//===- LoopSimplify.cpp - Loop Canonicalization Pass ----------------------===// // // The LLVM Compiler Infrastructure // // This file was developed by the LLVM research group and is distributed under // the University of Illinois Open Source License. See LICENSE.TXT for details. // //===----------------------------------------------------------------------===// // // This pass performs several transformations to transform natural loops into a // simpler form, which makes subsequent analyses and transformations simpler and // more effective. // // Loop pre-header insertion guarantees that there is a single, non-critical // entry edge from outside of the loop to the loop header. This simplifies a // number of analyses and transformations, such as LICM. // // Loop exit-block insertion guarantees that all exit blocks from the loop // (blocks which are outside of the loop that have predecessors inside of the // loop) only have predecessors from inside of the loop (and are thus dominated // by the loop header). This simplifies transformations such as store-sinking // that are built into LICM. // // This pass also guarantees that loops will have exactly one backedge. // // Note that the simplifycfg pass will clean up blocks which are split out but // end up being unnecessary, so usage of this pass should not pessimize // generated code. // // This pass obviously modifies the CFG, but updates loop information and // dominator information. // //===----------------------------------------------------------------------===// #include "llvm/Transforms/Scalar.h" #include "llvm/Constant.h" #include "llvm/Instructions.h" #include "llvm/Function.h" #include "llvm/Type.h" #include "llvm/Analysis/AliasAnalysis.h" #include "llvm/Analysis/Dominators.h" #include "llvm/Analysis/LoopInfo.h" #include "llvm/Support/CFG.h" #include "llvm/ADT/SetOperations.h" #include "llvm/ADT/SetVector.h" #include "llvm/ADT/Statistic.h" #include "llvm/ADT/DepthFirstIterator.h" using namespace llvm; namespace { Statistic<> NumInserted("loopsimplify", "Number of pre-header or exit blocks inserted"); Statistic<> NumNested("loopsimplify", "Number of nested loops split out"); struct LoopSimplify : public FunctionPass { // AA - If we have an alias analysis object to update, this is it, otherwise // this is null. AliasAnalysis *AA; LoopInfo *LI; virtual bool runOnFunction(Function &F); virtual void getAnalysisUsage(AnalysisUsage &AU) const { // We need loop information to identify the loops... AU.addRequired(); AU.addRequired(); AU.addRequired(); AU.addPreserved(); AU.addPreserved(); AU.addPreserved(); AU.addPreserved(); AU.addPreserved(); AU.addPreserved(); AU.addPreservedID(BreakCriticalEdgesID); // No critical edges added. } private: bool ProcessLoop(Loop *L); BasicBlock *SplitBlockPredecessors(BasicBlock *BB, const char *Suffix, const std::vector &Preds); BasicBlock *RewriteLoopExitBlock(Loop *L, BasicBlock *Exit); void InsertPreheaderForLoop(Loop *L); Loop *SeparateNestedLoop(Loop *L); void InsertUniqueBackedgeBlock(Loop *L); void UpdateDomInfoForRevectoredPreds(BasicBlock *NewBB, std::vector &PredBlocks); }; RegisterOpt X("loopsimplify", "Canonicalize natural loops", true); } // Publically exposed interface to pass... const PassInfo *llvm::LoopSimplifyID = X.getPassInfo(); FunctionPass *llvm::createLoopSimplifyPass() { return new LoopSimplify(); } /// runOnFunction - Run down all loops in the CFG (recursively, but we could do /// it in any convenient order) inserting preheaders... /// bool LoopSimplify::runOnFunction(Function &F) { bool Changed = false; LI = &getAnalysis(); AA = getAnalysisToUpdate(); for (LoopInfo::iterator I = LI->begin(), E = LI->end(); I != E; ++I) Changed |= ProcessLoop(*I); return Changed; } /// ProcessLoop - Walk the loop structure in depth first order, ensuring that /// all loops have preheaders. /// bool LoopSimplify::ProcessLoop(Loop *L) { bool Changed = false; // Canonicalize inner loops before