaterT *Updater;

	using Traits = SSAUpdaterTraits<UpdaterT>;
	using BlkT = typename Traits::BlkT;
	using ValT = typename Traits::ValT;
	using PhiT = typename Traits::PhiT;

	/// BBInfo - Per-basic block information used internally by SSAUpdaterImpl.
	/// The predecessors of each block are cached here since pred_iterator is
	/// slow and we need to iterate over the blocks at least a few times.
	class BBInfo {
	public:
	// Back-pointer to the corresponding block.
	BlkT *BB;

	// Value to use in this block.
	ValT AvailableVal;

	// Block that defines the available value.
	BBInfo *DefBB;

	// Postorder number.
	int BlkNum = 0;

	// Immediate dominator.
	BBInfo *IDom = nullptr;

	// Number of predecessor blocks.
	unsigned NumPreds = 0;

	// Array[NumPreds] of predecessor blocks.
	BBInfo **Preds = nullptr;

	// Marker for existing PHIs that match.
	PhiT *PHITag = nullptr;

	BBInfo(BlkT *ThisBB, ValT V)
	: BB(ThisBB), AvailableVal(V), DefBB(V ? this : nullptr) {}
	};

	using AvailableValsTy = DenseMap<BlkT *, ValT>;

	AvailableValsTy *AvailableVals;

	SmallVectorImpl<PhiT > InsertedPHIs;

	using BlockListTy = SmallVectorImpl<BBInfo *>;
	using BBMapTy = DenseMap<BlkT , BBInfo >;

	BBMapTy BBMap;
	BumpPtrAllocator Allocator;

	public:
	explicit SSAUpdaterImpl(UpdaterT U, AvailableValsTy A,
	SmallVectorImpl<PhiT > Ins) :
	Updater(U), AvailableVals(A), InsertedPHIs(Ins) {}

	/// GetValue - 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.
	ValT GetValue(BlkT *BB) {
	SmallVector<BBInfo *, 100> BlockList;
	BBInfo *PseudoEntry = BuildBlockList(BB, &BlockList);

	// Special case: bail out if BB is unreachable.
	if (BlockList.size() == 0) {
	ValT V = Traits::GetUndefVal(BB, Updater);
	(*AvailableVals)[BB] = V;
	return V;
	}

	FindDominators(&BlockList, PseudoEntry);
	FindPHIPlacement(&BlockList);
	FindAvailableVals(&BlockList);

	return BBMap[BB]->DefBB->AvailableVal;
	}

	/// 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.
	BBInfo BuildBlockList(BlkT BB, BlockListTy *BlockList) {
	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<BlkT *, 10> Preds;
	while (!WorkList.empty()) {
	Info = WorkList.pop_back_val();
	Preds.clear();
	Traits::FindPredecessorBlocks(Info->BB, &Preds);
	Info->NumPreds = Preds.size();
	if (Info->NumPreds == 0)
	Info->Preds = nullptr;
	else
	Info->Preds = static_cast<BBInfo **>(Allocator.Allocate(
	Info->NumPreds * sizeof(BBInfo ), alignof(BBInfo )));

	for (unsigned p = 0; p != Info->NumPreds; ++p) {
	BlkT *Pred = Preds[p];
	// Check if BBMap already has a BBInfo for the predecessor block.
	typename BBMapTy::value_type &BBMapBucket =
	BBMap.FindAndConstruct(Pred);
	if (BBMapBucket.second) {
	Info->Preds[p] = BBMapBucket.second;
	continue;
	}

	// Create a new BBInfo for the predecessor.
	ValT 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);
	}
	}

	// 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(nullptr, 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);
	}

	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 (typename Traits::BlkSucc_iterator SI =
	Traits::BlkSucc_begin(Info->BB),
	E = Traits::BlkSucc_end(Info->BB); SI != E; ++SI) {
	BBInfo SuccInfo = BBMap[SI];
	if (!SuccInfo \|\| SuccInfo->BlkNum)
	continue;
	SuccInfo->BlkNum = -1;
	WorkList.push_back(SuccInfo);
	}
	}
	PseudoEntry->BlkNum = BlkNum;
	return PseudoEntry;
	}

	/// 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.
	BBInfo IntersectDominators(BBInfo Blk1, 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 FindDominators(BlockListTy BlockList, BBInfo PseudoEntry) {
	bool Changed;
	do {
	Changed = false;
	// Iterate over the list in reverse order, i.e., forward on CFG edges.
	for (typename BlockListTy::reverse_iterator I = BlockList->rbegin(),
	E = BlockList->rend(); I != E; ++I) {
	BBInfo Info = I;
	BBInfo *NewIDom = nullptr;

	// Iterate through the block's predecessors.
	for (unsigned p = 0; p != Info->NumPreds; ++p) {
	BBInfo *Pred = Info->Preds[p];

