LoopDeletion.cpp revision 210299
1193323Sed//===- LoopDeletion.cpp - Dead Loop Deletion Pass ---------------===// 2193323Sed// 3193323Sed// The LLVM Compiler Infrastructure 4193323Sed// 5193323Sed// This file is distributed under the University of Illinois Open Source 6193323Sed// License. See LICENSE.TXT for details. 7193323Sed// 8193323Sed//===----------------------------------------------------------------------===// 9193323Sed// 10193323Sed// This file implements the Dead Loop Deletion Pass. This pass is responsible 11193323Sed// for eliminating loops with non-infinite computable trip counts that have no 12193323Sed// side effects or volatile instructions, and do not contribute to the 13193323Sed// computation of the function's return value. 14193323Sed// 15193323Sed//===----------------------------------------------------------------------===// 16193323Sed 17193323Sed#define DEBUG_TYPE "loop-delete" 18193323Sed#include "llvm/Transforms/Scalar.h" 19193323Sed#include "llvm/Analysis/LoopPass.h" 20193323Sed#include "llvm/Analysis/ScalarEvolution.h" 21193323Sed#include "llvm/ADT/Statistic.h" 22193323Sed#include "llvm/ADT/SmallVector.h" 23193323Sedusing namespace llvm; 24193323Sed 25193323SedSTATISTIC(NumDeleted, "Number of loops deleted"); 26193323Sed 27193323Sednamespace { 28198090Srdivacky class LoopDeletion : public LoopPass { 29193323Sed public: 30193323Sed static char ID; // Pass ID, replacement for typeid 31193323Sed LoopDeletion() : LoopPass(&ID) {} 32193323Sed 33193323Sed // Possibly eliminate loop L if it is dead. 34193323Sed bool runOnLoop(Loop* L, LPPassManager& LPM); 35193323Sed 36193323Sed bool IsLoopDead(Loop* L, SmallVector<BasicBlock*, 4>& exitingBlocks, 37198090Srdivacky SmallVector<BasicBlock*, 4>& exitBlocks, 38198090Srdivacky bool &Changed, BasicBlock *Preheader); 39198090Srdivacky 40193323Sed virtual void getAnalysisUsage(AnalysisUsage& AU) const { 41193323Sed AU.addRequired<ScalarEvolution>(); 42193323Sed AU.addRequired<DominatorTree>(); 43193323Sed AU.addRequired<LoopInfo>(); 44193323Sed AU.addRequiredID(LoopSimplifyID); 45193323Sed AU.addRequiredID(LCSSAID); 46193323Sed 47193323Sed AU.addPreserved<ScalarEvolution>(); 48193323Sed AU.addPreserved<DominatorTree>(); 49193323Sed AU.addPreserved<LoopInfo>(); 50193323Sed AU.addPreservedID(LoopSimplifyID); 51193323Sed AU.addPreservedID(LCSSAID); 52193323Sed AU.addPreserved<DominanceFrontier>(); 53193323Sed } 54193323Sed }; 55193323Sed} 56193323Sed 57193323Sedchar LoopDeletion::ID = 0; 58193323Sedstatic RegisterPass<LoopDeletion> X("loop-deletion", "Delete dead loops"); 59193323Sed 60193323SedPass* llvm::createLoopDeletionPass() { 61193323Sed return new LoopDeletion(); 62193323Sed} 63193323Sed 64193323Sed/// IsLoopDead - Determined if a loop is dead. This assumes that we've already 65193323Sed/// checked for unique exit and exiting blocks, and that the code is in LCSSA 66193323Sed/// form. 67193323Sedbool LoopDeletion::IsLoopDead(Loop* L, 68193323Sed SmallVector<BasicBlock*, 4>& exitingBlocks, 69198090Srdivacky SmallVector<BasicBlock*, 4>& exitBlocks, 70198090Srdivacky bool &Changed, BasicBlock *Preheader) { 71193323Sed BasicBlock* exitingBlock = exitingBlocks[0]; 72193323Sed BasicBlock* exitBlock = exitBlocks[0]; 73193323Sed 74193323Sed // Make sure that all PHI entries coming from the loop are loop invariant. 75193323Sed // Because the code is in LCSSA form, any values used outside of the loop 76193323Sed // must pass through a PHI in the exit block, meaning that this check is 77193323Sed // sufficient to guarantee that no loop-variant values are used outside 78193323Sed // of the loop. 79193323Sed BasicBlock::iterator BI = exitBlock->begin(); 80193323Sed while (PHINode* P = dyn_cast<PHINode>(BI)) { 81193323Sed Value* incoming = P->getIncomingValueForBlock(exitingBlock); 82193323Sed if (Instruction* I = dyn_cast<Instruction>(incoming)) 83198090Srdivacky if (!L->makeLoopInvariant(I, Changed, Preheader->getTerminator())) 84193323Sed return false; 85193323Sed 86210299Sed ++BI; 87193323Sed } 88193323Sed 89193323Sed // Make sure that no instructions in the block have potential side-effects. 