LoopDeletion.cpp revision 2ab36d350293c77fc8941ce1023e4899df7e3a82
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_BEGIN(LoopDeletion, "loop-deletion", 59 "Delete dead loops", false, false) 60INITIALIZE_PASS_DEPENDENCY(DominatorTree) 61INITIALIZE_PASS_DEPENDENCY(LoopInfo) 62INITIALIZE_PASS_DEPENDENCY(ScalarEvolution) 63INITIALIZE_PASS_DEPENDENCY(LoopSimplify) 64INITIALIZE_PASS_DEPENDENCY(LCSSA) 65INITIALIZE_PASS_DEPENDENCY(DominanceFrontier) 66INITIALIZE_PASS_END(LoopDeletion, "loop-deletion", 67 "Delete dead loops", false, false) 68 69Pass* llvm::createLoopDeletionPass() { 70 return new LoopDeletion(); 71} 72 73/// IsLoopDead - Determined if a loop is dead. This assumes that we've already 74/// checked for unique exit and exiting blocks, and that the code is in LCSSA 75/// form. 76bool LoopDeletion::IsLoopDead(Loop* L, 77 SmallVector<BasicBlock*, 4>& exitingBlocks, 78 SmallVector<BasicBlock*, 4>& exitBlocks, 79 bool &Changed, BasicBlock *Preheader) { 80 BasicBlock* exitingBlock = exitingBlocks[0]; 81 BasicBlock* exitBlock = exitBlocks[0]; 82 83 // Make sure that all PHI entries coming from the loop are loop invariant. 84 // Because the code is in LCSSA form, any values used outside of the loop 85 // must pass through a PHI in the exit block, meaning that this check is 86 // sufficient to guarantee that no loop-variant values are used outside 87 // of the loop. 88 BasicBlock::iterator BI = exitBlock->begin(); 89 while (PHINode* P = dyn_cast<PHINode>(BI)) { 90 Value* incoming = P->getIncomingValueForBlock(exitingBlock); 91 if (Instruction* I = dyn_cast<Instruction>(incoming)) 92 if (!L->makeLoopInvariant(I, Changed, Preheader->getTerminator())) 93 return false; 94 95 ++BI; 96 } 97 98 // Make sure that no instructions in the block have potential side-effects. 99 // This includes instructions that could write to memory, and loads that are 100 // marked volatile. This could be made more aggressive by using aliasing 101 // information to identify readonly and readnone calls. 102 for (Loop::block_iterator LI = L->block_begin(), LE = L->block_end(); 103 LI != LE; ++LI) { 104 for (BasicBlock::iterator BI = (*LI)->begin(), BE = (*LI)->end(); 105 BI != BE; ++BI) { 106 if (BI->mayHaveSideEffects()) 107 return false; 108 } 109 } 110 111 return true; 112} 113 114/// runOnLoop - Remove dead loops, by which we mean loops that do not impact the 115/// observable behavior of the program other than finite running time. Note 116/// we do ensure that this never remove a loop that might be infinite, as doing 117/// so could change the halting/non-halting nature of a program. 118/// NOTE: This entire process relies pretty heavily on LoopSimplify and LCSSA 119/// in order to make various safety checks work. 120bool LoopDeletion::runOnLoop(Loop* L, LPPassManager& LPM) { 121 // We can only remove the loop if there is a preheader that we can 122 // branch from after removing it. 123 BasicBlock* preheader = L->getLoopPreheader(); 124 if (!preheader) 125 return false; 126 127 // If LoopSimplify form is not available, stay out of trouble. 128 if (!L->hasDedicatedExits()) 129 return false; 130 131 // We can't remove loops that contain subloops. If the subloops were dead, 132 // they would already have been removed in earlier executions of this pass. 133 if (L->begin() != L->end()) 134 return false; 135 136 SmallVector<BasicBlock*, 4> exitingBlocks; 137 L->getExitingBlocks(exitingBlocks); 138 139 SmallVector<BasicBlock*, 4> exitBlocks; 140 L->getUniqueExitBlocks(exitBlocks); 141 142 // We require that the loop only have a single exit block. Otherwise, we'd 143 // be in the situation of needing to be able to solve statically which exit 144 // block will be branched to, or trying to preserve the branching logic in 145 // a loop invariant manner. 146 if (exitBlocks.size() != 1) 147 return false; 148 149 // Loops with multiple exits are too complicated to handle correctly. 