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BreakCriticalEdges.cpp

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00001 //===- BreakCriticalEdges.cpp - Critical Edge Elimination Pass ------------===//
00002 // 
00003 //                     The LLVM Compiler Infrastructure
00004 //
00005 // This file was developed by the LLVM research group and is distributed under
00006 // the University of Illinois Open Source License. See LICENSE.TXT for details.
00007 // 
00008 //===----------------------------------------------------------------------===//
00009 //
00010 // BreakCriticalEdges pass - Break all of the critical edges in the CFG by
00011 // inserting a dummy basic block.  This pass may be "required" by passes that
00012 // cannot deal with critical edges.  For this usage, the structure type is
00013 // forward declared.  This pass obviously invalidates the CFG, but can update
00014 // forward dominator (set, immediate dominators, tree, and frontier)
00015 // information.
00016 //
00017 //===----------------------------------------------------------------------===//
00018 
00019 #include "llvm/Transforms/Scalar.h"
00020 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
00021 #include "llvm/Analysis/Dominators.h"
00022 #include "llvm/Function.h"
00023 #include "llvm/Instructions.h"
00024 #include "llvm/Support/CFG.h"
00025 #include "llvm/ADT/Statistic.h"
00026 using namespace llvm;
00027 
00028 namespace {
00029   Statistic<> NumBroken("break-crit-edges", "Number of blocks inserted");
00030 
00031   struct BreakCriticalEdges : public FunctionPass {
00032     virtual bool runOnFunction(Function &F);
00033     
00034     virtual void getAnalysisUsage(AnalysisUsage &AU) const {
00035       AU.addPreserved<DominatorSet>();
00036       AU.addPreserved<ImmediateDominators>();
00037       AU.addPreserved<DominatorTree>();
00038       AU.addPreserved<DominanceFrontier>();
00039 
00040       // No loop canonicalization guarantees are broken by this pass.
00041       AU.addPreservedID(LoopSimplifyID);
00042     }
00043   };
00044 
00045   RegisterOpt<BreakCriticalEdges> X("break-crit-edges",
00046                                     "Break critical edges in CFG");
00047 }
00048 
00049 // Publically exposed interface to pass...
00050 const PassInfo *llvm::BreakCriticalEdgesID = X.getPassInfo();
00051 FunctionPass *llvm::createBreakCriticalEdgesPass() {
00052   return new BreakCriticalEdges();
00053 }
00054 
00055 // runOnFunction - Loop over all of the edges in the CFG, breaking critical
00056 // edges as they are found.
00057 //
00058 bool BreakCriticalEdges::runOnFunction(Function &F) {
00059   bool Changed = false;
00060   for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I) {
00061     TerminatorInst *TI = I->getTerminator();
00062     if (TI->getNumSuccessors() > 1)
00063       for (unsigned i = 0, e = TI->getNumSuccessors(); i != e; ++i)
00064         if (SplitCriticalEdge(TI, i, this)) {
00065           ++NumBroken;
00066           Changed = true;
00067         }
00068   }
00069 
00070   return Changed;
00071 }
00072 
00073 //===----------------------------------------------------------------------===//
00074 //    Implementation of the external critical edge manipulation functions
00075 //===----------------------------------------------------------------------===//
00076 
00077 // isCriticalEdge - Return true if the specified edge is a critical edge.
00078 // Critical edges are edges from a block with multiple successors to a block
00079 // with multiple predecessors.
00080 //
00081 bool llvm::isCriticalEdge(const TerminatorInst *TI, unsigned SuccNum) {
00082   assert(SuccNum < TI->getNumSuccessors() && "Illegal edge specification!");
00083   if (TI->getNumSuccessors() == 1) return false;
00084 
00085   const BasicBlock *Dest = TI->getSuccessor(SuccNum);
00086   pred_const_iterator I = pred_begin(Dest), E = pred_end(Dest);
00087 
00088   // If there is more than one predecessor, this is a critical edge...
00089   assert(I != E && "No preds, but we have an edge to the block?");
00090   ++I;        // Skip one edge due to the incoming arc from TI.
