//===- LoopSimplify.cpp - Loop Canonicalization Pass ----------------------===// // // The LLVM Compiler Infrastructure // // This file 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. // //===----------------------------------------------------------------------===// #define DEBUG_TYPE "loopsimplify" #include "llvm/Transforms/Scalar.h" #include "llvm/Constants.h" #include "llvm/Instructions.h" #include "llvm/Function.h" #include "llvm/LLVMContext.h" #include "llvm/Type.h" #include "llvm/Analysis/AliasAnalysis.h" #include "llvm/Analysis/Dominators.h" #include "llvm/Analysis/LoopInfo.h" #include "llvm/Transforms/Utils/BasicBlockUtils.h" #include "llvm/Transforms/Utils/Local.h" #include "llvm/Support/CFG.h" #include "llvm/Support/Compiler.h" #include "llvm/ADT/SetOperations.h" #include "llvm/ADT/SetVector.h" #include "llvm/ADT/Statistic.h" #include "llvm/ADT/DepthFirstIterator.h" using namespace llvm; STATISTIC(NumInserted, "Number of pre-header or exit blocks inserted"); STATISTIC(NumNested , "Number of nested loops split out"); namespace { struct VISIBILITY_HIDDEN LoopSimplify : public FunctionPass { static char ID; // Pass identification, replacement for typeid LoopSimplify() : FunctionPass(&ID) {} // AA - If we have an alias analysis object to update, this is it, otherwise // this is null. AliasAnalysis *AA; LoopInfo *LI; DominatorTree *DT; virtual bool runOnFunction(Function &F); virtual void getAnalysisUsage(AnalysisUsage &AU) const { // We need loop information to identify the loops... AU.addRequired(); AU.addRequired(); AU.addPreserved(); AU.addPreserved(); AU.addPreserved(); AU.addPreserved(); AU.addPreservedID(BreakCriticalEdgesID); // No critical edges added. } /// verifyAnalysis() - Verify loop nest. void verifyAnalysis() const { #ifndef NDEBUG LoopInfo *NLI = &getAnalysis(); for (LoopInfo::iterator I = NLI->begin(), E = NLI->end(); I != E; ++I) (*I)->verifyLoop(); #endif } private: bool ProcessLoop(Loop *L); BasicBlock *RewriteLoopExitBlock(Loop *L, BasicBlock *Exit); BasicBlock *InsertPreheaderForLoop(Loop *L); Loop *SeparateNestedLoop(Loop *L); void InsertUniqueBackedgeBlock(Loop *L, BasicBlock *Preheader); void PlaceSplitBlockCarefully(BasicBlock *NewBB, SmallVectorImpl &SplitPreds, Loop *L); }; } char LoopSimplify::ID = 0; static RegisterPass X("loopsimplify", "Canonicalize natural loops", true); // Publically exposed interface to pass... const PassInfo *const llvm::LoopSimplifyID = &X; 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 = getAnalysisIfAvailable(); DT = &getAnalysis(); // Check to see that no blocks (other than the header) in loops have // predecessors that are not in loops. 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! for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB) { // This case can only occur for unreachable blocks. Blocks that are // unreachable can't be in loops, so filter those blocks out. if (LI->getLoopFor(BB)) continue; bool BlockUnreachable = false; TerminatorInst *TI = BB->getTerminator(); // Check to see if any successors of this block are non-loop-header loops // that are not the header. for (unsigned i = 0, e = TI->getNumSuccessors(); i != e; ++i) { // If this successor is not in a loop, BB is clearly ok. Loop *L = LI->getLoopFor(TI->getSuccessor(i)); if (!L) continue; // If the succ is the loop header, and if L is a top-level loop, then this // is an entrance into a loop through the header, which is also ok. if (L->getHeader() == TI->getSuccessor(i) && L->getParentLoop() == 0) continue; // Otherwise, this is an entrance into a loop from some place invalid. // Either the loop structure is invalid and this is not a natural loop (in // which case the compiler is buggy somewhere else) or BB is unreachable. BlockUnreachable = true; break; } // If this block is ok, check the next one. if (!BlockUnreachable) continue; // Otherwise, this block is dead. To clean up the CFG and to allow later // loop transformations to ignore this case, we delete the edges into the // loop by replacing the terminator. // Remove PHI entries from the successors. for (unsigned i = 0, e = TI->getNumSuccessors(); i != e; ++i) TI->getSuccessor(i)->removePredecessor(BB); // Add a new unreachable instruction before the old terminator. new UnreachableInst(TI->getContext(), TI); // Delete the dead terminator. if (AA) AA->deleteValue(TI); if (!TI->use_empty()) TI->replaceAllUsesWith(UndefValue::get(TI->getType())); TI->eraseFromParent(); Changed |= true; } 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; ReprocessLoop: // 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); assert(L->getBlocks()[0] == L->getHeader() && "Header isn't first block in loop?"); // Does the loop already have a preheader? If so, don't insert one. BasicBlock *Preheader = L->getLoopPreheader(); if (!Preheader) { Preheader = 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. SmallVector 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. unsigned NumBackedges = L->getNumBackEdges(); if (NumBackedges != 1) { // If this is really a nested loop, rip it out into a child loop. Don't do // this for loops with a giant number of backedges, just factor them into a // common backedge instead. if (NumBackedges < 8) { if (Loop *NL = SeparateNestedLoop(L)) { ++NumNested; // This is a big restructuring change, reprocess the whole loop. ProcessLoop(NL); Changed = true; // GCC doesn't tail recursion eliminate this. goto ReprocessLoop; } } // If we either couldn't, or didn't want to, identify nesting of the loops, // insert a new block that all backedges target, then make it jump to the // loop header. InsertUniqueBackedgeBlock(L, Preheader); 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()) { if (AA) AA->deleteValue(PN); PN->replaceAllUsesWith(V); PN->eraseFromParent(); } // If this loop has muliple exits and the exits all go to the same // block, attempt to merge the exits. This helps several passes, such // as LoopRotation, which do not support loops with multiple exits. // SimplifyCFG also does this (and this code uses the same utility // function), however this code is loop-aware, where SimplifyCFG is // not. That gives it the advantage of being able to hoist // loop-invariant instructions out of the way to open up more // opportunities, and the disadvantage of having the responsibility // to preserve dominator information. if (ExitBlocks.size() > 1 && L->getUniqueExitBlock()) { SmallVector ExitingBlocks; L->getExitingBlocks(ExitingBlocks); for (unsigned i = 0, e = ExitingBlocks.size(); i != e; ++i) { BasicBlock *ExitingBlock = ExitingBlocks[i]; if (!ExitingBlock->getSinglePredecessor()) continue; BranchInst *BI = dyn_cast(ExitingBlock->getTerminator()); if (!BI || !BI->isConditional()) continue; CmpInst *CI = dyn_cast(BI->getCondition()); if (!CI || CI->getParent() != ExitingBlock) continue; // Attempt to hoist out all instructions except for the // comparison and the branch. bool AllInvariant = true; for (BasicBlock::iterator I = ExitingBlock->begin(); &*I != BI; ) { Instruction *Inst = I++; if (Inst == CI) continue; if (!L->makeLoopInvariant(Inst, Changed, Preheader->getTerminator())) { AllInvariant = false; break; } } if (!AllInvariant) continue; // The block has now been cleared of all instructions except for // a comparison and a conditional branch. SimplifyCFG may be able // to fold it now. if (!FoldBranchToCommonDest(BI)) continue; // Success. The block is now dead, so remove it from the loop, // update the dominator tree and dominance frontier, and delete it. assert(pred_begin(ExitingBlock) == pred_end(ExitingBlock)); Changed = true; LI->removeBlock(ExitingBlock); DominanceFrontier *DF = getAnalysisIfAvailable(); DomTreeNode *Node = DT->getNode(ExitingBlock); const std::vector *> &Children = Node->getChildren(); for (unsigned k = 0, g = Children.size(); k != g; ++k) { DT->changeImmediateDominator(Children[k], Node->getIDom()); if (DF) DF->changeImmediateDominator(Children[k]->getBlock(), Node->getIDom()->getBlock(), DT); } DT->eraseNode(ExitingBlock); if (DF) DF->removeBlock(ExitingBlock); BI->getSuccessor(0)->removePredecessor(ExitingBlock); BI->getSuccessor(1)->removePredecessor(ExitingBlock); ExitingBlock->eraseFromParent(); } } return Changed; } /// 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. /// BasicBlock *LoopSimplify::InsertPreheaderForLoop(Loop *L) { BasicBlock *Header = L->getHeader(); // Compute the set of predecessors of the loop that are not in the loop. SmallVector 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, &OutsideBlocks[0], OutsideBlocks.size(), ".preheader", this); //===--------------------------------------------------------------------===// // 