LoopDeletion.cpp revision ce665bd2e2b581ab0858d1afe359192bac96b868
1//===- LoopDeletion.cpp - Dead Loop Deletion Pass ---------------===// 2// 3// The LLVM Compiler Infrastructure 4// 5// This file is distributed under the University of Illinois Open Source 6// License. See LICENSE.TXT for details. 7// 8//===----------------------------------------------------------------------===// 9// 10// This file implements the Dead Loop Deletion Pass. This pass is responsible 11// for eliminating loops with non-infinite computable trip counts that have no 12// side effects or volatile instructions, and do not contribute to the 13// computation of the function's return value. 14// 15//===----------------------------------------------------------------------===// 16 17#define DEBUG_TYPE "loop-delete" 18#include "llvm/Transforms/Scalar.h" 19#include "llvm/Analysis/LoopPass.h" 20#include "llvm/Analysis/ScalarEvolution.h" 21#include "llvm/ADT/Statistic.h" 22#include "llvm/ADT/SmallVector.h" 23using namespace llvm; 24 25STATISTIC(NumDeleted, "Number of loops deleted"); 26 27namespace { 28 class LoopDeletion : public LoopPass { 29 public: 30 static char ID; // Pass ID, replacement for typeid 31 LoopDeletion() : LoopPass(ID) {} 32 33 // Possibly eliminate loop L if it is dead. 34 bool runOnLoop(Loop* L, LPPassManager& LPM); 35 36 bool IsLoopDead(Loop* L, SmallVector<BasicBlock*, 4>& exitingBlocks, 37 SmallVector<BasicBlock*, 4>& exitBlocks, 38 bool &Changed, BasicBlock *Preheader); 39 40 virtual void getAnalysisUsage(AnalysisUsage& AU) const { 41 AU.addRequired<DominatorTree>(); 42 AU.addRequired<LoopInfo>(); 43 AU.addRequired<ScalarEvolution>(); 44 AU.addRequiredID(LoopSimplifyID); 45 AU.addRequiredID(LCSSAID); 46 47 AU.addPreserved<ScalarEvolution>(); 48 AU.addPreserved<DominatorTree>(); 49 AU.addPreserved<LoopInfo>(); 50 AU.addPreservedID(LoopSimplifyID); 51 AU.addPreservedID(LCSSAID); 52 AU.addPreserved<DominanceFrontier>(); 53 } 54 }; 55} 56 57char LoopDeletion::ID = 0; 58INITIALIZE_PASS(LoopDeletion, "loop-deletion", 59 "Delete dead loops", false, false) 60 61Pass* llvm::createLoopDeletionPass() { 62 return new LoopDeletion(); 63} 64 65/// IsLoopDead - Determined if a loop is dead. This assumes that we've already 66/// checked for unique exit and exiting blocks, and that the code is in LCSSA 67/// form. 68bool LoopDeletion::IsLoopDead(Loop* L, 69 SmallVector<BasicBlock*, 4>& exitingBlocks, 70 SmallVector<BasicBlock*, 4>& exitBlocks, 71 bool &Changed, BasicBlock *Preheader) { 72 BasicBlock* exitingBlock = exitingBlocks[0]; 73 BasicBlock* exitBlock = exitBlocks[0]; 74 75 // Make sure that all PHI entries coming from the loop are loop invariant. 76 // Because the code is in LCSSA form, any values used outside of the loop 77 // must pass through a PHI in the exit block, meaning that this check is 78 // sufficient to guarantee that no loop-variant values are used outside 79 // of the loop. 80 BasicBlock::iterator BI = exitBlock->begin(); 81 while (PHINode* P = dyn_cast<PHINode>(BI)) { 82 Value* incoming = P->getIncomingValueForBlock(exitingBlock); 83 if (Instruction* I = dyn_cast<Instruction>(incoming)) 84 if (!L->makeLoopInvariant(I, Changed, Preheader->getTerminator())) 85 return false; 86 87 ++BI; 88 } 89 90 // Make sure that no instructions in the block have potential side-effects. 91 // This includes instructions that could write to memory, and loads that are 92 // marked volatile. This could be made more aggressive by using aliasing 93 // information to identify readonly and readnone calls. 94 for (Loop::block_iterator LI = L->block_begin(), LE = L->block_end(); 95 LI != LE; ++LI) { 96 for (BasicBlock::iterator BI = (*LI)->begin(), BE = (*LI)->end(); 97 BI != BE; ++BI) { 98 if (BI->mayHaveSideEffects()) 99 return false; 100 } 101 } 102 103 return true; 104} 105 106/// runOnLoop - Remove dead loops, by which we mean loops that do not impact the 107/// observable behavior of the program other than finite running time. Note 108/// we do ensure that this never remove a loop that might be infinite, as doing 109/// so could change the halting/non-halting nature of a program. 110/// NOTE: This entire process relies pretty heavily on LoopSimplify and LCSSA 111/// in order to make various safety checks work. 112bool LoopDeletion::runOnLoop(Loop* L, LPPassManager& LPM) { 113 // We can only remove the loop if there is a preheader that we can 114 // branch from after removing it. 115 BasicBlock* preheader = L->getLoopPreheader(); 116 if (!preheader) 117 return false; 118 119 // If LoopSimplify form is not available, stay out of trouble. 