00091   return I != E;
00092 }
00093 
00094 // SplitCriticalEdge - If this edge is a critical edge, insert a new node to
00095 // split the critical edge.  This will update DominatorSet, ImmediateDominator,
00096 // DominatorTree, and DominatorFrontier information if it is available, thus
00097 // calling this pass will not invalidate either of them.  This returns true if
00098 // the edge was split, false otherwise.
00099 //
00100 bool llvm::SplitCriticalEdge(TerminatorInst *TI, unsigned SuccNum, Pass *P) {
00101   if (!isCriticalEdge(TI, SuccNum)) return false;
00102   BasicBlock *TIBB = TI->getParent();
00103   BasicBlock *DestBB = TI->getSuccessor(SuccNum);
00104 
00105   // Create a new basic block, linking it into the CFG.
00106   BasicBlock *NewBB = new BasicBlock(TIBB->getName() + "." +
00107                                      DestBB->getName() + "_crit_edge");
00108   // Create our unconditional branch...
00109   new BranchInst(DestBB, NewBB);
00110   
00111   // Branch to the new block, breaking the edge...
00112   TI->setSuccessor(SuccNum, NewBB);
00113 
00114   // Insert the block into the function... right after the block TI lives in.
00115   Function &F = *TIBB->getParent();
00116   F.getBasicBlockList().insert(TIBB->getNext(), NewBB);
00117 
00118   // If there are any PHI nodes in DestBB, we need to update them so that they
00119   // merge incoming values from NewBB instead of from TIBB.
00120   //
00121   for (BasicBlock::iterator I = DestBB->begin(); isa<PHINode>(I); ++I) {
00122     PHINode *PN = cast<PHINode>(I);
00123     // We no longer enter through TIBB, now we come in through NewBB.  Revector
00124     // exactly one entry in the PHI node that used to come from TIBB to come
00125     // from NewBB.
00126     Value *InVal = PN->removeIncomingValue(TIBB, false);
00127     PN->addIncoming(InVal, NewBB);
00128   }
00129 
00130   // If we don't have a pass object, we can't update anything...
00131   if (P == 0) return true;
00132 
00133   // Now update analysis information.  These are the analyses that we are
00134   // currently capable of updating...
00135   //
00136 
00137   // Should we update DominatorSet information?
00138   if (DominatorSet *DS = P->getAnalysisToUpdate<DominatorSet>()) {
00139     // The blocks that dominate the new one are the blocks that dominate TIBB
00140     // plus the new block itself.
00141     DominatorSet::DomSetType DomSet = DS->getDominators(TIBB);
00142     DomSet.insert(NewBB);  // A block always dominates itself.
00143     DS->addBasicBlock(NewBB, DomSet);
00144   }
00145 
00146   // Should we update ImmediateDominator information?
00147   if (ImmediateDominators *ID = P->getAnalysisToUpdate<ImmediateDominators>()) {
00148     // TIBB is the new immediate dominator for NewBB.  NewBB doesn't dominate
00149     // anything.
00150     ID->addNewBlock(NewBB, TIBB);
00151   }
00152   
00153   // Should we update DominatorTree information?
00154   if (DominatorTree *DT = P->getAnalysisToUpdate<DominatorTree>()) {
00155     DominatorTree::Node *TINode = DT->getNode(TIBB);
00156     
00157     // The new block is not the immediate dominator for any other nodes, but
00158     // TINode is the immediate dominator for the new node.
00159     //
00160     if (TINode)        // Don't break unreachable code!
00161       DT->createNewNode(NewBB, TINode);
00162   }
00163 
00164   // Should we update DominanceFrontier information?
00165   if (DominanceFrontier *DF = P->getAnalysisToUpdate<DominanceFrontier>()) {
00166     // Since the new block is dominated by its only predecessor TIBB,
00167     // it cannot be in any block's dominance frontier.  Its dominance
00168     // frontier is {DestBB}.
00169     DominanceFrontier::DomSetType NewDFSet;
00170     NewDFSet.insert(DestBB);
00171     DF->addBasicBlock(NewBB, NewDFSet);
00172   }
00173   return true;
00174 }