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/ADT/SmallVector.h" 20#include "llvm/ADT/Statistic.h" 21#include "llvm/Analysis/Dominators.h" 22#include "llvm/Analysis/LoopPass.h" 23#include "llvm/Analysis/ScalarEvolution.h" 24using namespace llvm; 25 26STATISTIC(NumDeleted, "Number of loops deleted"); 27 28namespace { 29 class LoopDeletion : public LoopPass { 30 public: 31 static char ID; // Pass ID, replacement for typeid 32 LoopDeletion() : LoopPass(ID) { 33 initializeLoopDeletionPass(*PassRegistry::getPassRegistry()); 34 } 35 36 // Possibly eliminate loop L if it is dead. 37 bool runOnLoop(Loop *L, LPPassManager &LPM); 38 39 virtual void getAnalysisUsage(AnalysisUsage &AU) const { 40 AU.addRequired<DominatorTree>(); 41 AU.addRequired<LoopInfo>(); 42 AU.addRequired<ScalarEvolution>(); 43 AU.addRequiredID(LoopSimplifyID); 44 AU.addRequiredID(LCSSAID); 45 46 AU.addPreserved<ScalarEvolution>(); 47 AU.addPreserved<DominatorTree>(); 48 AU.addPreserved<LoopInfo>(); 49 AU.addPreservedID(LoopSimplifyID); 50 AU.addPreservedID(LCSSAID); 51 } 52 53 private: 54 bool isLoopDead(Loop *L, SmallVectorImpl<BasicBlock *> &exitingBlocks, 55 SmallVectorImpl<BasicBlock *> &exitBlocks, 56 bool &Changed, BasicBlock *Preheader); 57 58 }; 59} 60 61char LoopDeletion::ID = 0; 62INITIALIZE_PASS_BEGIN(LoopDeletion, "loop-deletion", 63 "Delete dead loops", false, false) 64INITIALIZE_PASS_DEPENDENCY(DominatorTree) 65INITIALIZE_PASS_DEPENDENCY(LoopInfo) 66INITIALIZE_PASS_DEPENDENCY(ScalarEvolution) 67INITIALIZE_PASS_DEPENDENCY(LoopSimplify) 68INITIALIZE_PASS_DEPENDENCY(LCSSA) 69INITIALIZE_PASS_END(LoopDeletion, "loop-deletion", 70 "Delete dead loops", false, false) 71 72Pass *llvm::createLoopDeletionPass() { 73 return new LoopDeletion(); 74} 75 76/// isLoopDead - Determined if a loop is dead. This assumes that we've already 77/// checked for unique exit and exiting blocks, and that the code is in LCSSA 78/// form. 79bool LoopDeletion::isLoopDead(Loop *L, 80 SmallVectorImpl<BasicBlock *> &exitingBlocks, 81 SmallVectorImpl<BasicBlock *> &exitBlocks, 82 bool &Changed, BasicBlock *Preheader) { 83 BasicBlock *exitBlock = exitBlocks[0]; 84 85 // Make sure that all PHI entries coming from the loop are loop invariant. 86 // Because the code is in LCSSA form, any values used outside of the loop 87 // must pass through a PHI in the exit block, meaning that this check is 88 // sufficient to guarantee that no loop-variant values are used outside 89 // of the loop. 90 BasicBlock::iterator BI = exitBlock->begin(); 91 while (PHINode *P = dyn_cast<PHINode>(BI)) { 92 Value *incoming = P->getIncomingValueForBlock(exitingBlocks[0]); 93 94 // Make sure all exiting blocks produce the same incoming value for the exit 95 // block. If there are different incoming values for different exiting 96 // blocks, then it is impossible to statically determine which value should 97 // be used. 98 for (unsigned i = 1, e = exitingBlocks.size(); i < e; ++i) { 99 if (incoming != P->getIncomingValueForBlock(exitingBlocks[i])) 100 return false; 101 } 102 103 if (Instruction *I = dyn_cast<Instruction>(incoming)) 104 if (!L->makeLoopInvariant(I, Changed, Preheader->getTerminator())) 105 return false; 106 107 ++BI; 108 } 109 110 // Make sure that no instructions in the block have potential side-effects. 111 // This includes instructions that could write to memory, and loads that are 112 // marked volatile. This could be made more aggressive by using aliasing 113 // information to identify readonly and readnone calls. 114 for (Loop::block_iterator LI = L->block_begin(), LE = L->block_end(); 115 LI != LE; ++LI) { 116 for (BasicBlock::iterator BI = (*LI)->begin(), BE = (*LI)->end(); 117 BI != BE; ++BI) { 118 if (BI->mayHaveSideEffects()) 119 return false; 120 } 121 } 122 123 return true; 124} 125 126/// runOnLoop - Remove dead loops, by which we mean loops that do not impact the 127/// observable behavior of the program other than finite running time. Note 128/// we do ensure that this never remove a loop that might be infinite, as doing 129/// so could change the halting/non-halting nature of a program. 130/// NOTE: This entire process relies pretty heavily on LoopSimplify and LCSSA 131/// in order to make various safety checks work. 132bool LoopDeletion::runOnLoop(Loop *L, LPPassManager &LPM) { 133 // We can only remove the loop if there is a preheader that we can 134 // branch from after removing it. 135 BasicBlock *preheader = L->getLoopPreheader(); 136 if (!preheader) 137 return false; 138 139 // If LoopSimplify form is not available, stay out of trouble. 