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->getBase()); // Make sure that NewBB is put someplace intelligent, which doesn't mess up // code layout too horribly. PlaceSplitBlockCarefully(NewBB, OutsideBlocks, L); return 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) { SmallVector 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, &LoopBlocks[0], LoopBlocks.size(), ".loopexit", this); // 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->getBase()); 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 *InputBB, BasicBlock *StopBlock, std::set &Blocks) { std::vector WorkList; WorkList.push_back(InputBB); do { BasicBlock *BB = WorkList.back(); WorkList.pop_back(); if (Blocks.insert(BB).second && BB != StopBlock) // If BB is not already processed and it is not a stop block then // insert its predecessor in the work list for (pred_iterator I = pred_begin(BB), E = pred_end(BB); I != E; ++I) { BasicBlock *WBB = *I; WorkList.push_back(WBB); } } while(!WorkList.empty()); } /// 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, DominatorTree *DT, AliasAnalysis *AA) { for (BasicBlock::iterator I = L->getHeader()->begin(); isa(I); ) { PHINode *PN = cast(I); ++I; if (Value *V = PN->hasConstantValue()) if (!isa(V) || DT->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; } // PlaceSplitBlockCarefully - If the block isn't already, move the new block to // right after some 'outside block' block. This prevents the preheader from // being placed inside the loop body, e.g. when the loop hasn't been rotated. void LoopSimplify::PlaceSplitBlockCarefully(BasicBlock *NewBB, SmallVectorImpl &SplitPreds, Loop *L) { // Check to see if NewBB is already well placed. Function::iterator BBI = NewBB; --BBI; for (unsigned i = 0, e = SplitPreds.size(); i != e; ++i) { if (&*BBI == SplitPreds[i]) return; } // If it isn't already after an outside block, move it after one. This is // always good as it makes the uncond branch from the outside block into a // fall-through. // Figure out *which* outside block to put this after. Prefer an outside // block that neighbors a BB actually in the loop. BasicBlock *FoundBB = 0; for (unsigned i = 0, e = SplitPreds.size(); i != e; ++i) { Function::iterator BBI = SplitPreds[i]; if (++BBI != NewBB->getParent()->end() && L->contains(BBI)) { FoundBB = SplitPreds[i]; break; } } // If our heuristic for a *good* bb to place this after doesn't find // anything, just pick something. It's likely better than leaving it within // the loop. if (!FoundBB) FoundBB = SplitPreds[0]; NewBB->moveAfter(FoundBB); } /// 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, DT, 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. SmallVector 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, &OuterLoopPreds[0], OuterLoopPreds.size(), ".outer", this); // Make sure that NewBB is put someplace intelligent, which doesn't mess up // code layout too horribly. PlaceSplitBlockCarefully(NewBB, OuterLoopPreds, L); // 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->getBase()); // L is now a subloop of our outer loop. NewOuter->addChildLoop(L); for (Loop::block_iterator I = L->block_begin(), E = L->block_end(); I != E; ++I) NewOuter->addBlockEntry(*I); // Determine which blocks should stay in L and which should be moved out to // the Outer loop now. std::set BlocksInL; for (pred_iterator PI = pred_begin(Header), E = pred_end(Header); PI!=E; ++PI) if (DT->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. const std::vector &SubLoops = L->getSubLoops(); for (size_t I = 0; I != SubLoops.size(); ) if (BlocksInL.count(SubLoops[I]->getHeader())) ++I; // Loop remains in L else NewOuter->addChildLoop(L->removeChildLoop(SubLoops.begin() + 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, BasicBlock *Preheader) { assert(L->getNumBackEdges() > 1 && "Must have > 1 backedge!"); // Get information about the loop 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 = BasicBlock::Create(Header->getContext(), Header->getName()+".backedge", F); BranchInst *BETerminator = BranchInst::Create(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 = PHINode::Create(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->getBase()); // Update dominator information DT->splitBlock(BEBlock); if (DominanceFrontier *DF = getAnalysisIfAvailable()) DF->splitBlock(BEBlock); }