outer loops. Inner loop canonicalization // can provide work for the outer loop to canonicalize. for (Loop::iterator I = L->begin(), E = L->end(); I != E; ++I) Changed |= ProcessLoop(*I); // Check to see that no blocks (other than the header) in the loop have // predecessors that are not in the loop. This is not valid for natural // loops, but can occur if the blocks are unreachable. Since they are // unreachable we can just shamelessly destroy their terminators to make them // not branch into the loop! assert(L->getBlocks()[0] == L->getHeader() && "Header isn't first block in loop?"); for (unsigned i = 1, e = L->getBlocks().size(); i != e; ++i) { BasicBlock *LoopBB = L->getBlocks()[i]; Retry: for (pred_iterator PI = pred_begin(LoopBB), E = pred_end(LoopBB); PI != E; ++PI) if (!L->contains(*PI)) { // This predecessor is not in the loop. Kill its terminator! BasicBlock *DeadBlock = *PI; for (succ_iterator SI = succ_begin(DeadBlock), E = succ_end(DeadBlock); SI != E; ++SI) (*SI)->removePredecessor(DeadBlock); // Remove PHI node entries // Delete the dead terminator. if (AA) AA->deleteValue(&DeadBlock->back()); DeadBlock->getInstList().pop_back(); Value *RetVal = 0; if (LoopBB->getParent()->getReturnType() != Type::VoidTy) RetVal = Constant::getNullValue(LoopBB->getParent()->getReturnType()); new ReturnInst(RetVal, DeadBlock); goto Retry; // We just invalidated the pred_iterator. Retry. } } // Does the loop already have a preheader? If so, don't modify the loop... if (L->getLoopPreheader() == 0) { InsertPreheaderForLoop(L); NumInserted++; Changed = true; } // Next, check to make sure that all exit nodes of the loop only have // predecessors that are inside of the loop. This check guarantees that the // loop preheader/header will dominate the exit blocks. If the exit block has // predecessors from outside of the loop, split the edge now. std::vector ExitBlocks; L->getExitBlocks(ExitBlocks); SetVector ExitBlockSet(ExitBlocks.begin(), ExitBlocks.end()); for (SetVector::iterator I = ExitBlockSet.begin(), E = ExitBlockSet.end(); I != E; ++I) { BasicBlock *ExitBlock = *I; for (pred_iterator PI = pred_begin(ExitBlock), PE = pred_end(ExitBlock); PI != PE; ++PI) // Must be exactly this loop: no subloops, parent loops, or non-loop preds // allowed. if (!L->contains(*PI)) { RewriteLoopExitBlock(L, ExitBlock); NumInserted++; Changed = true; break; } } // If the header has more than two predecessors at this point (from the // preheader and from multiple backedges), we must adjust the loop. if (L->getNumBackEdges() != 1) { // If this is really a nested loop, rip it out into a child loop. if (Loop *NL = SeparateNestedLoop(L)) { ++NumNested; // This is a big restructuring change, reprocess the whole loop. ProcessLoop(NL); return true; } InsertUniqueBackedgeBlock(L); NumInserted++; Changed = true; } // Scan over the PHI nodes in the loop header. Since they now have only two // incoming values (the loop is canonicalized), we may have simplified the PHI // down to 'X = phi [X, Y]', which should be replaced with 'Y'. PHINode *PN; for (BasicBlock::iterator I = L->getHeader()->begin(); (PN = dyn_cast(I++)); ) if (Value *V = PN->hasConstantValue()) { PN->replaceAllUsesWith(V); PN->eraseFromParent(); } return Changed; } /// SplitBlockPredecessors - Split the specified block into two blocks. We want /// to move the predecessors specified in the Preds list to point to the new /// block, leaving the remaining predecessors pointing to BB. This method /// updates the SSA PHINode's, but no other analyses. /// BasicBlock *LoopSimplify::SplitBlockPredecessors(BasicBlock *BB, const char *Suffix, const std::vector &Preds) { // Create new basic block, insert right before the original block... BasicBlock *NewBB = new BasicBlock(BB->getName()+Suffix, BB->getParent(), BB); // The preheader first gets an unconditional branch to the loop header... BranchInst *BI = new BranchInst(BB, NewBB); // For every PHI node in the