lib/Transforms/Utils/BreakCriticalEdges.cpp - platform/external/llvm - Git at Google

 //===- BreakCriticalEdges.cpp - Critical Edge Elimination Pass ------------===//
 //
 //                     The LLVM Compiler Infrastructure
 //
 // This file is distributed under the University of Illinois Open Source
 // License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
 //
 // BreakCriticalEdges pass - Break all of the critical edges in the CFG by
 // inserting a dummy basic block.  This pass may be "required" by passes that
 // cannot deal with critical edges.  For this usage, the structure type is
 // forward declared.  This pass obviously invalidates the CFG, but can update
 // dominator trees.
 //
 //===----------------------------------------------------------------------===//

 #define DEBUG_TYPE "break-crit-edges"
 #include "llvm/Transforms/Scalar.h"
 #include "llvm/ADT/SmallVector.h"
 #include "llvm/ADT/Statistic.h"
 #include "llvm/Analysis/Dominators.h"
 #include "llvm/Analysis/LoopInfo.h"
 #include "llvm/Analysis/ProfileInfo.h"
 #include "llvm/IR/Function.h"
 #include "llvm/IR/Instructions.h"
 #include "llvm/IR/Type.h"
 #include "llvm/Support/CFG.h"
 #include "llvm/Support/ErrorHandling.h"
 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
 using namespace llvm;

 STATISTIC(NumBroken, "Number of blocks inserted");

 namespace {
   struct BreakCriticalEdges : public FunctionPass {
     static char ID; // Pass identification, replacement for typeid
     BreakCriticalEdges() : FunctionPass(ID) {
       initializeBreakCriticalEdgesPass(*PassRegistry::getPassRegistry());
     }

     virtual bool runOnFunction(Function &F);

     virtual void getAnalysisUsage(AnalysisUsage &AU) const {
       AU.addPreserved<DominatorTree>();
       AU.addPreserved<LoopInfo>();
       AU.addPreserved<ProfileInfo>();

       // No loop canonicalization guarantees are broken by this pass.
       AU.addPreservedID(LoopSimplifyID);
     }
   };
 }

 char BreakCriticalEdges::ID = 0;
 INITIALIZE_PASS(BreakCriticalEdges, "break-crit-edges",
                 "Break critical edges in CFG", false, false)

 // Publicly exposed interface to pass...
 char &llvm::BreakCriticalEdgesID = BreakCriticalEdges::ID;
 FunctionPass *llvm::createBreakCriticalEdgesPass() {
   return new BreakCriticalEdges();
 }

 // runOnFunction - Loop over all of the edges in the CFG, breaking critical
 // edges as they are found.
 //
 bool BreakCriticalEdges::runOnFunction(Function &F) {
   bool Changed = false;
   for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I) {
     TerminatorInst *TI = I->getTerminator();
     if (TI->getNumSuccessors() > 1 && !isa<IndirectBrInst>(TI))
       for (unsigned i = 0, e = TI->getNumSuccessors(); i != e; ++i)
         if (SplitCriticalEdge(TI, i, this)) {
           ++NumBroken;
           Changed = true;
         }
   }

   return Changed;
 }

 //===----------------------------------------------------------------------===//
 //    Implementation of the external critical edge manipulation functions
 //===----------------------------------------------------------------------===//

 // isCriticalEdge - Return true if the specified edge is a critical edge.
 // Critical edges are edges from a block with multiple successors to a block
 // with multiple predecessors.
 //
 bool llvm::isCriticalEdge(const TerminatorInst *TI, unsigned SuccNum,
                           bool AllowIdenticalEdges) {
   assert(SuccNum < TI->getNumSuccessors() && "Illegal edge specification!");
   if (TI->getNumSuccessors() == 1) return false;

   const BasicBlock *Dest = TI->getSuccessor(SuccNum);
   const_pred_iterator I = pred_begin(Dest), E = pred_end(Dest);

   // If there is more than one predecessor, this is a critical edge...
   assert(I != E && "No preds, but we have an edge to the block?");
   const BasicBlock *FirstPred = *I;
   ++I;        // Skip one edge due to the incoming arc from TI.
   if (!AllowIdenticalEdges)
     return I != E;

   // If AllowIdenticalEdges is true, then we allow this edge to be considered
   // non-critical iff all preds come from TI's block.
   while (I != E) {
     const BasicBlock *P = *I;
     if (P != FirstPred)
       return true;
     // Note: leave this as is until no one ever compiles with either gcc 4.0.1
     // or Xcode 2. This seems to work around the pred_iterator assert in PR 2207
     E = pred_end(P);
     ++I;
   }
   return false;
 }