	// Treat an unreachable predecessor as a definition with 'undef'.
	if (Pred->BlkNum == 0) {
	Pred->AvailableVal = Traits::GetUndefVal(Pred->BB, Updater);
	(*AvailableVals)[Pred->BB] = Pred->AvailableVal;
	Pred->DefBB = Pred;
	Pred->BlkNum = PseudoEntry->BlkNum;
	PseudoEntry->BlkNum++;
	}

	if (!NewIDom)
	NewIDom = Pred;
	else
	NewIDom = IntersectDominators(NewIDom, Pred);
	}

	// Check if the IDom value has changed.
	if (NewIDom && 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.
	bool IsDefInDomFrontier(const BBInfo Pred, const BBInfo IDom) {
	for (; Pred != IDom; Pred = Pred->IDom) {
	if (Pred->DefBB == Pred)
	return true;
	}
	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 FindPHIPlacement(BlockListTy *BlockList) {
	bool Changed;
	do {
	Changed = false;
	// Iterate over the list in reverse order, i.e., forward on CFG edges.
	for (typename 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 FindAvailableVals(BlockListTy *BlockList) {
	// 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 (typename 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;

	ValT PHI = Traits::CreateEmptyPHI(Info->BB, Info->NumPreds, Updater);
	Info->AvailableVal = PHI;
	(*AvailableVals)[Info->BB] = PHI;
	}

	// Now go back through the worklist in reverse order to fill in the
	// arguments for any new PHIs added in the forward traversal.
	for (typename BlockListTy::reverse_iterator I = BlockList->rbegin(),
	E = BlockList->rend(); I != E; ++I) {
	BBInfo Info = I;

	if (Info->DefBB != Info) {
	// Record the available value to speed up subsequent uses of this
	// SSAUpdater for the same value.
	(*AvailableVals)[Info->BB] = Info->DefBB->AvailableVal;
	continue;
	}

	// Check if this block contains a newly added PHI.
	PhiT *PHI = Traits::ValueIsNewPHI(Info->AvailableVal, Updater);
	if (!PHI)
	continue;

	// Iterate through the block's predecessors.
	for (unsigned p = 0; p != Info->NumPreds; ++p) {
	BBInfo *PredInfo = Info->Preds[p];
	BlkT *Pred = PredInfo->BB;
	// Skip to the nearest preceding definition.
	if (PredInfo->DefBB != PredInfo)
	PredInfo = PredInfo->DefBB;
	Traits::AddPHIOperand(PHI, PredInfo->AvailableVal, Pred);
	}

	LLVM_DEBUG(dbgs() << " Inserted PHI: " << *PHI << "\n");

	// If the client wants to know about all new instructions, tell it.
	if (InsertedPHIs) InsertedPHIs->push_back(PHI);
	}
	}

	/// FindExistingPHI - Look through the PHI nodes in a block to see if any of
	/// them match what is needed.
	void FindExistingPHI(BlkT BB, BlockListTy BlockList) {
	for (auto &SomePHI : BB->phis()) {
	if (CheckIfPHIMatches(&SomePHI)) {
	RecordMatchingPHIs(BlockList);
	break;
	}
	// Match failed: clear all the PHITag values.
	for (typename BlockListTy::iterator I = BlockList->begin(),
	E = BlockList->end(); I != E; ++I)
	(*I)->PHITag = nullptr;
	}
	}

	/// CheckIfPHIMatches - Check if a PHI node matches the placement and values
	/// in the BBMap.
	bool CheckIfPHIMatches(PhiT *PHI) {
	SmallVector<PhiT *, 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 (typename Traits::PHI_iterator I = Traits::PHI_begin(PHI),
	E = Traits::PHI_end(PHI); I != E; ++I) {
	ValT IncomingVal = I.getIncomingValue();
	BBInfo *PredInfo = BBMap[I.getIncomingBlock()];
	// 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.
	PhiT *IncomingPHIVal = Traits::ValueIsPHI(IncomingVal, Updater);
	if (!IncomingPHIVal \|\| 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;
	}

	/// RecordMatchingPHIs - For each PHI node that matches, record it in both
	/// the BBMap and the AvailableVals mapping.
	void RecordMatchingPHIs(BlockListTy *BlockList) {
	for (typename BlockListTy::iterator I = BlockList->begin(),
	E = BlockList->end(); I != E; ++I)
	if (PhiT PHI = (I)->PHITag) {
	BlkT *BB = PHI->getParent();
	ValT PHIVal = Traits::GetPHIValue(PHI);
	(*AvailableVals)[BB] = PHIVal;
	BBMap[BB]->AvailableVal = PHIVal;
	}
	}
	};

	} // end namespace llvm

	#undef DEBUG_TYPE // "ssaupdater"

	#endif // LLVM_TRANSFORMS_UTILS_SSAUPDATERIMPL_H