lib/Transforms/Utils/BreakCriticalEdges.cpp - 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
 // forward dominator (set, immediate dominators, tree, and frontier)
 // information.
 //
 //===----------------------------------------------------------------------===//

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

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

 namespace {
   struct VISIBILITY_HIDDEN BreakCriticalEdges : public FunctionPass {
     static char ID; // Pass identification, replacement for typeid
     BreakCriticalEdges() : FunctionPass(&ID) {}

     virtual bool runOnFunction(Function &F);

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

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

 char BreakCriticalEdges::ID = 0;
 static RegisterPass<BreakCriticalEdges>
 X("break-crit-edges", "Break critical edges in CFG");

 // Publically exposed interface to pass...
 const PassInfo *const llvm::BreakCriticalEdgesID = &X;
 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)
       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);
   pred_const_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) {
     if (*I != 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(*I);
     ++I;
   }
   return false;
 }

 /// SplitCriticalEdge - If this edge is a critical edge, insert a new node to
 /// split the critical edge.  This will update DominatorTree and
 /// DominatorFrontier  information if it is available, thus calling this pass
 /// will not invalidate  any of them.  This returns true if the edge was split,
 /// false otherwise.  This ensures that all edges to that dest go to one block
 /// instead of each going to a different block.
 //
 bool llvm::SplitCriticalEdge(TerminatorInst *TI, unsigned SuccNum, Pass *P,
                              bool MergeIdenticalEdges) {
   if (!isCriticalEdge(TI, SuccNum, MergeIdenticalEdges)) return false;
   BasicBlock *TIBB = TI->getParent();
   BasicBlock *DestBB = TI->getSuccessor(SuccNum);

   // 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::Create(DestBB, NewBB);

   // 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.
   //
   for (BasicBlock::iterator I = DestBB->begin(); isa<PHINode>(I); ++I) {
     PHINode *PN = cast<PHINode>(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.
     int 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);

       // 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 true;

   // 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;

   for (pred_iterator I = pred_begin(DestBB), E = pred_end(DestBB); I != E; ++I)
     if (*I != NewBB)
       OtherPreds.push_back(*I);

   bool NewBBDominatesDestBB = true;

   // Should we update DominatorTree information?
   if (DominatorTree *DT = P->getAnalysisIfAvailable<DominatorTree>()) {
     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);
       }
     }
   }

   // Should we update DominanceFrontier information?
   if (DominanceFrontier *DF = P->getAnalysisIfAvailable<DominanceFrontier>()) {
     // If NewBBDominatesDestBB hasn't been computed yet, do so with DF.
     if (!OtherPreds.empty()) {
       // FIXME: IMPLEMENT THIS!
       llvm_unreachable("Requiring domfrontiers but not idom/domtree/domset."
                        " not implemented yet!");
     }

     // Since the new block is dominated by its only predecessor TIBB,
     // it cannot be in any block's dominance frontier.  If NewBB dominates
     // DestBB, its dominance frontier is the same as DestBB's, otherwise it is
     // just {DestBB}.
     DominanceFrontier::DomSetType NewDFSet;
     if (NewBBDominatesDestBB) {
       DominanceFrontier::iterator I = DF->find(DestBB);
       if (I != DF->end()) {
         DF->addBasicBlock(NewBB, I->second);

         if (I->second.count(DestBB)) {
           // However NewBB's frontier does not include DestBB.
           DominanceFrontier::iterator NF = DF->find(NewBB);
           DF->removeFromFrontier(NF, DestBB);
         }
       }
       else
         DF->addBasicBlock(NewBB, DominanceFrontier::DomSetType());
     } else {
       DominanceFrontier::DomSetType NewDFSet;
       NewDFSet.insert(DestBB);
       DF->addBasicBlock(NewBB, NewDFSet);
     }
   }

   // Update LoopInfo if it is around.
   if (LoopInfo *LI = P->getAnalysisIfAvailable<LoopInfo>()) {
     // 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 *TIL = LI->getLoopFor(TIBB))
       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->getHeader())) {
           // Edge from an outer loop to an inner loop.  Add to the outer loop.
           TIL->addBasicBlockToLoop(NewBB, LI->getBase());
         } else if (DestLoop->contains(TIL->getHeader())) {
           // 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());
         }
       }
   }
   return true;
 }
	//===- 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
	// forward dominator (set, immediate dominators, tree, and frontier)
	// information.
	//
	//===----------------------------------------------------------------------===//