 /// createPHIsForSplitLoopExit - When a loop exit edge is split, LCSSA form
 /// may require new PHIs in the new exit block. This function inserts the
 /// new PHIs, as needed. Preds is a list of preds inside the loop, SplitBB
 /// is the new loop exit block, and DestBB is the old loop exit, now the
 /// successor of SplitBB.
 static void createPHIsForSplitLoopExit(ArrayRef<BasicBlock *> Preds,
                                        BasicBlock *SplitBB,
                                        BasicBlock *DestBB) {
   // SplitBB shouldn't have anything non-trivial in it yet.
   assert((SplitBB->getFirstNonPHI() == SplitBB->getTerminator() ||
           SplitBB->isLandingPad()) && "SplitBB has non-PHI nodes!");

   // For each PHI in the destination block.
   for (BasicBlock::iterator I = DestBB->begin();
        PHINode *PN = dyn_cast<PHINode>(I); ++I) {
     unsigned Idx = PN->getBasicBlockIndex(SplitBB);
     Value *V = PN->getIncomingValue(Idx);

     // If the input is a PHI which already satisfies LCSSA, don't create
     // a new one.
     if (const PHINode *VP = dyn_cast<PHINode>(V))
       if (VP->getParent() == SplitBB)
         continue;

     // Otherwise a new PHI is needed. Create one and populate it.
     PHINode *NewPN =
       PHINode::Create(PN->getType(), Preds.size(), "split",
                       SplitBB->isLandingPad() ?
                       SplitBB->begin() : SplitBB->getTerminator());
     for (unsigned i = 0, e = Preds.size(); i != e; ++i)
       NewPN->addIncoming(V, Preds[i]);

     // Update the original PHI.
     PN->setIncomingValue(Idx, NewPN);
   }
 }

 /// SplitCriticalEdge - If this edge is a critical edge, insert a new node to
 /// split the critical edge.  This will update DominatorTree information if it
 /// is available, thus calling this pass will not invalidate either of them.
 /// This returns the new block if the edge was split, null otherwise.
 ///
 /// If MergeIdenticalEdges is true (not the default), *all* edges from TI to the
 /// specified successor will be merged into the same critical edge block.
 /// This is most commonly interesting with switch instructions, which may
 /// have many edges to any one destination.  This ensures that all edges to that
 /// dest go to one block instead of each going to a different block, but isn't
 /// the standard definition of a "critical edge".
 ///
 /// It is invalid to call this function on a critical edge that starts at an
 /// IndirectBrInst.  Splitting these edges will almost always create an invalid
 /// program because the address of the new block won't be the one that is jumped
 /// to.
 ///
 BasicBlock *llvm::SplitCriticalEdge(TerminatorInst *TI, unsigned SuccNum,
                                     Pass *P, bool MergeIdenticalEdges,
                                     bool DontDeleteUselessPhis,
                                     bool SplitLandingPads) {
   if (!isCriticalEdge(TI, SuccNum, MergeIdenticalEdges)) return 0;

   assert(!isa<IndirectBrInst>(TI) &&
          "Cannot split critical edge from IndirectBrInst");

   BasicBlock *TIBB = TI->getParent();
   BasicBlock *DestBB = TI->getSuccessor(SuccNum);

   // Splitting the critical edge to a landing pad block is non-trivial. Don't do
   // it in this generic function.
   if (DestBB->isLandingPad()) return 0;

   // Create a new basic block, linking it into the CFG.
   BasicBlock *NewBB = BasicBlock::Create(TI->getContext(),
                       TIBB->getName() + "." + DestBB->getName() + "_crit_edge");
   // Create our unconditional branch.
   BranchInst *NewBI = BranchInst::Create(DestBB, NewBB);
   NewBI->setDebugLoc(TI->getDebugLoc());

   // Branch to the new block, breaking the edge.
   TI->setSuccessor(SuccNum, NewBB);

   // Insert the block into the function... right after the block TI lives in.
   Function &F = *TIBB->getParent();
   Function::iterator FBBI = TIBB;
   F.getBasicBlockList().insert(++FBBI, NewBB);

