1 //===-- Sink.cpp - Code Sinking -------------------------------------------===// 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 pass moves instructions into successor blocks, when possible, so that 11 // they aren't executed on paths where their results aren't needed. 12 // 13 //===----------------------------------------------------------------------===// 14 15 #include "llvm/Transforms/Scalar.h" 16 #include "llvm/ADT/Statistic.h" 17 #include "llvm/Analysis/AliasAnalysis.h" 18 #include "llvm/Analysis/LoopInfo.h" 19 #include "llvm/Analysis/ValueTracking.h" 20 #include "llvm/IR/CFG.h" 21 #include "llvm/IR/DataLayout.h" 22 #include "llvm/IR/Dominators.h" 23 #include "llvm/IR/IntrinsicInst.h" 24 #include "llvm/Support/Debug.h" 25 #include "llvm/Support/raw_ostream.h" 26 using namespace llvm; 27 28 #define DEBUG_TYPE "sink" 29 30 STATISTIC(NumSunk, "Number of instructions sunk"); 31 STATISTIC(NumSinkIter, "Number of sinking iterations"); 32 33 namespace { 34 class Sinking : public FunctionPass { 35 DominatorTree *DT; 36 LoopInfo *LI; 37 AliasAnalysis *AA; 38 const DataLayout *DL; 39 40 public: 41 static char ID; // Pass identification 42 Sinking() : FunctionPass(ID) { 43 initializeSinkingPass(*PassRegistry::getPassRegistry()); 44 } 45 46 bool runOnFunction(Function &F) override; 47 48 void getAnalysisUsage(AnalysisUsage &AU) const override { 49 AU.setPreservesCFG(); 50 FunctionPass::getAnalysisUsage(AU); 51 AU.addRequired<AliasAnalysis>(); 52 AU.addRequired<DominatorTreeWrapperPass>(); 53 AU.addRequired<LoopInfo>(); 54 AU.addPreserved<DominatorTreeWrapperPass>(); 55 AU.addPreserved<LoopInfo>(); 56 } 57 private: 58 bool ProcessBlock(BasicBlock &BB); 59 bool SinkInstruction(Instruction *I, SmallPtrSet<Instruction *, 8> &Stores); 60 bool AllUsesDominatedByBlock(Instruction *Inst, BasicBlock *BB) const; 61 bool IsAcceptableTarget(Instruction *Inst, BasicBlock *SuccToSinkTo) const; 62 }; 63 } // end anonymous namespace 64 65 char Sinking::ID = 0; 66 INITIALIZE_PASS_BEGIN(Sinking, "sink", "Code sinking", false, false) 67 INITIALIZE_PASS_DEPENDENCY(LoopInfo) 68 INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass) 69 INITIALIZE_AG_DEPENDENCY(AliasAnalysis) 70 INITIALIZE_PASS_END(Sinking, "sink", "Code sinking", false, false) 71 72 FunctionPass *llvm::createSinkingPass() { return new Sinking(); } 73 74 /// AllUsesDominatedByBlock - Return true if all uses of the specified value 75 /// occur in blocks dominated by the specified block. 76 bool Sinking::AllUsesDominatedByBlock(Instruction *Inst, 77 BasicBlock *BB) const { 78 // Ignoring debug uses is necessary so debug info doesn't affect the code. 79 // This may leave a referencing dbg_value in the original block, before 80 // the definition of the vreg. Dwarf generator handles this although the 81 // user might not get the right info at runtime. 82 for (Use &U : Inst->uses()) { 83 // Determine the block of the use. 84 Instruction *UseInst = cast<Instruction>(U.getUser()); 85 BasicBlock *UseBlock = UseInst->getParent(); 86 if (PHINode *PN = dyn_cast<PHINode>(UseInst)) { 87 // PHI nodes use the operand in the predecessor block, not the block with 88 // the PHI. 89 unsigned Num = PHINode::getIncomingValueNumForOperand(U.getOperandNo()); 90 UseBlock = PN->getIncomingBlock(Num); 91 } 92 // Check that it dominates. 93 if (!DT->dominates(BB, UseBlock)) 94 return false; 95 } 96 return true; 97 } 98 99 bool Sinking::runOnFunction(Function &F) { 100 DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree(); 101 LI = &getAnalysis<LoopInfo>(); 102 AA = &getAnalysis<AliasAnalysis>(); 103 DataLayoutPass *DLP = getAnalysisIfAvailable<DataLayoutPass>(); 104 DL = DLP ? &DLP->getDataLayout() : nullptr; 105 106 bool MadeChange, EverMadeChange = false; 107 108 do { 109 MadeChange = false; 110 DEBUG(dbgs() << "Sinking iteration " << NumSinkIter << "\n"); 111 // Process all basic blocks. 112 for (Function::iterator I = F.begin(), E = F.end(); 113 I != E; ++I) 114 MadeChange |= ProcessBlock(*I); 115 EverMadeChange |= MadeChange; 116 NumSinkIter++; 117 } while (MadeChange); 118 119 return EverMadeChange; 120 } 121 122 bool Sinking::ProcessBlock(BasicBlock &BB) { 123 // Can't sink anything out of a block that has less than two successors. 124 if (BB.getTerminator()->getNumSuccessors() <= 1 || BB.empty()) return false; 125 126 // Don't bother sinking code out of unreachable blocks. In addition to being 127 // unprofitable, it can also lead to infinite looping, because in an 128 // unreachable loop there may be nowhere to stop. 129 if (!DT->isReachableFromEntry(&BB)) return false; 130 131 bool MadeChange = false; 132 133 // Walk the basic block bottom-up. Remember if we saw a store. 