140 if (!L->hasDedicatedExits()) 141 return false; 142 143 // We can't remove loops that contain subloops. If the subloops were dead, 144 // they would already have been removed in earlier executions of this pass. 145 if (L->begin() != L->end()) 146 return false; 147 148 SmallVector<BasicBlock*, 4> exitingBlocks; 149 L->getExitingBlocks(exitingBlocks); 150 151 SmallVector<BasicBlock*, 4> exitBlocks; 152 L->getUniqueExitBlocks(exitBlocks); 153 154 // We require that the loop only have a single exit block. Otherwise, we'd 155 // be in the situation of needing to be able to solve statically which exit 156 // block will be branched to, or trying to preserve the branching logic in 157 // a loop invariant manner. 158 if (exitBlocks.size() != 1) 159 return false; 160 161 // Finally, we have to check that the loop really is dead. 162 bool Changed = false; 163 if (!isLoopDead(L, exitingBlocks, exitBlocks, Changed, preheader)) 164 return Changed; 165 166 // Don't remove loops for which we can't solve the trip count. 167 // They could be infinite, in which case we'd be changing program behavior. 168 ScalarEvolution &SE = getAnalysis<ScalarEvolution>(); 169 const SCEV *S = SE.getMaxBackedgeTakenCount(L); 170 if (isa<SCEVCouldNotCompute>(S)) 171 return Changed; 172 173 // Now that we know the removal is safe, remove the loop by changing the 174 // branch from the preheader to go to the single exit block. 175 BasicBlock *exitBlock = exitBlocks[0]; 176 177 // Because we're deleting a large chunk of code at once, the sequence in which 178 // we remove things is very important to avoid invalidation issues. Don't 179 // mess with this unless you have good reason and know what you're doing. 180 181 // Tell ScalarEvolution that the loop is deleted. Do this before 182 // deleting the loop so that ScalarEvolution can look at the loop 183 // to determine what it needs to clean up. 184 SE.forgetLoop(L); 185 186 // Connect the preheader directly to the exit block. 187 TerminatorInst *TI = preheader->getTerminator(); 188 TI->replaceUsesOfWith(L->getHeader(), exitBlock); 189 190 // Rewrite phis in the exit block to get their inputs from 191 // the preheader instead of the exiting block. 192 BasicBlock *exitingBlock = exitingBlocks[0]; 193 BasicBlock::iterator BI = exitBlock->begin(); 194 while (PHINode *P = dyn_cast<PHINode>(BI)) { 195 int j = P->getBasicBlockIndex(exitingBlock); 196 assert(j >= 0 && "Can't find exiting block in exit block's phi node!"); 197 P->setIncomingBlock(j, preheader); 198 for (unsigned i = 1; i < exitingBlocks.size(); ++i) 199 P->removeIncomingValue(exitingBlocks[i]); 200 ++BI; 201 } 202 203 // Update the dominator tree and remove the instructions and blocks that will 204 // be deleted from the reference counting scheme. 205 DominatorTree &DT = getAnalysis<DominatorTree>(); 206 SmallVector<DomTreeNode*, 8> ChildNodes; 207 for (Loop::block_iterator LI = L->block_begin(), LE = L->block_end(); 208 LI != LE; ++LI) { 209 // Move all of the block's children to be children of the preheader, which 210 // allows us to remove the domtree entry for the block. 211 ChildNodes.insert(ChildNodes.begin(), DT[*LI]->begin(), DT[*LI]->end()); 212 for (SmallVectorImpl<DomTreeNode *>::iterator DI = ChildNodes.begin(), 213 DE = ChildNodes.end(); DI != DE; ++DI) { 214 DT.changeImmediateDominator(*DI, DT[preheader]); 215 } 216 217 ChildNodes.clear(); 218 DT.eraseNode(*LI); 219 220 // Remove the block from the reference counting scheme, so that we can 221 // delete it freely later. 222 (*LI)->dropAllReferences(); 223 } 224 225 // Erase the instructions and the blocks without having to worry 226 // about ordering because we already dropped the references. 227 // NOTE: This iteration is safe because erasing the block does not remove its 228 // entry from the loop's block list. We do that in the next section. 229 for (Loop::block_iterator LI = L->block_begin(), LE = L->block_end(); 230 LI != LE; ++LI) 231 (*LI)->eraseFromParent(); 232 233 // Finally, the blocks from loopinfo. This has to happen late because 234 // otherwise our loop iterators won't work. 235 LoopInfo &loopInfo = getAnalysis<LoopInfo>(); 236 SmallPtrSet<BasicBlock*, 8> blocks; 237 blocks.insert(L->block_begin(), L->block_end()); 238 for (SmallPtrSet<BasicBlock*,8>::iterator I = blocks.begin(), 239 E = blocks.end(); I != E; ++I) 240 loopInfo.removeBlock(*I); 241 242 // The last step is to inform the loop pass manager that we've 243 // eliminated this loop. 244 LPM.deleteLoopFromQueue(L); 245 Changed = true; 246 247 ++NumDeleted; 248 249 return Changed; 250} 251