   // If there are any PHI nodes in DestBB, we need to update them so that they
   // merge incoming values from NewBB instead of from TIBB.
   {
     unsigned BBIdx = 0;
     for (BasicBlock::iterator I = DestBB->begin(); isa<PHINode>(I); ++I) {
       // We no longer enter through TIBB, now we come in through NewBB.
       // Revector exactly one entry in the PHI node that used to come from
       // TIBB to come from NewBB.
       PHINode *PN = cast<PHINode>(I);

       // Reuse the previous value of BBIdx if it lines up.  In cases where we
       // have multiple phi nodes with *lots* of predecessors, this is a speed
       // win because we don't have to scan the PHI looking for TIBB.  This
       // happens because the BB list of PHI nodes are usually in the same
       // order.
       if (PN->getIncomingBlock(BBIdx) != TIBB)
         BBIdx = PN->getBasicBlockIndex(TIBB);
       PN->setIncomingBlock(BBIdx, NewBB);
     }
   }

   // If there are any other edges from TIBB to DestBB, update those to go
   // through the split block, making those edges non-critical as well (and
   // reducing the number of phi entries in the DestBB if relevant).
   if (MergeIdenticalEdges) {
     for (unsigned i = SuccNum+1, e = TI->getNumSuccessors(); i != e; ++i) {
       if (TI->getSuccessor(i) != DestBB) continue;

       // Remove an entry for TIBB from DestBB phi nodes.
       DestBB->removePredecessor(TIBB, DontDeleteUselessPhis);

       // We found another edge to DestBB, go to NewBB instead.
       TI->setSuccessor(i, NewBB);
     }
   }


   // If we don't have a pass object, we can't update anything...
   if (P == 0) return NewBB;

   DominatorTree *DT = P->getAnalysisIfAvailable<DominatorTree>();
   LoopInfo *LI = P->getAnalysisIfAvailable<LoopInfo>();
   ProfileInfo *PI = P->getAnalysisIfAvailable<ProfileInfo>();

   // If we have nothing to update, just return.
   if (DT == 0 && LI == 0 && PI == 0)
     return NewBB;

   // Now update analysis information.  Since the only predecessor of NewBB is
   // the TIBB, TIBB clearly dominates NewBB.  TIBB usually doesn't dominate
   // anything, as there are other successors of DestBB.  However, if all other
   // predecessors of DestBB are already dominated by DestBB (e.g. DestBB is a
   // loop header) then NewBB dominates DestBB.
   SmallVector<BasicBlock*, 8> OtherPreds;

   // If there is a PHI in the block, loop over predecessors with it, which is
   // faster than iterating pred_begin/end.
   if (PHINode *PN = dyn_cast<PHINode>(DestBB->begin())) {
     for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
       if (PN->getIncomingBlock(i) != NewBB)
         OtherPreds.push_back(PN->getIncomingBlock(i));
   } else {
     for (pred_iterator I = pred_begin(DestBB), E = pred_end(DestBB);
          I != E; ++I) {
       BasicBlock *P = *I;
       if (P != NewBB)
         OtherPreds.push_back(P);
     }
   }

   bool NewBBDominatesDestBB = true;

   // Should we update DominatorTree information?
   if (DT) {
     DomTreeNode *TINode = DT->getNode(TIBB);

     // The new block is not the immediate dominator for any other nodes, but
     // TINode is the immediate dominator for the new node.
     //
     if (TINode) {       // Don't break unreachable code!
       DomTreeNode *NewBBNode = DT->addNewBlock(NewBB, TIBB);
       DomTreeNode *DestBBNode = 0;

       // If NewBBDominatesDestBB hasn't been computed yet, do so with DT.
       if (!OtherPreds.empty()) {
         DestBBNode = DT->getNode(DestBB);
         while (!OtherPreds.empty() && NewBBDominatesDestBB) {
           if (DomTreeNode *OPNode = DT->getNode(OtherPreds.back()))
             NewBBDominatesDestBB = DT->dominates(DestBBNode, OPNode);
           OtherPreds.pop_back();
         }
         OtherPreds.clear();
       }

       // If NewBBDominatesDestBB, then NewBB dominates DestBB, otherwise it
       // doesn't dominate anything.
       if (NewBBDominatesDestBB) {
         if (!DestBBNode) DestBBNode = DT->getNode(DestBB);
         DT->changeImmediateDominator(DestBBNode, NewBBNode);
       }
     }
   }