120 if (!L->hasDedicatedExits()) 121 return false; 122 123 // We can't remove loops that contain subloops. If the subloops were dead, 124 // they would already have been removed in earlier executions of this pass. 125 if (L->begin() != L->end()) 126 return false; 127 128 SmallVector<BasicBlock*, 4> exitingBlocks; 129 L->getExitingBlocks(exitingBlocks); 130 131 SmallVector<BasicBlock*, 4> exitBlocks; 132 L->getUniqueExitBlocks(exitBlocks); 133 134 // We require that the loop only have a single exit block. Otherwise, we'd 135 // be in the situation of needing to be able to solve statically which exit 136 // block will be branched to, or trying to preserve the branching logic in 137 // a loop invariant manner. 138 if (exitBlocks.size() != 1) 139 return false; 140 141 // Loops with multiple exits are too complicated to handle correctly. 142 if (exitingBlocks.size() != 1) 143 return false; 144 145 // Finally, we have to check that the loop really is dead. 146 bool Changed = false; 147 if (!IsLoopDead(L, exitingBlocks, exitBlocks, Changed, preheader)) 148 return Changed; 149 150 // Don't remove loops for which we can't solve the trip count. 151 // They could be infinite, in which case we'd be changing program behavior. 152 ScalarEvolution& SE = getAnalysis<ScalarEvolution>(); 153 const SCEV *S = SE.getMaxBackedgeTakenCount(L); 154 if (isa<SCEVCouldNotCompute>(S)) 155 return Changed; 156 157 // Now that we know the removal is safe, remove the loop by changing the 158 // branch from the preheader to go to the single exit block. 159 BasicBlock* exitBlock = exitBlocks[0]; 160 BasicBlock* exitingBlock = exitingBlocks[0]; 161 162 // Because we're deleting a large chunk of code at once, the sequence in which 163 // we remove things is very important to avoid invalidation issues. Don't 164 // mess with this unless you have good reason and know what you're doing. 165 166 // Tell ScalarEvolution that the loop is deleted. Do this before 167 // deleting the loop so that ScalarEvolution can look at the loop 168 // to determine what it needs to clean up. 169 SE.forgetLoop(L); 170 171 // Connect the preheader directly to the exit block. 172 TerminatorInst* TI = preheader->getTerminator(); 173 TI->replaceUsesOfWith(L->getHeader(), exitBlock); 174 175 // Rewrite phis in the exit block to get their inputs from 176 // the preheader instead of the exiting block. 177 BasicBlock::iterator BI = exitBlock->begin(); 178 while (PHINode* P = dyn_cast<PHINode>(BI)) { 179 P->replaceUsesOfWith(exitingBlock, preheader); 180 ++BI; 181 } 182 183 // Update the dominator tree and remove the instructions and blocks that will 184 // be deleted from the reference counting scheme. 185 DominatorTree& DT = getAnalysis<DominatorTree>(); 186 DominanceFrontier* DF = getAnalysisIfAvailable<DominanceFrontier>(); 187 SmallPtrSet<DomTreeNode*, 8> ChildNodes; 188 for (Loop::block_iterator LI = L->block_begin(), LE = L->block_end(); 189 LI != LE; ++LI) { 190 // Move all of the block's children to be children of the preheader, which 191 // allows us to remove the domtree entry for the block. 192 ChildNodes.insert(DT[*LI]->begin(), DT[*LI]->end()); 193 for (SmallPtrSet<DomTreeNode*, 8>::iterator DI = ChildNodes.begin(), 194 DE = ChildNodes.end(); DI != DE; ++DI) { 195 DT.changeImmediateDominator(*DI, DT[preheader]); 196 if (DF) DF->changeImmediateDominator((*DI)->getBlock(), preheader, &DT); 197 } 198 199 ChildNodes.clear(); 200 DT.eraseNode(*LI); 201 if (DF) DF->removeBlock(*LI); 202 203 // Remove the block from the reference counting scheme, so that we can 204 // delete it freely later. 205 (*LI)->dropAllReferences(); 206 } 207 208 // Erase the instructions and the blocks without having to worry 209 // about ordering because we already dropped the references. 210 // NOTE: This iteration is safe because erasing the block does not remove its 211 // entry from the loop's block list. We do that in the next section. 212 for (Loop::block_iterator LI = L->block_begin(), LE = L->block_end(); 213 LI != LE; ++LI) 214 (*LI)->eraseFromParent(); 215 216 // Finally, the blocks from loopinfo. This has to happen late because 217 // otherwise our loop iterators won't work. 218 LoopInfo& loopInfo = getAnalysis<LoopInfo>(); 219 SmallPtrSet<BasicBlock*, 8> blocks; 220 blocks.insert(L->block_begin(), L->block_end()); 221 for (SmallPtrSet<BasicBlock*,8>::iterator I = blocks.begin(), 222 E = blocks.end(); I != E; ++I) 223 loopInfo.removeBlock(*I); 224 225 // The last step is to inform the loop pass manager that we've 226 // eliminated this loop. 227 LPM.deleteLoopFromQueue(L); 228 Changed = true; 229 230 ++NumDeleted; 231 232 return Changed; 233} 234