block, insert a PHI node into NewBB where the // incoming values from the out of loop edges are moved to NewBB. We have two // possible cases here. If the loop is dead, we just insert dummy entries // into the PHI nodes for the new edge. If the loop is not dead, we move the // incoming edges in BB into new PHI nodes in NewBB. // if (!Preds.empty()) { // Is the loop not obviously dead? // Check to see if the values being merged into the new block need PHI // nodes. If so, insert them. for (BasicBlock::iterator I = BB->begin(); isa(I); ) { PHINode *PN = cast(I); ++I; // Check to see if all of the values coming in are the same. If so, we // don't need to create a new PHI node. Value *InVal = PN->getIncomingValueForBlock(Preds[0]); for (unsigned i = 1, e = Preds.size(); i != e; ++i) if (InVal != PN->getIncomingValueForBlock(Preds[i])) { InVal = 0; break; } // If the values coming into the block are not the same, we need a PHI. if (InVal == 0) { // Create the new PHI node, insert it into NewBB at the end of the block PHINode *NewPHI = new PHINode(PN->getType(), PN->getName()+".ph", BI); if (AA) AA->copyValue(PN, NewPHI); // Move all of the edges from blocks outside the loop to the new PHI for (unsigned i = 0, e = Preds.size(); i != e; ++i) { Value *V = PN->removeIncomingValue(Preds[i], false); NewPHI->addIncoming(V, Preds[i]); } InVal = NewPHI; } else { // Remove all of the edges coming into the PHI nodes from outside of the // block. for (unsigned i = 0, e = Preds.size(); i != e; ++i) PN->removeIncomingValue(Preds[i], false); } // Add an incoming value to the PHI node in the loop for the preheader // edge. PN->addIncoming(InVal, NewBB); // Can we eliminate this phi node now? if (Value *V = PN->hasConstantValue(true)) { if (!isa(V) || getAnalysis().dominates(cast(V), PN)) { PN->replaceAllUsesWith(V); if (AA) AA->deleteValue(PN); BB->getInstList().erase(PN); } } } // Now that the PHI nodes are updated, actually move the edges from // Preds to point to NewBB instead of BB. // for (unsigned i = 0, e = Preds.size(); i != e; ++i) { TerminatorInst *TI = Preds[i]->getTerminator(); for (unsigned s = 0, e = TI->getNumSuccessors(); s != e; ++s) if (TI->getSuccessor(s) == BB) TI->setSuccessor(s, NewBB); } } else { // Otherwise the loop is dead... for (BasicBlock::iterator I = BB->begin(); isa(I); ++I) { PHINode *PN = cast(I); // Insert dummy values as the incoming value... PN->addIncoming(Constant::getNullValue(PN->getType()), NewBB); } } return NewBB; } /// InsertPreheaderForLoop - Once we discover that a loop doesn't have a /// preheader, this method is called to insert one. This method has two phases: /// preheader insertion and analysis updating. /// void LoopSimplify::InsertPreheaderForLoop(Loop *L) { BasicBlock *Header = L->getHeader(); // Compute the set of predecessors of the loop that are not in the loop. std::vector OutsideBlocks; for (pred_iterator PI = pred_begin(Header), PE = pred_end(Header); PI != PE; ++PI) if (!L->contains(*PI)) // Coming in from outside the loop? OutsideBlocks.push_back(*PI); // Keep track of it... // Split out the loop pre-header BasicBlock *NewBB = SplitBlockPredecessors(Header, ".preheader", OutsideBlocks); //===--------------------------------------------------------------------===// // Update analysis results now that we have performed the transformation // // We know that we have loop information to update... update it now. if (Loop *Parent = L->getParentLoop()) Parent->addBasicBlockToLoop(NewBB, *LI); DominatorSet &DS = getAnalysis(); // Update dominator info DominatorTree &DT = getAnalysis(); // Update the dominator tree information. // The immediate dominator of the preheader is the immediate dominator of // the old header. DominatorTree::Node *PHDomTreeNode = DT.createNewNode(NewBB, DT.getNode(Header)->getIDom()); BasicBlock *oldHeaderIDom = DT.getNode(Header)->getIDom()->getBlock(); // Change the header node so that PNHode is the new immediate dominator