   // Update LoopInfo if it is around.
   if (LI) {
     if (Loop *TIL = LI->getLoopFor(TIBB)) {
       // If one or the other blocks were not in a loop, the new block is not
       // either, and thus LI doesn't need to be updated.
       if (Loop *DestLoop = LI->getLoopFor(DestBB)) {
         if (TIL == DestLoop) {
           // Both in the same loop, the NewBB joins loop.
           DestLoop->addBasicBlockToLoop(NewBB, LI->getBase());
         } else if (TIL->contains(DestLoop)) {
           // Edge from an outer loop to an inner loop.  Add to the outer loop.
           TIL->addBasicBlockToLoop(NewBB, LI->getBase());
         } else if (DestLoop->contains(TIL)) {
           // Edge from an inner loop to an outer loop.  Add to the outer loop.
           DestLoop->addBasicBlockToLoop(NewBB, LI->getBase());
         } else {
           // Edge from two loops with no containment relation.  Because these
           // are natural loops, we know that the destination block must be the
           // header of its loop (adding a branch into a loop elsewhere would
           // create an irreducible loop).
           assert(DestLoop->getHeader() == DestBB &&
                  "Should not create irreducible loops!");
           if (Loop *P = DestLoop->getParentLoop())
             P->addBasicBlockToLoop(NewBB, LI->getBase());
         }
       }
       // If TIBB is in a loop and DestBB is outside of that loop, split the
       // other exit blocks of the loop that also have predecessors outside
       // the loop, to maintain a LoopSimplify guarantee.
       if (!TIL->contains(DestBB) &&
           P->mustPreserveAnalysisID(LoopSimplifyID)) {
         assert(!TIL->contains(NewBB) &&
                "Split point for loop exit is contained in loop!");

         // Update LCSSA form in the newly created exit block.
         if (P->mustPreserveAnalysisID(LCSSAID))
           createPHIsForSplitLoopExit(TIBB, NewBB, DestBB);

         // For each unique exit block...
         // FIXME: This code is functionally equivalent to the corresponding
         // loop in LoopSimplify.
         SmallVector<BasicBlock *, 4> ExitBlocks;
         TIL->getExitBlocks(ExitBlocks);
         for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i) {
           // Collect all the preds that are inside the loop, and note
           // whether there are any preds outside the loop.
           SmallVector<BasicBlock *, 4> Preds;
           bool HasPredOutsideOfLoop = false;
           BasicBlock *Exit = ExitBlocks[i];
           for (pred_iterator I = pred_begin(Exit), E = pred_end(Exit);
                I != E; ++I) {
             BasicBlock *P = *I;
             if (TIL->contains(P)) {
               if (isa<IndirectBrInst>(P->getTerminator())) {
                 Preds.clear();
                 break;
               }
               Preds.push_back(P);
             } else {
               HasPredOutsideOfLoop = true;
             }
           }
           // If there are any preds not in the loop, we'll need to split
           // the edges. The Preds.empty() check is needed because a block
           // may appear multiple times in the list. We can't use
           // getUniqueExitBlocks above because that depends on LoopSimplify
           // form, which we're in the process of restoring!
           if (!Preds.empty() && HasPredOutsideOfLoop) {
             if (!Exit->isLandingPad()) {
               BasicBlock *NewExitBB =
                 SplitBlockPredecessors(Exit, Preds, "split", P);
               if (P->mustPreserveAnalysisID(LCSSAID))
                 createPHIsForSplitLoopExit(Preds, NewExitBB, Exit);
             } else if (SplitLandingPads) {
               SmallVector<BasicBlock*, 8> NewBBs;
               SplitLandingPadPredecessors(Exit, Preds,
                                           ".split1", ".split2",
                                           P, NewBBs);
               if (P->mustPreserveAnalysisID(LCSSAID))
                 createPHIsForSplitLoopExit(Preds, NewBBs[0], Exit);
             }
           }
         }
       }
       // LCSSA form was updated above for the case where LoopSimplify is
       // available, which means that all predecessors of loop exit blocks
       // are within the loop. Without LoopSimplify form, it would be
       // necessary to insert a new phi.
       assert((!P->mustPreserveAnalysisID(LCSSAID) ||
               P->mustPreserveAnalysisID(LoopSimplifyID)) &&
              "SplitCriticalEdge doesn't know how to update LCCSA form "
              "without LoopSimplify!");
     }
   }