90193323Sed // This includes instructions that could write to memory, and loads that are 91193323Sed // marked volatile. This could be made more aggressive by using aliasing 92193323Sed // information to identify readonly and readnone calls. 93193323Sed for (Loop::block_iterator LI = L->block_begin(), LE = L->block_end(); 94193323Sed LI != LE; ++LI) { 95193323Sed for (BasicBlock::iterator BI = (*LI)->begin(), BE = (*LI)->end(); 96193323Sed BI != BE; ++BI) { 97193323Sed if (BI->mayHaveSideEffects()) 98193323Sed return false; 99193323Sed } 100193323Sed } 101193323Sed 102193323Sed return true; 103193323Sed} 104193323Sed 105193323Sed/// runOnLoop - Remove dead loops, by which we mean loops that do not impact the 106193323Sed/// observable behavior of the program other than finite running time. Note 107193323Sed/// we do ensure that this never remove a loop that might be infinite, as doing 108193323Sed/// so could change the halting/non-halting nature of a program. 109193323Sed/// NOTE: This entire process relies pretty heavily on LoopSimplify and LCSSA 110193323Sed/// in order to make various safety checks work. 111193323Sedbool LoopDeletion::runOnLoop(Loop* L, LPPassManager& LPM) { 112193323Sed // We can only remove the loop if there is a preheader that we can 113193323Sed // branch from after removing it. 114193323Sed BasicBlock* preheader = L->getLoopPreheader(); 115193323Sed if (!preheader) 116193323Sed return false; 117193323Sed 118199481Srdivacky // If LoopSimplify form is not available, stay out of trouble. 119199481Srdivacky if (!L->hasDedicatedExits()) 120199481Srdivacky return false; 121199481Srdivacky 122193323Sed // We can't remove loops that contain subloops. If the subloops were dead, 123193323Sed // they would already have been removed in earlier executions of this pass. 124193323Sed if (L->begin() != L->end()) 125193323Sed return false; 126193323Sed 127193323Sed SmallVector<BasicBlock*, 4> exitingBlocks; 128193323Sed L->getExitingBlocks(exitingBlocks); 129193323Sed 130193323Sed SmallVector<BasicBlock*, 4> exitBlocks; 131193323Sed L->getUniqueExitBlocks(exitBlocks); 132193323Sed 133193323Sed // We require that the loop only have a single exit block. Otherwise, we'd 134193323Sed // be in the situation of needing to be able to solve statically which exit 135193323Sed // block will be branched to, or trying to preserve the branching logic in 136193323Sed // a loop invariant manner. 137193323Sed if (exitBlocks.size() != 1) 138193323Sed return false; 139193323Sed 140198892Srdivacky // Loops with multiple exits are too complicated to handle correctly. 141198892Srdivacky if (exitingBlocks.size() != 1) 142193323Sed return false; 143193323Sed 144193323Sed // Finally, we have to check that the loop really is dead. 145198090Srdivacky bool Changed = false; 146198090Srdivacky if (!IsLoopDead(L, exitingBlocks, exitBlocks, Changed, preheader)) 147198090Srdivacky return Changed; 148193323Sed 149193323Sed // Don't remove loops for which we can't solve the trip count. 150193323Sed // They could be infinite, in which case we'd be changing program behavior. 151193323Sed ScalarEvolution& SE = getAnalysis<ScalarEvolution>(); 152198892Srdivacky const SCEV *S = SE.getMaxBackedgeTakenCount(L); 153193323Sed if (isa<SCEVCouldNotCompute>(S)) 154198090Srdivacky return Changed; 155193323Sed 156193323Sed // Now that we know the removal is safe, remove the loop by changing the 157193323Sed // branch from the preheader to go to the single exit block. 