DT.changeImmediateDominator(DT.getNode(Header), PHDomTreeNode); { // The blocks that dominate NewBB are the blocks that dominate Header, // minus Header, plus NewBB. DominatorSet::DomSetType DomSet = DS.getDominators(Header); DomSet.erase(Header); // Header does not dominate us... DS.addBasicBlock(NewBB, DomSet); // The newly created basic block dominates all nodes dominated by Header. for (df_iterator DFI = df_begin(PHDomTreeNode), E = df_end(PHDomTreeNode); DFI != E; ++DFI) DS.addDominator((*DFI)->getBlock(), NewBB); } // Update immediate dominator information if we have it... if (ImmediateDominators *ID = getAnalysisToUpdate()) { // Whatever i-dominated the header node now immediately dominates NewBB ID->addNewBlock(NewBB, ID->get(Header)); // The preheader now is the immediate dominator for the header node... ID->setImmediateDominator(Header, NewBB); } // Update ET Forest information if we have it... if (ETForest *EF = getAnalysisToUpdate()) { // Whatever i-dominated the header node now immediately dominates NewBB EF->addNewBlock(NewBB, oldHeaderIDom); // The preheader now is the immediate dominator for the header node... EF->setImmediateDominator(Header, NewBB); } // Update dominance frontier information... if (DominanceFrontier *DF = getAnalysisToUpdate()) { // The DF(NewBB) is just (DF(Header)-Header), because NewBB dominates // everything that Header does, and it strictly dominates Header in // addition. assert(DF->find(Header) != DF->end() && "Header node doesn't have DF set?"); DominanceFrontier::DomSetType NewDFSet = DF->find(Header)->second; NewDFSet.erase(Header); DF->addBasicBlock(NewBB, NewDFSet); // Now we must loop over all of the dominance frontiers in the function, // replacing occurrences of Header with NewBB in some cases. If a block // dominates a (now) predecessor of NewBB, but did not strictly dominate // Header, it will have Header in it's DF set, but should now have NewBB in // its set. for (unsigned i = 0, e = OutsideBlocks.size(); i != e; ++i) { // Get all of the dominators of the predecessor... const DominatorSet::DomSetType &PredDoms = DS.getDominators(OutsideBlocks[i]); for (DominatorSet::DomSetType::const_iterator PDI = PredDoms.begin(), PDE = PredDoms.end(); PDI != PDE; ++PDI) { BasicBlock *PredDom = *PDI; // If the loop header is in DF(PredDom), then PredDom didn't dominate // the header but did dominate a predecessor outside of the loop. Now // we change this entry to include the preheader in the DF instead of // the header. DominanceFrontier::iterator DFI = DF->find(PredDom); assert(DFI != DF->end() && "No dominance frontier for node?"); if (DFI->second.count(Header)) { DF->removeFromFrontier(DFI, Header); DF->addToFrontier(DFI, NewBB); } } } } } /// RewriteLoopExitBlock - Ensure that the loop preheader dominates all exit /// blocks. This method is used to split exit blocks that have predecessors /// outside of the loop. BasicBlock *LoopSimplify::RewriteLoopExitBlock(Loop *L, BasicBlock *Exit) { std::vector LoopBlocks; for (pred_iterator I = pred_begin(Exit), E = pred_end(Exit); I != E; ++I) if (L->contains(*I)) LoopBlocks.push_back(*I); assert(!LoopBlocks.empty() && "No edges coming in from outside the loop?"); BasicBlock *NewBB = SplitBlockPredecessors(Exit, ".loopexit", LoopBlocks); // Update Loop Information - we know that the new block will be in whichever // loop the Exit block is in. Note that it may not be in that immediate loop, // if the successor is some other loop header. In that case, we continue // walking up the loop tree to find a loop that contains both the successor // block and the predecessor block. Loop *SuccLoop = LI->getLoopFor(Exit); while (SuccLoop && !SuccLoop->contains(L->getHeader())) SuccLoop = SuccLoop->getParentLoop(); if (SuccLoop) SuccLoop->addBasicBlockToLoop(NewBB, *LI); // Update dominator information (set, immdom, domtree, and domfrontier) UpdateDomInfoForRevectoredPreds(NewBB, LoopBlocks); return NewBB; } /// AddBlockAndPredsToSet - Add the specified block, and all of its /// predecessors, to the specified set, if it's not already in there. Stop /// predecessor traversal when we reach StopBlock. static void AddBlockAndPredsToSet(BasicBlock *BB, BasicBlock *StopBlock, std::set &Blocks) { if (!Blocks.insert(BB).second) return; // already processed. if (BB == StopBlock) return; // Stop here! for (pred_iterator I = pred_begin(BB), E = pred_end(BB); I != E; ++I) AddBlockAndPredsToSet(*I, StopBlock, Blocks); } /// FindPHIToPartitionLoops - The first part of loop-nestification is to find a /// PHI node that tells us how to partition the loops. static PHINode *FindPHIToPartitionLoops(Loop *L, DominatorSet &DS, AliasAnalysis *AA) { for (BasicBlock::iterator I = L->getHeader()->begin(); isa(I); ) { PHINode *PN = cast(I); ++I; if (Value *V = PN->hasConstantValue()) if (!isa(V) || DS.dominates(cast(V), PN)) { // This is a degenerate PHI already, don't modify it! PN->replaceAllUsesWith(V); if (AA) AA->deleteValue(PN); PN->eraseFromParent(); continue; } // Scan this PHI node looking for a use of the PHI node by itself. for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) if (PN->getIncomingValue(i) == PN && L->contains(PN->getIncomingBlock(i))) // We found something tasty to remove. return PN; } return 0; } /// SeparateNestedLoop - If this loop has multiple backedges, try to pull one of /// them out into a nested loop. This is important for code that looks like /// this: /// /// Loop: /// ... /// br cond, Loop, Next /// ... /// br cond2, Loop, Out /// /// To identify this common case, we look at the PHI nodes in the header of the /// loop. PHI nodes with unchanging values on one backedge correspond to values /// that change in the "outer" loop, but not in the "inner" loop. /// /// If we are able to separate out a loop, return the new outer loop that was /// created. /// Loop *LoopSimplify::SeparateNestedLoop(Loop *L) { PHINode *PN = FindPHIToPartitionLoops(L, getAnalysis(), AA); if (PN == 0) return 0; // No known way to partition. // Pull out all predecessors that have varying values in the loop. This // handles the case when a PHI node has multiple instances of itself as // arguments. std::vector OuterLoopPreds; for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) if (PN->getIncomingValue(i) != PN || !L->contains(PN->getIncomingBlock(i))) OuterLoopPreds.push_back(PN->getIncomingBlock(i)); BasicBlock *Header = L->getHeader(); BasicBlock *NewBB = SplitBlockPredecessors(Header, ".outer", OuterLoopPreds); // Update dominator information (set, immdom, domtree, and domfrontier) UpdateDomInfoForRevectoredPreds(NewBB, OuterLoopPreds); // Create the new outer loop. Loop *NewOuter = new Loop(); // Change the parent loop to use the outer loop as its child now. if (Loop *Parent = L->getParentLoop()) Parent->replaceChildLoopWith(L, NewOuter); else LI->changeTopLevelLoop(L, NewOuter); // This block is going to be our new header block: add it to this loop and all // parent loops. NewOuter->addBasicBlockToLoop(NewBB, *LI); // L is now a subloop of our outer loop. NewOuter->addChildLoop(L); for (unsigned i = 0, e = L->getBlocks().size(); i != e; ++i) NewOuter->addBlockEntry(L->getBlocks()[i]); // Determine which blocks should stay in L and which should be moved out to // the Outer loop now. DominatorSet &DS = getAnalysis(); std::set BlocksInL; for (pred_iterator PI = pred_begin(Header), E = pred_end(Header); PI!