   // Update ProfileInfo if it is around.
   if (PI)
     PI->splitEdge(TIBB, DestBB, NewBB, MergeIdenticalEdges);

   return NewBB;
 }
	//===- BreakCriticalEdges.cpp - Critical Edge Elimination Pass ------------===//
	//
	// The LLVM Compiler Infrastructure
	//
	// This file is distributed under the University of Illinois Open Source
	// License. See LICENSE.TXT for details.
	//
	//===----------------------------------------------------------------------===//
	//
	// BreakCriticalEdges pass - Break all of the critical edges in the CFG by
	// inserting a dummy basic block. This pass may be "required" by passes that
	// cannot deal with critical edges. For this usage, the structure type is
	// forward declared. This pass obviously invalidates the CFG, but can update
	// dominator trees.
	//
	//===----------------------------------------------------------------------===//

	#define DEBUG_TYPE "break-crit-edges"
	#include "llvm/Transforms/Scalar.h"
	#include "llvm/ADT/SmallVector.h"
	#include "llvm/ADT/Statistic.h"
	#include "llvm/Analysis/Dominators.h"
	#include "llvm/Analysis/LoopInfo.h"
	#include "llvm/Analysis/ProfileInfo.h"
	#include "llvm/IR/Function.h"
	#include "llvm/IR/Instructions.h"
	#include "llvm/IR/Type.h"
	#include "llvm/Support/CFG.h"
	#include "llvm/Support/ErrorHandling.h"
	#include "llvm/Transforms/Utils/BasicBlockUtils.h"
	using namespace llvm;

	STATISTIC(NumBroken, "Number of blocks inserted");

	namespace {
	struct BreakCriticalEdges : public FunctionPass {
	static char ID; // Pass identification, replacement for typeid
	BreakCriticalEdges() : FunctionPass(ID) {
	initializeBreakCriticalEdgesPass(*PassRegistry::getPassRegistry());
	}

	virtual bool runOnFunction(Function &F);

	virtual void getAnalysisUsage(AnalysisUsage &AU) const {
	AU.addPreserved<DominatorTree>();
	AU.addPreserved<LoopInfo>();
	AU.addPreserved<ProfileInfo>();

	// No loop canonicalization guarantees are broken by this pass.
	AU.addPreservedID(LoopSimplifyID);
	}
	};
	}

	char BreakCriticalEdges::ID = 0;
	INITIALIZE_PASS(BreakCriticalEdges, "break-crit-edges",
	"Break critical edges in CFG", false, false)

	// Publicly exposed interface to pass...
	char &llvm::BreakCriticalEdgesID = BreakCriticalEdges::ID;
	FunctionPass *llvm::createBreakCriticalEdgesPass() {
	return new BreakCriticalEdges();
	}

	// runOnFunction - Loop over all of the edges in the CFG, breaking critical
	// edges as they are found.
	//
	bool BreakCriticalEdges::runOnFunction(Function &F) {
	bool Changed = false;
	for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I) {
	TerminatorInst *TI = I->getTerminator();
	if (TI->getNumSuccessors() > 1 && !isa<IndirectBrInst>(TI))
	for (unsigned i = 0, e = TI->getNumSuccessors(); i != e; ++i)
	if (SplitCriticalEdge(TI, i, this)) {
	++NumBroken;
	Changed = true;
	}
	}

	return Changed;
	}

	//===----------------------------------------------------------------------===//
	// Implementation of the external critical edge manipulation functions
	//===----------------------------------------------------------------------===//

	// isCriticalEdge - Return true if the specified edge is a critical edge.
	// Critical edges are edges from a block with multiple successors to a block
	// with multiple predecessors.
	//
	bool llvm::isCriticalEdge(const TerminatorInst *TI, unsigned SuccNum,
	bool AllowIdenticalEdges) {
	assert(SuccNum < TI->getNumSuccessors() && "Illegal edge specification!");
	if (TI->getNumSuccessors() == 1) return false;

	const BasicBlock *Dest = TI->getSuccessor(SuccNum);
	const_pred_iterator I = pred_begin(Dest), E = pred_end(Dest);

	// If there is more than one predecessor, this is a critical edge...
	assert(I != E && "No preds, but we have an edge to the block?");
	const BasicBlock FirstPred = I;
	++I; // Skip one edge due to the incoming arc from TI.
	if (!AllowIdenticalEdges)
	return I != E;

