LoopDeletion.cpp revision 198892
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<ScalarEvolution>(); 42 AU.addRequired<DominatorTree>(); 43 AU.addRequired<LoopInfo>(); 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; 58static RegisterPass<LoopDeletion> X("loop-deletion", "Delete dead loops"); 59 60Pass* llvm::createLoopDeletionPass() { 61 return new LoopDeletion(); 62} 63 64/// IsLoopDead - Determined if a loop is dead. This assumes that we've already 65/// checked for unique exit and exiting blocks, and that the code is in LCSSA 66/// form. 67bool LoopDeletion::IsLoopDead(Loop* L, 68 SmallVector<BasicBlock*, 4>& exitingBlocks, 69 SmallVector<BasicBlock*, 4>& exitBlocks, 70 bool &Changed, BasicBlock *Preheader) { 71 BasicBlock* exitingBlock = exitingBlocks[0]; 72 BasicBlock* exitBlock = exitBlocks[0]; 73 74 // Make sure that all PHI entries coming from the loop are loop invariant. 75 // Because the code is in LCSSA form, any values used outside of the loop 76 // must pass through a PHI in the exit block, meaning that this check is 77 // sufficient to guarantee that no loop-variant values are used outside 78 // of the loop. 79 BasicBlock::iterator BI = exitBlock->begin(); 80 while (PHINode* P = dyn_cast<PHINode>(BI)) { 81 Value* incoming = P->getIncomingValueForBlock(exitingBlock); 82 if (Instruction* I = dyn_cast<Instruction>(incoming)) 83 if (!L->makeLoopInvariant(I, Changed, Preheader->getTerminator())) 84 return false; 85 86 BI++; 87 } 88 89 // Make sure that no instructions in the block have potential side-effects. 90 // This includes instructions that could write to memory, and loads that are 91 // marked volatile. This could be made more aggressive by using aliasing 92 // information to identify readonly and readnone calls. 93 for (Loop::block_iterator LI = L->block_begin(), LE = L->block_end(); 94 LI != LE; ++LI) { 95 for (BasicBlock::iterator BI = (*LI)->begin(), BE = (*LI)->end(); 96 BI != BE; ++BI) { 97 if (BI->mayHaveSideEffects()) 98 return false; 99 } 100 } 101 102 return true; 103} 104 105/// runOnLoop - Remove dead loops, by which we mean loops that do not impact the 106/// observable behavior of the program other than finite running time. Note 107/// we do ensure that this never remove a loop that might be infinite, as doing 108/// so could change the halting/non-halting nature of a program. 109/// NOTE: This entire process relies pretty heavily on LoopSimplify and LCSSA 110/// in order to make various safety checks work. 111bool LoopDeletion::runOnLoop(Loop* L, LPPassManager& LPM) { 112 // We can only remove the loop if there is a preheader that we can 113 // branch from after removing it. 114 BasicBlock* preheader = L->getLoopPreheader(); 115 if (!preheader) 116 return false; 117 118 // We can't remove loops that contain subloops. If the subloops were dead, 119 // they would already have been removed in earlier executions of this pass. 120 if (L->begin() != L->end()) 121 return false; 122 123 SmallVector<BasicBlock*, 4> exitingBlocks; 124 L->getExitingBlocks(exitingBlocks); 125 126 SmallVector<BasicBlock*, 4> exitBlocks; 127 L->getUniqueExitBlocks(exitBlocks); 128 129 // We require that the loop only have a single exit block. Otherwise, we'd 130 // be in the situation of needing to be able to solve statically which exit 131 // block will be branched to, or trying to preserve the branching logic in 132 // a loop invariant manner. 133 if (exitBlocks.size() != 1) 134 return false; 135 136 // Loops with multiple exits are too complicated to handle correctly. 