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

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

	namespace {
	struct VISIBILITY_HIDDEN BreakCriticalEdges : public FunctionPass {
	static char ID; // Pass identification, replacement for typeid
	BreakCriticalEdges() : FunctionPass(&ID) {}

	virtual bool runOnFunction(Function &F);

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

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

	char BreakCriticalEdges::ID = 0;
	static RegisterPass<BreakCriticalEdges>
	X("break-crit-edges", "Break critical edges in CFG");

	// Publically exposed interface to pass...
	const PassInfo *const llvm::BreakCriticalEdgesID = &X;
	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)
	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);
	pred_const_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) {
	if (*I != 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(*I);
	++I;
	}
	return false;
	}

	/// SplitCriticalEdge - If this edge is a critical edge, insert a new node to
	/// split the critical edge. This will update DominatorTree and
	/// DominatorFrontier information if it is available, thus calling this pass
	/// will not invalidate any of them. This returns true if the edge was split,
	/// false otherwise. This ensures that all edges to that dest go to one block
	/// instead of each going to a different block.
	//
	bool llvm::SplitCriticalEdge(TerminatorInst TI, unsigned SuccNum, Pass P,
	bool MergeIdenticalEdges) {
	if (!isCriticalEdge(TI, SuccNum, MergeIdenticalEdges)) return false;
	BasicBlock *TIBB = TI->getParent();
	BasicBlock *DestBB = TI->getSuccessor(SuccNum);

	// 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::Create(DestBB, NewBB);

	// 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.
	//
	for (BasicBlock::iterator I = DestBB->begin(); isa<PHINode>(I); ++I) {
	PHINode *PN = cast<PHINode>(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.
	int 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);

	// 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 true;

	// 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;

	for (pred_iterator I = pred_begin(DestBB), E = pred_end(DestBB); I != E; ++I)
	if (*I != NewBB)
	OtherPreds.push_back(*I);

	bool NewBBDominatesDestBB = true;

	// Should we update DominatorTree information?
	if (DominatorTree *DT = P->getAnalysisIfAvailable<DominatorTree>()) {
	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);
	}
	}
	}

	// Should we update DominanceFrontier information?
	if (DominanceFrontier *DF = P->getAnalysisIfAvailable<DominanceFrontier>()) {
	// If NewBBDominatesDestBB hasn't been computed yet, do so with DF.
	if (!OtherPreds.empty()) {
	// FIXME: IMPLEMENT THIS!
	llvm_unreachable("Requiring domfrontiers but not idom/domtree/domset."
	" not implemented yet!");
	}

	// Since the new block is dominated by its only predecessor TIBB,
	// it cannot be in any block's dominance frontier. If NewBB dominates
	// DestBB, its dominance frontier is the same as DestBB's, otherwise it is
	// just {DestBB}.
	DominanceFrontier::DomSetType NewDFSet;
	if (NewBBDominatesDestBB) {
	DominanceFrontier::iterator I = DF->find(DestBB);
	if (I != DF->end()) {
	DF->addBasicBlock(NewBB, I->second);

	if (I->second.count(DestBB)) {
	// However NewBB's frontier does not include DestBB.
	DominanceFrontier::iterator NF = DF->find(NewBB);
	DF->removeFromFrontier(NF, DestBB);
	}
	}
	else
	DF->addBasicBlock(NewBB, DominanceFrontier::DomSetType());
	} else {
	DominanceFrontier::DomSetType NewDFSet;
	NewDFSet.insert(DestBB);
	DF->addBasicBlock(NewBB, NewDFSet);
	}
	}

	// Update LoopInfo if it is around.
	if (LoopInfo *LI = P->getAnalysisIfAvailable<LoopInfo>()) {
	// 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 *TIL = LI->getLoopFor(TIBB))
	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->getHeader())) {
	// Edge from an outer loop to an inner loop. Add to the outer loop.
	TIL->addBasicBlockToLoop(NewBB, LI->getBase());
	} else if (DestLoop->contains(TIL->getHeader())) {
	// 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());
	}
	}
	}
	return true;
	}