	// If AllowIdenticalEdges is true, then we allow this edge to be considered
	// non-critical iff all preds come from TI's block.
	while (I != E) {
	const BasicBlock P = I;
	if (P != FirstPred)
	return true;
	// Note: leave this as is until no one ever compiles with either gcc 4.0.1
	// or Xcode 2. This seems to work around the pred_iterator assert in PR 2207
	E = pred_end(P);
	++I;
	}
	return false;
	}

	/// createPHIsForSplitLoopExit - When a loop exit edge is split, LCSSA form
	/// may require new PHIs in the new exit block. This function inserts the
	/// new PHIs, as needed. Preds is a list of preds inside the loop, SplitBB
	/// is the new loop exit block, and DestBB is the old loop exit, now the
	/// successor of SplitBB.
	static void createPHIsForSplitLoopExit(ArrayRef<BasicBlock *> Preds,
	BasicBlock *SplitBB,
	BasicBlock *DestBB) {
	// SplitBB shouldn't have anything non-trivial in it yet.
	assert((SplitBB->getFirstNonPHI() == SplitBB->getTerminator() \|\|
	SplitBB->isLandingPad()) && "SplitBB has non-PHI nodes!");

	// For each PHI in the destination block.
	for (BasicBlock::iterator I = DestBB->begin();
	PHINode *PN = dyn_cast<PHINode>(I); ++I) {
	unsigned Idx = PN->getBasicBlockIndex(SplitBB);
	Value *V = PN->getIncomingValue(Idx);

	// If the input is a PHI which already satisfies LCSSA, don't create
	// a new one.
	if (const PHINode *VP = dyn_cast<PHINode>(V))
	if (VP->getParent() == SplitBB)
	continue;

	// Otherwise a new PHI is needed. Create one and populate it.
	PHINode *NewPN =
	PHINode::Create(PN->getType(), Preds.size(), "split",
	SplitBB->isLandingPad() ?
	SplitBB->begin() : SplitBB->getTerminator());
	for (unsigned i = 0, e = Preds.size(); i != e; ++i)
	NewPN->addIncoming(V, Preds[i]);

	// Update the original PHI.
	PN->setIncomingValue(Idx, NewPN);
	}
	}

	/// SplitCriticalEdge - If this edge is a critical edge, insert a new node to
	/// split the critical edge. This will update DominatorTree information if it
	/// is available, thus calling this pass will not invalidate either of them.
	/// This returns the new block if the edge was split, null otherwise.
	///
	/// If MergeIdenticalEdges is true (not the default), all edges from TI to the
	/// specified successor will be merged into the same critical edge block.
	/// This is most commonly interesting with switch instructions, which may
	/// have many edges to any one destination. This ensures that all edges to that
	/// dest go to one block instead of each going to a different block, but isn't
	/// the standard definition of a "critical edge".
	///
	/// It is invalid to call this function on a critical edge that starts at an
	/// IndirectBrInst. Splitting these edges will almost always create an invalid
	/// program because the address of the new block won't be the one that is jumped
	/// to.
	///
	BasicBlock llvm::SplitCriticalEdge(TerminatorInst TI, unsigned SuccNum,
	Pass *P, bool MergeIdenticalEdges,
	bool DontDeleteUselessPhis,
	bool SplitLandingPads) {
	if (!isCriticalEdge(TI, SuccNum, MergeIdenticalEdges)) return 0;

	assert(!isa<IndirectBrInst>(TI) &&
	"Cannot split critical edge from IndirectBrInst");

	BasicBlock *TIBB = TI->getParent();
	BasicBlock *DestBB = TI->getSuccessor(SuccNum);

	// Splitting the critical edge to a landing pad block is non-trivial. Don't do
	// it in this generic function.
	if (DestBB->isLandingPad()) return 0;

	// Create a new basic block, linking it into the CFG.
	BasicBlock *NewBB = BasicBlock::Create(TI->getContext(),
	TIBB->getName() + "." + DestBB->getName() + "_crit_edge");
	// Create our unconditional branch.
	BranchInst *NewBI = BranchInst::Create(DestBB, NewBB);
	NewBI->setDebugLoc(TI->getDebugLoc());

	// Branch to the new block, breaking the edge.
	TI->setSuccessor(SuccNum, NewBB);

	// Insert the block into the function... right after the block TI lives in.
	Function &F = *TIBB->getParent();
	Function::iterator FBBI = TIBB;
	F.getBasicBlockList().insert(++FBBI, NewBB);

