LLVM API Documentation

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/Analysis/LoopInfo.h"
00023 #include "llvm/Function.h"
00024 #include "llvm/Instructions.h"
00025 #include "llvm/Type.h"
00026 #include "llvm/Support/CFG.h"
00027 #include "llvm/Support/Visibility.h"
00028 #include "llvm/ADT/Statistic.h"
00029 using namespace llvm;
00030 
00031 namespace {
00032   Statistic<> NumBroken("break-crit-edges", "Number of blocks inserted");
00033 
00034   struct VISIBILITY_HIDDEN BreakCriticalEdges : public FunctionPass {
00035     virtual bool runOnFunction(Function &F);
00036 
00037     virtual void getAnalysisUsage(AnalysisUsage &AU) const {
00038       AU.addPreserved<ETForest>();
00039       AU.addPreserved<DominatorSet>();
00040       AU.addPreserved<ImmediateDominators>();
00041       AU.addPreserved<DominatorTree>();
00042       AU.addPreserved<DominanceFrontier>();
00043       AU.addPreserved<LoopInfo>();
00044 
00045       // No loop canonicalization guarantees are broken by this pass.
00046       AU.addPreservedID(LoopSimplifyID);
00047     }
00048   };
00049 
00050   RegisterOpt<BreakCriticalEdges> X("break-crit-edges",
00051                                     "Break critical edges in CFG");
00052 }
00053 
00054 // Publically exposed interface to pass...
00055 const PassInfo *llvm::BreakCriticalEdgesID = X.getPassInfo();
00056 FunctionPass *llvm::createBreakCriticalEdgesPass() {
00057   return new BreakCriticalEdges();
00058 }
00059 
00060 // runOnFunction - Loop over all of the edges in the CFG, breaking critical
00061 // edges as they are found.
00062 //
00063 bool BreakCriticalEdges::runOnFunction(Function &F) {
00064   bool Changed = false;
00065   for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I) {
00066     TerminatorInst *TI = I->getTerminator();
00067     if (TI->getNumSuccessors() > 1)
00068       for (unsigned i = 0, e = TI->getNumSuccessors(); i != e; ++i)
00069         if (SplitCriticalEdge(TI, i, this)) {
00070           ++NumBroken;
00071           Changed = true;
00072         }
00073   }
00074 
00075   return Changed;
00076 }
00077 
00078 //===----------------------------------------------------------------------===//
00079 //    Implementation of the external critical edge manipulation functions
00080 //===----------------------------------------------------------------------===//
00081 
00082 // isCriticalEdge - Return true if the specified edge is a critical edge.
00083 // Critical edges are edges from a block with multiple successors to a block
00084 // with multiple predecessors.
00085 //
00086 bool llvm::isCriticalEdge(const TerminatorInst *TI, unsigned SuccNum) {
00087   assert(SuccNum < TI->getNumSuccessors() && "Illegal edge specification!");
00088   if (TI->getNumSuccessors() == 1) return false;
00089 
00090   const BasicBlock *Dest = TI->getSuccessor(SuccNum);
00091   pred_const_iterator I = pred_begin(Dest), E = pred_end(Dest);
00092 
00093   // If there is more than one predecessor, this is a critical edge...
00094   assert(I != E && "No preds, but we have an edge to the block?");
00095   ++I;        // Skip one edge due to the incoming arc from TI.
00096   return I != E;
00097 }
00098 
00099 // SplitCriticalEdge - If this edge is a critical edge, insert a new node to
00100 // split the critical edge.  This will update DominatorSet, ImmediateDominator,
00101 // DominatorTree, and DominatorFrontier information if it is available, thus
00102 // calling this pass will not invalidate either of them.  This returns true if
00103 // the edge was split, false otherwise.
00104 //
00105 bool llvm::SplitCriticalEdge(TerminatorInst *TI, unsigned SuccNum, Pass *P) {
00106   if (!isCriticalEdge(TI, SuccNum)) return false;
00107   BasicBlock *TIBB = TI->getParent();
00108   BasicBlock *DestBB = TI->getSuccessor(SuccNum);
00109 
00110   // Create a new basic block, linking it into the CFG.
00111   BasicBlock *NewBB = new BasicBlock(TIBB->getName() + "." +
00112                                      DestBB->getName() + "_crit_edge");
00113   // Create our unconditional branch...
00114   new BranchInst(DestBB, NewBB);
00115 
00116   // Branch to the new block, breaking the edge...
00117   TI->setSuccessor(SuccNum, NewBB);
00118 
00119   // Insert the block into the function... right after the block TI lives in.