=E; ++PI) if (DS.dominates(Header, *PI)) AddBlockAndPredsToSet(*PI, Header, BlocksInL); // Scan all of the loop children of L, moving them to OuterLoop if they are // not part of the inner loop. for (Loop::iterator I = L->begin(); I != L->end(); ) if (BlocksInL.count((*I)->getHeader())) ++I; // Loop remains in L else NewOuter->addChildLoop(L->removeChildLoop(I)); // Now that we know which blocks are in L and which need to be moved to // OuterLoop, move any blocks that need it. for (unsigned i = 0; i != L->getBlocks().size(); ++i) { BasicBlock *BB = L->getBlocks()[i]; if (!BlocksInL.count(BB)) { // Move this block to the parent, updating the exit blocks sets L->removeBlockFromLoop(BB); if ((*LI)[BB] == L) LI->changeLoopFor(BB, NewOuter); --i; } } return NewOuter; } /// InsertUniqueBackedgeBlock - This method is called when the specified loop /// has more than one backedge in it. If this occurs, revector all of these /// backedges to target a new basic block and have that block branch to the loop /// header. This ensures that loops have exactly one backedge. /// void LoopSimplify::InsertUniqueBackedgeBlock(Loop *L) { assert(L->getNumBackEdges() > 1 && "Must have > 1 backedge!"); // Get information about the loop BasicBlock *Preheader = L->getLoopPreheader(); BasicBlock *Header = L->getHeader(); Function *F = Header->getParent(); // Figure out which basic blocks contain back-edges to the loop header. std::vector BackedgeBlocks; for (pred_iterator I = pred_begin(Header), E = pred_end(Header); I != E; ++I) if (*I != Preheader) BackedgeBlocks.push_back(*I); // Create and insert the new backedge block... BasicBlock *BEBlock = new BasicBlock(Header->getName()+".backedge", F); BranchInst *BETerminator = new BranchInst(Header, BEBlock); // Move the new backedge block to right after the last backedge block. Function::iterator InsertPos = BackedgeBlocks.back(); ++InsertPos; F->getBasicBlockList().splice(InsertPos, F->getBasicBlockList(), BEBlock); // Now that the block has been inserted into the function, create PHI nodes in // the backedge block which correspond to any PHI nodes in the header block. for (BasicBlock::iterator I = Header->begin(); isa(I); ++I) { PHINode *PN = cast(I); PHINode *NewPN = new PHINode(PN->getType(), PN->getName()+".be", BETerminator); NewPN->reserveOperandSpace(BackedgeBlocks.size()); if (AA) AA->copyValue(PN, NewPN); // Loop over the PHI node, moving all entries except the one for the // preheader over to the new PHI node. unsigned PreheaderIdx = ~0U; bool HasUniqueIncomingValue = true; Value *UniqueValue = 0; for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) { BasicBlock *IBB = PN->getIncomingBlock(i); Value *IV = PN->getIncomingValue(i); if (IBB == Preheader) { PreheaderIdx = i; } else { NewPN->addIncoming(IV, IBB); if (HasUniqueIncomingValue) { if (UniqueValue == 0) UniqueValue = IV; else if (UniqueValue != IV) HasUniqueIncomingValue = false; } } } // Delete all of the incoming values from the old PN except the preheader's assert(PreheaderIdx != ~0U && "PHI has no preheader entry??"); if (PreheaderIdx != 0) { PN->setIncomingValue(0, PN->getIncomingValue(PreheaderIdx)); PN->setIncomingBlock(0, PN->getIncomingBlock(PreheaderIdx)); } // Nuke all entries except the zero'th. for (unsigned i = 0, e = PN->getNumIncomingValues()-1; i != e; ++i) PN->removeIncomingValue(e-i, false); // Finally, add the newly constructed PHI node as the entry for the BEBlock. PN->addIncoming(NewPN, BEBlock); // As an optimization, if all incoming values in the new PhiNode (which is a // subset of the incoming values of the old PHI node) have the same value, // eliminate the PHI Node. if (HasUniqueIncomingValue) { NewPN->replaceAllUsesWith(UniqueValue); if (AA) AA->deleteValue(NewPN); BEBlock->getInstList().erase(NewPN); } } // Now that all of the PHI nodes have been inserted and adjusted, modify the // backedge blocks to just to the BEBlock instead of the header. for (unsigned i = 0, e = BackedgeBlocks.size(); i != e; ++i) { TerminatorInst *TI = BackedgeBlocks[i]->getTerminator(); for (unsigned Op = 0, e = TI->getNumSuccessors(); Op != e; ++Op) if (TI->getSuccessor(Op) == Header) TI->setSuccessor(Op, BEBlock); } //===--- Update all analyses which we must preserve now -----------------===// // Update Loop Information - we know that this block is now in the current // loop and all parent loops. L->addBasicBlockToLoop(BEBlock, *LI); // Update dominator information (set, immdom, domtree, and