	// If there are any PHI nodes in DestBB, we need to update them so that they
	// merge incoming values from NewBB instead of from TIBB.
	{
	unsigned BBIdx = 0;
	for (BasicBlock::iterator I = DestBB->begin(); isa<PHINode>(I); ++I) {
	// We no longer enter through TIBB, now we come in through NewBB.
	// Revector exactly one entry in the PHI node that used to come from
	// TIBB to come from NewBB.
	PHINode *PN = cast<PHINode>(I);

	// Reuse the previous value of BBIdx if it lines up. In cases where we
	// have multiple phi nodes with lots of predecessors, this is a speed
	// win because we don't have to scan the PHI looking for TIBB. This
	// happens because the BB list of PHI nodes are usually in the same
	// order.
	if (PN->getIncomingBlock(BBIdx) != TIBB)
	BBIdx = PN->getBasicBlockIndex(TIBB);
	PN->setIncomingBlock(BBIdx, NewBB);
	}
	}

	// If there are any other edges from TIBB to DestBB, update those to go
	// through the split block, making those edges non-critical as well (and
	// reducing the number of phi entries in the DestBB if relevant).
	if (MergeIdenticalEdges) {
	for (unsigned i = SuccNum+1, e = TI->getNumSuccessors(); i != e; ++i) {
	if (TI->getSuccessor(i) != DestBB) continue;

	// Remove an entry for TIBB from DestBB phi nodes.
	DestBB->removePredecessor(TIBB, DontDeleteUselessPhis);

	// We found another edge to DestBB, go to NewBB instead.
	TI->setSuccessor(i, NewBB);
	}
	}



	// If we don't have a pass object, we can't update anything...
	if (P == 0) return NewBB;

	DominatorTree *DT = P->getAnalysisIfAvailable<DominatorTree>();
	LoopInfo *LI = P->getAnalysisIfAvailable<LoopInfo>();
	ProfileInfo *PI = P->getAnalysisIfAvailable<ProfileInfo>();

	// If we have nothing to update, just return.
	if (DT == 0 && LI == 0 && PI == 0)
	return NewBB;

	// Now update analysis information. Since the only predecessor of NewBB is
	// the TIBB, TIBB clearly dominates NewBB. TIBB usually doesn't dominate
	// anything, as there are other successors of DestBB. However, if all other
	// predecessors of DestBB are already dominated by DestBB (e.g. DestBB is a
	// loop header) then NewBB dominates DestBB.
	SmallVector<BasicBlock*, 8> OtherPreds;

	// If there is a PHI in the block, loop over predecessors with it, which is
	// faster than iterating pred_begin/end.
	if (PHINode *PN = dyn_cast<PHINode>(DestBB->begin())) {
	for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
	if (PN->getIncomingBlock(i) != NewBB)
	OtherPreds.push_back(PN->getIncomingBlock(i));
	} else {
	for (pred_iterator I = pred_begin(DestBB), E = pred_end(DestBB);
	I != E; ++I) {
	BasicBlock P = I;
	if (P != NewBB)
	OtherPreds.push_back(P);
	}
	}

	bool NewBBDominatesDestBB = true;

	// Should we update DominatorTree information?
	if (DT) {
	DomTreeNode *TINode = DT->getNode(TIBB);

	// The new block is not the immediate dominator for any other nodes, but
	// TINode is the immediate dominator for the new node.
	//
	if (TINode) { // Don't break unreachable code!
	DomTreeNode *NewBBNode = DT->addNewBlock(NewBB, TIBB);
	DomTreeNode *DestBBNode = 0;

	// If NewBBDominatesDestBB hasn't been computed yet, do so with DT.
	if (!OtherPreds.empty()) {
	DestBBNode = DT->getNode(DestBB);
	while (!OtherPreds.empty() && NewBBDominatesDestBB) {
	if (DomTreeNode *OPNode = DT->getNode(OtherPreds.back()))
	NewBBDominatesDestBB = DT->dominates(DestBBNode, OPNode);
	OtherPreds.pop_back();
	}
	OtherPreds.clear();
	}

	// If NewBBDominatesDestBB, then NewBB dominates DestBB, otherwise it
	// doesn't dominate anything.
	if (NewBBDominatesDestBB) {
	if (!DestBBNode) DestBBNode = DT->getNode(DestBB);
	DT->changeImmediateDominator(DestBBNode, NewBBNode);
	}
	}
	}