150 if (exitingBlocks.size() != 1) 151 return false; 152 153 // Finally, we have to check that the loop really is dead. 154 bool Changed = false; 155 if (!IsLoopDead(L, exitingBlocks, exitBlocks, Changed, preheader)) 156 return Changed; 157 158 // Don't remove loops for which we can't solve the trip count. 159 // They could be infinite, in which case we'd be changing program behavior. 160 ScalarEvolution& SE = getAnalysis<ScalarEvolution>(); 161 const SCEV *S = SE.getMaxBackedgeTakenCount(L); 162 if (isa<SCEVCouldNotCompute>(S)) 163 return Changed; 164 165 // Now that we know the removal is safe, remove the loop by changing the 166 // branch from the preheader to go to the single exit block. 167 BasicBlock* exitBlock = exitBlocks[0]; 168 BasicBlock* exitingBlock = exitingBlocks[0]; 169 170 // Because we're deleting a large chunk of code at once, the sequence in which 171 // we remove things is very important to avoid invalidation issues. Don't 172 // mess with this unless you have good reason and know what you're doing. 173 174 // Tell ScalarEvolution that the loop is deleted. Do this before 175 // deleting the loop so that ScalarEvolution can look at the loop 176 // to determine what it needs to clean up. 177 SE.forgetLoop(L); 178 179 // Connect the preheader directly to the exit block. 180 TerminatorInst* TI = preheader->getTerminator(); 181 TI->replaceUsesOfWith(L->getHeader(), exitBlock); 182 183 // Rewrite phis in the exit block to get their inputs from 184 // the preheader instead of the exiting block. 185 BasicBlock::iterator BI = exitBlock->begin(); 186 while (PHINode* P = dyn_cast<PHINode>(BI)) { 187 P->replaceUsesOfWith(exitingBlock, preheader); 188 ++BI; 189 } 190 191 // Update the dominator tree and remove the instructions and blocks that will 192 // be deleted from the reference counting scheme. 193 DominatorTree& DT = getAnalysis<DominatorTree>(); 194 DominanceFrontier* DF = getAnalysisIfAvailable<DominanceFrontier>(); 195 SmallPtrSet<DomTreeNode*, 8> ChildNodes; 196 for (Loop::block_iterator LI = L->block_begin(), LE = L->block_end(); 197 LI != LE; ++LI) { 198 // Move all of the block's children to be children of the preheader, which 199 // allows us to remove the domtree entry for the block. 200 ChildNodes.insert(DT[*LI]->begin(), DT[*LI]->end()); 201 for (SmallPtrSet<DomTreeNode*, 8>::iterator DI = ChildNodes.begin(), 202 DE = ChildNodes.end(); DI != DE; ++DI) { 203 DT.changeImmediateDominator(*DI, DT[preheader]); 204 if (DF) DF->changeImmediateDominator((*DI)->getBlock(), preheader, &DT); 205 } 206 207 ChildNodes.clear(); 208 DT.eraseNode(*LI); 209 if (DF) DF->removeBlock(*LI); 210 211 // Remove the block from the reference counting scheme, so that we can 212 // delete it freely later. 213 (*LI)->dropAllReferences(); 214 } 215 216 // Erase the instructions and the blocks without having to worry 217 // about ordering because we already dropped the references. 218 // NOTE: This iteration is safe because erasing the block does not remove its 219 // entry from the loop's block list. We do that in the next section. 220 for (Loop::block_iterator LI = L->block_begin(), LE = L->block_end(); 221 LI != LE; ++LI) 222 (*LI)->eraseFromParent(); 223 224 // Finally, the blocks from loopinfo. This has to happen late because 225 // otherwise our loop iterators won't work. 226 LoopInfo& loopInfo = getAnalysis<LoopInfo>(); 227 SmallPtrSet<BasicBlock*, 8> blocks; 228 blocks.insert(L->block_begin(), L->block_end()); 229 for (SmallPtrSet<BasicBlock*,8>::iterator I = blocks.begin(), 230 E = blocks.end(); I != E; ++I) 231 loopInfo.removeBlock(*I); 232 233 // The last step is to inform the loop pass manager that we've 234 // eliminated this loop. 235 LPM.deleteLoopFromQueue(L); 236 Changed = true; 237 238 ++NumDeleted; 239 240 return Changed; 241} 242