domfrontier) UpdateDomInfoForRevectoredPreds(BEBlock, BackedgeBlocks); } /// UpdateDomInfoForRevectoredPreds - This method is used to update the four /// different kinds of dominator information (dominator sets, immediate /// dominators, dominator trees, and dominance frontiers) after a new block has /// been added to the CFG. /// /// This only supports the case when an existing block (known as "NewBBSucc"), /// had some of its predecessors factored into a new basic block. This /// transformation inserts a new basic block ("NewBB"), with a single /// unconditional branch to NewBBSucc, and moves some predecessors of /// "NewBBSucc" to now branch to NewBB. These predecessors are listed in /// PredBlocks, even though they are the same as /// pred_begin(NewBB)/pred_end(NewBB). /// void LoopSimplify::UpdateDomInfoForRevectoredPreds(BasicBlock *NewBB, std::vector &PredBlocks) { assert(!PredBlocks.empty() && "No predblocks??"); assert(succ_begin(NewBB) != succ_end(NewBB) && ++succ_begin(NewBB) == succ_end(NewBB) && "NewBB should have a single successor!"); BasicBlock *NewBBSucc = *succ_begin(NewBB); DominatorSet &DS = getAnalysis(); // Update dominator information... The blocks that dominate NewBB are the // intersection of the dominators of predecessors, plus the block itself. // DominatorSet::DomSetType NewBBDomSet = DS.getDominators(PredBlocks[0]); for (unsigned i = 1, e = PredBlocks.size(); i != e; ++i) set_intersect(NewBBDomSet, DS.getDominators(PredBlocks[i])); NewBBDomSet.insert(NewBB); // All blocks dominate themselves... DS.addBasicBlock(NewBB, NewBBDomSet); // The newly inserted basic block will dominate existing basic blocks iff the // PredBlocks dominate all of the non-pred blocks. If all predblocks dominate // the non-pred blocks, then they all must be the same block! // bool NewBBDominatesNewBBSucc = true; { BasicBlock *OnePred = PredBlocks[0]; for (unsigned i = 1, e = PredBlocks.size(); i != e; ++i) if (PredBlocks[i] != OnePred) { NewBBDominatesNewBBSucc = false; break; } if (NewBBDominatesNewBBSucc) for (pred_iterator PI = pred_begin(NewBBSucc), E = pred_end(NewBBSucc); PI != E; ++PI) if (*PI != NewBB && !DS.dominates(NewBBSucc, *PI)) { NewBBDominatesNewBBSucc = false; break; } } // The other scenario where the new block can dominate its successors are when // all predecessors of NewBBSucc that are not NewBB are dominated by NewBBSucc // already. if (!NewBBDominatesNewBBSucc) { NewBBDominatesNewBBSucc = true; for (pred_iterator PI = pred_begin(NewBBSucc), E = pred_end(NewBBSucc); PI != E; ++PI) if (*PI != NewBB && !DS.dominates(NewBBSucc, *PI)) { NewBBDominatesNewBBSucc = false; break; } } // If NewBB dominates some blocks, then it will dominate all blocks that // NewBBSucc does. if (NewBBDominatesNewBBSucc) { BasicBlock *PredBlock = PredBlocks[0]; Function *F = NewBB->getParent(); for (Function::iterator I = F->begin(), E = F->end(); I != E; ++I) if (DS.dominates(NewBBSucc, I)) DS.addDominator(I, NewBB); } // Update immediate dominator information if we have it... BasicBlock *NewBBIDom = 0; if (ImmediateDominators *ID = getAnalysisToUpdate()) { // To find the immediate dominator of the new exit node, we trace up the // immediate dominators of a predecessor until we find a basic block that // dominates the exit block. // BasicBlock *Dom = PredBlocks[0]; // Some random predecessor... while (!NewBBDomSet.count(Dom)) { // Loop until we find a dominator... assert(Dom != 0 && "No shared dominator found???"); Dom = ID->get(Dom); } // Set the immediate dominator now... ID->addNewBlock(NewBB, Dom); NewBBIDom = Dom; // Reuse this if calculating DominatorTree info... // If NewBB strictly dominates other blocks, we need to update their idom's // now. The only block that need adjustment is the NewBBSucc block, whose // idom should currently be set to PredBlocks[0]. if (NewBBDominatesNewBBSucc) ID->setImmediateDominator(NewBBSucc, NewBB); } // Update DominatorTree information if it