	// Update LoopInfo if it is around.
	if (LI) {
	if (Loop *TIL = LI->getLoopFor(TIBB)) {
	// If one or the other blocks were not in a loop, the new block is not
	// either, and thus LI doesn't need to be updated.
	if (Loop *DestLoop = LI->getLoopFor(DestBB)) {
	if (TIL == DestLoop) {
	// Both in the same loop, the NewBB joins loop.
	DestLoop->addBasicBlockToLoop(NewBB, LI->getBase());
	} else if (TIL->contains(DestLoop)) {
	// Edge from an outer loop to an inner loop. Add to the outer loop.
	TIL->addBasicBlockToLoop(NewBB, LI->getBase());
	} else if (DestLoop->contains(TIL)) {
	// Edge from an inner loop to an outer loop. Add to the outer loop.
	DestLoop->addBasicBlockToLoop(NewBB, LI->getBase());
	} else {
	// Edge from two loops with no containment relation. Because these
	// are natural loops, we know that the destination block must be the
	// header of its loop (adding a branch into a loop elsewhere would
	// create an irreducible loop).
	assert(DestLoop->getHeader() == DestBB &&
	"Should not create irreducible loops!");
	if (Loop *P = DestLoop->getParentLoop())
	P->addBasicBlockToLoop(NewBB, LI->getBase());
	}
	}
	// If TIBB is in a loop and DestBB is outside of that loop, split the
	// other exit blocks of the loop that also have predecessors outside
	// the loop, to maintain a LoopSimplify guarantee.
	if (!TIL->contains(DestBB) &&
	P->mustPreserveAnalysisID(LoopSimplifyID)) {
	assert(!TIL->contains(NewBB) &&
	"Split point for loop exit is contained in loop!");

	// Update LCSSA form in the newly created exit block.
	if (P->mustPreserveAnalysisID(LCSSAID))
	createPHIsForSplitLoopExit(TIBB, NewBB, DestBB);

	// For each unique exit block...
	// FIXME: This code is functionally equivalent to the corresponding
	// loop in LoopSimplify.
	SmallVector<BasicBlock *, 4> ExitBlocks;
	TIL->getExitBlocks(ExitBlocks);
	for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i) {
	// Collect all the preds that are inside the loop, and note
	// whether there are any preds outside the loop.
	SmallVector<BasicBlock *, 4> Preds;
	bool HasPredOutsideOfLoop = false;
	BasicBlock *Exit = ExitBlocks[i];
	for (pred_iterator I = pred_begin(Exit), E = pred_end(Exit);
	I != E; ++I) {
	BasicBlock P = I;
	if (TIL->contains(P)) {
	if (isa<IndirectBrInst>(P->getTerminator())) {
	Preds.clear();
	break;
	}
	Preds.push_back(P);
	} else {
	HasPredOutsideOfLoop = true;
	}
	}
	// If there are any preds not in the loop, we'll need to split
	// the edges. The Preds.empty() check is needed because a block
	// may appear multiple times in the list. We can't use
	// getUniqueExitBlocks above because that depends on LoopSimplify
	// form, which we're in the process of restoring!
	if (!Preds.empty() && HasPredOutsideOfLoop) {
	if (!Exit->isLandingPad()) {
	BasicBlock *NewExitBB =
	SplitBlockPredecessors(Exit, Preds, "split", P);
	if (P->mustPreserveAnalysisID(LCSSAID))
	createPHIsForSplitLoopExit(Preds, NewExitBB, Exit);
	} else if (SplitLandingPads) {
	SmallVector<BasicBlock*, 8> NewBBs;
	SplitLandingPadPredecessors(Exit, Preds,
	".split1", ".split2",
	P, NewBBs);
	if (P->mustPreserveAnalysisID(LCSSAID))
	createPHIsForSplitLoopExit(Preds, NewBBs[0], Exit);
	}
	}
	}
	}
	// LCSSA form was updated above for the case where LoopSimplify is
	// available, which means that all predecessors of loop exit blocks
	// are within the loop. Without LoopSimplify form, it would be
	// necessary to insert a new phi.
	assert((!P->mustPreserveAnalysisID(LCSSAID) \|\|
	P->mustPreserveAnalysisID(LoopSimplifyID)) &&
	"SplitCriticalEdge doesn't know how to update LCCSA form "
	"without LoopSimplify!");
	}
	}

	// Update ProfileInfo if it is around.
	if (PI)
	PI->splitEdge(TIBB, DestBB, NewBB, MergeIdenticalEdges);

	return NewBB;
	}