00120   Function &F = *TIBB->getParent();
00121   F.getBasicBlockList().insert(TIBB->getNext(), NewBB);
00122 
00123   // If there are any PHI nodes in DestBB, we need to update them so that they
00124   // merge incoming values from NewBB instead of from TIBB.
00125   //
00126   for (BasicBlock::iterator I = DestBB->begin(); isa<PHINode>(I); ++I) {
00127     PHINode *PN = cast<PHINode>(I);
00128     // We no longer enter through TIBB, now we come in through NewBB.  Revector
00129     // exactly one entry in the PHI node that used to come from TIBB to come
00130     // from NewBB.
00131     int BBIdx = PN->getBasicBlockIndex(TIBB);
00132     PN->setIncomingBlock(BBIdx, NewBB);
00133   }
00134 
00135   // If we don't have a pass object, we can't update anything...
00136   if (P == 0) return true;
00137 
00138   // Now update analysis information.  These are the analyses that we are
00139   // currently capable of updating...
00140   //
00141 
00142   // Should we update DominatorSet information?
00143   if (DominatorSet *DS = P->getAnalysisToUpdate<DominatorSet>()) {
00144     // The blocks that dominate the new one are the blocks that dominate TIBB
00145     // plus the new block itself.
00146     DominatorSet::DomSetType DomSet = DS->getDominators(TIBB);
00147     DomSet.insert(NewBB);  // A block always dominates itself.
00148     DS->addBasicBlock(NewBB, DomSet);
00149   }
00150 
00151   // Should we update ImmediateDominator information?
00152   if (ImmediateDominators *ID = P->getAnalysisToUpdate<ImmediateDominators>()) {
00153     // TIBB is the new immediate dominator for NewBB.  NewBB doesn't dominate
00154     // anything.
00155     ID->addNewBlock(NewBB, TIBB);
00156   }
00157 
00158   // Update the forest?
00159   if (ETForest *EF = P->getAnalysisToUpdate<ETForest>())
00160     EF->addNewBlock(NewBB, TIBB);
00161 
00162   // Should we update DominatorTree information?
00163   if (DominatorTree *DT = P->getAnalysisToUpdate<DominatorTree>()) {
00164     DominatorTree::Node *TINode = DT->getNode(TIBB);
00165 
00166     // The new block is not the immediate dominator for any other nodes, but
00167     // TINode is the immediate dominator for the new node.
00168     //
00169     if (TINode)        // Don't break unreachable code!
00170       DT->createNewNode(NewBB, TINode);
00171   }
00172 
00173   // Should we update DominanceFrontier information?
00174   if (DominanceFrontier *DF = P->getAnalysisToUpdate<DominanceFrontier>()) {
00175     // Since the new block is dominated by its only predecessor TIBB,
00176     // it cannot be in any block's dominance frontier.  Its dominance
00177     // frontier is {DestBB}.
00178     DominanceFrontier::DomSetType NewDFSet;
00179     NewDFSet.insert(DestBB);
00180     DF->addBasicBlock(NewBB, NewDFSet);
00181   }
00182   
00183   // Update LoopInfo if it is around.
00184   if (LoopInfo *LI = P->getAnalysisToUpdate<LoopInfo>()) {
00185     // If one or the other blocks were not in a loop, the new block is not
00186     // either, and thus LI doesn't need to be updated.
00187     if (Loop *TIL = LI->getLoopFor(TIBB))
00188       if (Loop *DestLoop = LI->getLoopFor(DestBB)) {
00189         if (TIL == DestLoop) {
00190           // Both in the same loop, the NewBB joins loop.
00191           DestLoop->addBasicBlockToLoop(NewBB, *LI);
00192         } else if (TIL->contains(DestLoop->getHeader())) {
00193           // Edge from an outer loop to an inner loop.  Add to the outer lopo.
00194           TIL->addBasicBlockToLoop(NewBB, *LI);
00195         } else if (DestLoop->contains(TIL->getHeader())) {
00196           // Edge from an inner loop to an outer loop.  Add to the outer lopo.
00197           DestLoop->addBasicBlockToLoop(NewBB, *LI);
00198         } else {
00199           // Edge from two loops with no containment relation.  Because these
00200           // are natural loops, we know that the destination block must be the
00201           // header of its loop (adding a branch into a loop elsewhere would
00202           // create an irreducible loop).
00203           assert(DestLoop->getHeader() == DestBB &&
00204                  "Should not create irreducible loops!");
00205           if (Loop *P = DestLoop->getParentLoop())
00206             P->addBasicBlockToLoop(NewBB, *LI);
00207         }
00208       }
00209     
00210   }
00211   return true;
00212 }