158193323Sed BasicBlock* exitBlock = exitBlocks[0]; 159193323Sed BasicBlock* exitingBlock = exitingBlocks[0]; 160193323Sed 161193323Sed // Because we're deleting a large chunk of code at once, the sequence in which 162193323Sed // we remove things is very important to avoid invalidation issues. Don't 163193323Sed // mess with this unless you have good reason and know what you're doing. 164198090Srdivacky 165198090Srdivacky // Tell ScalarEvolution that the loop is deleted. Do this before 166198090Srdivacky // deleting the loop so that ScalarEvolution can look at the loop 167198090Srdivacky // to determine what it needs to clean up. 168198892Srdivacky SE.forgetLoop(L); 169198090Srdivacky 170193323Sed // Connect the preheader directly to the exit block. 171193323Sed TerminatorInst* TI = preheader->getTerminator(); 172193323Sed TI->replaceUsesOfWith(L->getHeader(), exitBlock); 173193323Sed 174193323Sed // Rewrite phis in the exit block to get their inputs from 175193323Sed // the preheader instead of the exiting block. 176193323Sed BasicBlock::iterator BI = exitBlock->begin(); 177193323Sed while (PHINode* P = dyn_cast<PHINode>(BI)) { 178193323Sed P->replaceUsesOfWith(exitingBlock, preheader); 179210299Sed ++BI; 180193323Sed } 181193323Sed 182193323Sed // Update the dominator tree and remove the instructions and blocks that will 183193323Sed // be deleted from the reference counting scheme. 184193323Sed DominatorTree& DT = getAnalysis<DominatorTree>(); 185193323Sed DominanceFrontier* DF = getAnalysisIfAvailable<DominanceFrontier>(); 186193323Sed SmallPtrSet<DomTreeNode*, 8> ChildNodes; 187193323Sed for (Loop::block_iterator LI = L->block_begin(), LE = L->block_end(); 188193323Sed LI != LE; ++LI) { 189193323Sed // Move all of the block's children to be children of the preheader, which 190193323Sed // allows us to remove the domtree entry for the block. 191193323Sed ChildNodes.insert(DT[*LI]->begin(), DT[*LI]->end()); 192193323Sed for (SmallPtrSet<DomTreeNode*, 8>::iterator DI = ChildNodes.begin(), 193193323Sed DE = ChildNodes.end(); DI != DE; ++DI) { 194193323Sed DT.changeImmediateDominator(*DI, DT[preheader]); 195193323Sed if (DF) DF->changeImmediateDominator((*DI)->getBlock(), preheader, &DT); 196193323Sed } 197193323Sed 198193323Sed ChildNodes.clear(); 199193323Sed DT.eraseNode(*LI); 200193323Sed if (DF) DF->removeBlock(*LI); 201193323Sed 202193323Sed // Remove the block from the reference counting scheme, so that we can 203193323Sed // delete it freely later. 204193323Sed (*LI)->dropAllReferences(); 205193323Sed } 206193323Sed 207193323Sed // Erase the instructions and the blocks without having to worry 208193323Sed // about ordering because we already dropped the references. 209193323Sed // NOTE: This iteration is safe because erasing the block does not remove its 210193323Sed // entry from the loop's block list. We do that in the next section. 211193323Sed for (Loop::block_iterator LI = L->block_begin(), LE = L->block_end(); 212193323Sed LI != LE; ++LI) 213193323Sed (*LI)->eraseFromParent(); 214193323Sed 215193323Sed // Finally, the blocks from loopinfo. This has to happen late because 216193323Sed // otherwise our loop iterators won't work. 217193323Sed LoopInfo& loopInfo = getAnalysis<LoopInfo>(); 218193323Sed SmallPtrSet<BasicBlock*, 8> blocks; 219193323Sed blocks.insert(L->block_begin(), L->block_end()); 220193323Sed for (SmallPtrSet<BasicBlock*,8>::iterator I = blocks.begin(), 221193323Sed E = blocks.end(); I != E; ++I) 222193323Sed loopInfo.removeBlock(*I); 223193323Sed 224193323Sed // The last step is to inform the loop pass manager that we've 225193323Sed // eliminated this loop. 226193323Sed LPM.deleteLoopFromQueue(L); 227198090Srdivacky Changed = true; 228193323Sed 229210299Sed ++NumDeleted; 230193323Sed 231198090Srdivacky return Changed; 232193323Sed} 233