137 if (exitingBlocks.size() != 1) 138 return false; 139 140 // Finally, we have to check that the loop really is dead. 141 bool Changed = false; 142 if (!IsLoopDead(L, exitingBlocks, exitBlocks, Changed, preheader)) 143 return Changed; 144 145 // Don't remove loops for which we can't solve the trip count. 146 // They could be infinite, in which case we'd be changing program behavior. 147 ScalarEvolution& SE = getAnalysis<ScalarEvolution>(); 148 const SCEV *S = SE.getMaxBackedgeTakenCount(L); 149 if (isa<SCEVCouldNotCompute>(S)) 150 return Changed; 151 152 // Now that we know the removal is safe, remove the loop by changing the 153 // branch from the preheader to go to the single exit block. 154 BasicBlock* exitBlock = exitBlocks[0]; 155 BasicBlock* exitingBlock = exitingBlocks[0]; 156 157 // Because we're deleting a large chunk of code at once, the sequence in which 158 // we remove things is very important to avoid invalidation issues. Don't 159 // mess with this unless you have good reason and know what you're doing. 160 161 // Tell ScalarEvolution that the loop is deleted. Do this before 162 // deleting the loop so that ScalarEvolution can look at the loop 163 // to determine what it needs to clean up. 164 SE.forgetLoop(L); 165 166 // Connect the preheader directly to the exit block. 167 TerminatorInst* TI = preheader->getTerminator(); 168 TI->replaceUsesOfWith(L->getHeader(), exitBlock); 169 170 // Rewrite phis in the exit block to get their inputs from 171 // the preheader instead of the exiting block. 172 BasicBlock::iterator BI = exitBlock->begin(); 173 while (PHINode* P = dyn_cast<PHINode>(BI)) { 174 P->replaceUsesOfWith(exitingBlock, preheader); 175 BI++; 176 } 177 178 // Update the dominator tree and remove the instructions and blocks that will 179 // be deleted from the reference counting scheme. 180 DominatorTree& DT = getAnalysis<DominatorTree>(); 181 DominanceFrontier* DF = getAnalysisIfAvailable<DominanceFrontier>(); 182 SmallPtrSet<DomTreeNode*, 8> ChildNodes; 183 for (Loop::block_iterator LI = L->block_begin(), LE = L->block_end(); 184 LI != LE; ++LI) { 185 // Move all of the block's children to be children of the preheader, which 186 // allows us to remove the domtree entry for the block. 187 ChildNodes.insert(DT[*LI]->begin(), DT[*LI]->end()); 188 for (SmallPtrSet<DomTreeNode*, 8>::iterator DI = ChildNodes.begin(), 189 DE = ChildNodes.end(); DI != DE; ++DI) { 190 DT.changeImmediateDominator(*DI, DT[preheader]); 191 if (DF) DF->changeImmediateDominator((*DI)->getBlock(), preheader, &DT); 192 } 193 194 ChildNodes.clear(); 195 DT.eraseNode(*LI); 196 if (DF) DF->removeBlock(*LI); 197 198 // Remove the block from the reference counting scheme, so that we can 199 // delete it freely later. 200 (*LI)->dropAllReferences(); 201 } 202 203 // Erase the instructions and the blocks without having to worry 204 // about ordering because we already dropped the references. 205 // NOTE: This iteration is safe because erasing the block does not remove its 206 // entry from the loop's block list. We do that in the next section. 207 for (Loop::block_iterator LI = L->block_begin(), LE = L->block_end(); 208 LI != LE; ++LI) 209 (*LI)->eraseFromParent(); 210 211 // Finally, the blocks from loopinfo. This has to happen late because 212 // otherwise our loop iterators won't work. 213 LoopInfo& loopInfo = getAnalysis<LoopInfo>(); 214 SmallPtrSet<BasicBlock*, 8> blocks; 215 blocks.insert(L->block_begin(), L->block_end()); 216 for (SmallPtrSet<BasicBlock*,8>::iterator I = blocks.begin(), 217 E = blocks.end(); I != E; ++I) 218 loopInfo.removeBlock(*I); 219 220 // The last step is to inform the loop pass manager that we've 221 // eliminated this loop. 222 LPM.deleteLoopFromQueue(L); 223 Changed = true; 224 225 NumDeleted++; 226 227 return Changed; 228} 229