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