is active. if (DominatorTree *DT = getAnalysisToUpdate()) { // If we don't have ImmediateDominator info around, calculate the idom as // above. DominatorTree::Node *NewBBIDomNode; if (NewBBIDom) { NewBBIDomNode = DT->getNode(NewBBIDom); } else { NewBBIDomNode = DT->getNode(PredBlocks[0]); // Random pred while (!NewBBDomSet.count(NewBBIDomNode->getBlock())) { NewBBIDomNode = NewBBIDomNode->getIDom(); assert(NewBBIDomNode && "No shared dominator found??"); } NewBBIDom = NewBBIDomNode->getBlock(); } // Create the new dominator tree node... and set the idom of NewBB. DominatorTree::Node *NewBBNode = DT->createNewNode(NewBB, NewBBIDomNode); // If NewBB strictly dominates other blocks, then it is now the immediate // dominator of NewBBSucc. Update the dominator tree as appropriate. if (NewBBDominatesNewBBSucc) { DominatorTree::Node *NewBBSuccNode = DT->getNode(NewBBSucc); DT->changeImmediateDominator(NewBBSuccNode, NewBBNode); } } // Update ET-Forest information if it is active. if (ETForest *EF = getAnalysisToUpdate()) { EF->addNewBlock(NewBB, NewBBIDom); if (NewBBDominatesNewBBSucc) EF->setImmediateDominator(NewBBSucc, NewBB); } // Update dominance frontier information... if (DominanceFrontier *DF = getAnalysisToUpdate()) { // If NewBB dominates NewBBSucc, then DF(NewBB) is now going to be the // DF(PredBlocks[0]) without the stuff that the new block does not dominate // a predecessor of. if (NewBBDominatesNewBBSucc) { DominanceFrontier::iterator DFI = DF->find(PredBlocks[0]); if (DFI != DF->end()) { DominanceFrontier::DomSetType Set = DFI->second; // Filter out stuff in Set that we do not dominate a predecessor of. for (DominanceFrontier::DomSetType::iterator SetI = Set.begin(), E = Set.end(); SetI != E;) { bool DominatesPred = false; for (pred_iterator PI = pred_begin(*SetI), E = pred_end(*SetI); PI != E; ++PI) if (DS.dominates(NewBB, *PI)) DominatesPred = true; if (!DominatesPred) Set.erase(SetI++); else ++SetI; } DF->addBasicBlock(NewBB, Set); } } else { // DF(NewBB) is {NewBBSucc} because NewBB does not strictly dominate // NewBBSucc, but it does dominate itself (and there is an edge (NewBB -> // NewBBSucc)). NewBBSucc is the single successor of NewBB. DominanceFrontier::DomSetType NewDFSet; NewDFSet.insert(NewBBSucc); DF->addBasicBlock(NewBB, NewDFSet); } // Now we must loop over all of the dominance frontiers in the function, // replacing occurrences of NewBBSucc with NewBB in some cases. All // blocks that dominate a block in PredBlocks and contained NewBBSucc in // their dominance frontier must be updated to contain NewBB instead. // for (unsigned i = 0, e = PredBlocks.size(); i != e; ++i) { BasicBlock *Pred = PredBlocks[i]; // Get all of the dominators of the predecessor... const DominatorSet::DomSetType &PredDoms = DS.getDominators(Pred); for (DominatorSet::DomSetType::const_iterator PDI = PredDoms.begin(), PDE = PredDoms.end(); PDI != PDE; ++PDI) { BasicBlock *PredDom = *PDI; // If the NewBBSucc node is in DF(PredDom), then PredDom didn't // dominate NewBBSucc but did dominate a predecessor of it. Now we // change this entry to include NewBB in the DF instead of NewBBSucc. DominanceFrontier::iterator DFI = DF->find(PredDom); assert(DFI != DF->end() && "No dominance frontier for node?"); if (DFI->second.count(NewBBSucc)) { // If NewBBSucc should not stay in our dominator frontier, remove it. // We remove it unless there is a predecessor of NewBBSucc that we // dominate, but we don't strictly dominate NewBBSucc. bool ShouldRemove = true; if (PredDom == NewBBSucc || !DS.dominates(PredDom, NewBBSucc)) { // Okay, we know that PredDom does not strictly dominate NewBBSucc. // Check to see if it dominates any predecessors of NewBBSucc. for (pred_iterator PI = pred_begin(NewBBSucc), E = pred_end(NewBBSucc); PI != E; ++PI) if (DS.dominates(PredDom, *PI)) { ShouldRemove = false; break; } } if (ShouldRemove) DF->removeFromFrontier(DFI, NewBBSucc); DF->addToFrontier(DFI, NewBB); } } } } }