134 BasicBlock::iterator I = BB.end(); 135 --I; 136 bool ProcessedBegin = false; 137 SmallPtrSet<Instruction *, 8> Stores; 138 do { 139 Instruction *Inst = I; // The instruction to sink. 140 141 // Predecrement I (if it's not begin) so that it isn't invalidated by 142 // sinking. 143 ProcessedBegin = I == BB.begin(); 144 if (!ProcessedBegin) 145 --I; 146 147 if (isa<DbgInfoIntrinsic>(Inst)) 148 continue; 149 150 if (SinkInstruction(Inst, Stores)) 151 ++NumSunk, MadeChange = true; 152 153 // If we just processed the first instruction in the block, we're done. 154 } while (!ProcessedBegin); 155 156 return MadeChange; 157 } 158 159 static bool isSafeToMove(Instruction *Inst, AliasAnalysis *AA, 160 SmallPtrSet<Instruction *, 8> &Stores) { 161 162 if (Inst->mayWriteToMemory()) { 163 Stores.insert(Inst); 164 return false; 165 } 166 167 if (LoadInst *L = dyn_cast<LoadInst>(Inst)) { 168 AliasAnalysis::Location Loc = AA->getLocation(L); 169 for (SmallPtrSet<Instruction *, 8>::iterator I = Stores.begin(), 170 E = Stores.end(); I != E; ++I) 171 if (AA->getModRefInfo(*I, Loc) & AliasAnalysis::Mod) 172 return false; 173 } 174 175 if (isa<TerminatorInst>(Inst) || isa<PHINode>(Inst)) 176 return false; 177 178 return true; 179 } 180 181 /// IsAcceptableTarget - Return true if it is possible to sink the instruction 182 /// in the specified basic block. 183 bool Sinking::IsAcceptableTarget(Instruction *Inst, 184 BasicBlock *SuccToSinkTo) const { 185 assert(Inst && "Instruction to be sunk is null"); 186 assert(SuccToSinkTo && "Candidate sink target is null"); 187 188 // It is not possible to sink an instruction into its own block. This can 189 // happen with loops. 190 if (Inst->getParent() == SuccToSinkTo) 191 return false; 192 193 // If the block has multiple predecessors, this would introduce computation 194 // on different code paths. We could split the critical edge, but for now we 195 // just punt. 196 // FIXME: Split critical edges if not backedges. 197 if (SuccToSinkTo->getUniquePredecessor() != Inst->getParent()) { 198 // We cannot sink a load across a critical edge - there may be stores in 199 // other code paths. 200 if (!isSafeToSpeculativelyExecute(Inst, DL)) 201 return false; 202 203 // We don't want to sink across a critical edge if we don't dominate the 204 // successor. We could be introducing calculations to new code paths. 205 if (!DT->dominates(Inst->getParent(), SuccToSinkTo)) 206 return false; 207 208 // Don't sink instructions into a loop. 209 Loop *succ = LI->getLoopFor(SuccToSinkTo); 210 Loop *cur = LI->getLoopFor(Inst->getParent()); 211 if (succ != nullptr && succ != cur) 212 return false; 213 } 214 215 // Finally, check that all the uses of the instruction are actually 216 // dominated by the candidate 217 return AllUsesDominatedByBlock(Inst, SuccToSinkTo); 218 } 219 220 /// SinkInstruction - Determine whether it is safe to sink the specified machine 221 /// instruction out of its current block into a successor. 222 bool Sinking::SinkInstruction(Instruction *Inst, 223 SmallPtrSet<Instruction *, 8> &Stores) { 224 225 // Don't sink static alloca instructions. CodeGen assumes allocas outside the 226 // entry block are dynamically sized stack objects. 227 if (AllocaInst *AI = dyn_cast<AllocaInst>(Inst)) 228 if (AI->isStaticAlloca()) 229 return false; 230 231 // Check if it's safe to move the instruction. 232 if (!isSafeToMove(Inst, AA, Stores)) 233 return false; 234 235 // FIXME: This should include support for sinking instructions within the 236 // block they are currently in to shorten the live ranges. We often get 237 // instructions sunk into the top of a large block, but it would be better to 238 // also sink them down before their first use in the block. This xform has to 239 // be careful not to *increase* register pressure though, e.g. sinking 240 // "x = y + z" down if it kills y and z would increase the live ranges of y 241 // and z and only shrink the live range of x. 242 243 // SuccToSinkTo - This is the successor to sink this instruction to, once we 244 // decide. 245 BasicBlock *SuccToSinkTo = nullptr; 246 247 // Instructions can only be sunk if all their uses are in blocks 248 // dominated by one of the successors. 249 // Look at all the postdominators and see if we can sink it in one. 250 DomTreeNode *DTN = DT->getNode(Inst->getParent()); 251 for (DomTreeNode::iterator I = DTN->begin(), E = DTN->end(); 252 I != E && SuccToSinkTo == nullptr; ++I) { 253 BasicBlock *Candidate = (*I)->getBlock(); 254 if ((*I)->getIDom()->getBlock() == Inst->getParent() && 255 IsAcceptableTarget(Inst, Candidate)) 256 SuccToSinkTo = Candidate; 257 } 258 259 // If no suitable postdominator was found, look at all the successors and 260 // decide which one we should sink to, if any. 261 for (succ_iterator I = succ_begin(Inst->getParent()), 262 E = succ_end(Inst->getParent()); I != E && !SuccToSinkTo; ++I) { 263 if (IsAcceptableTarget(Inst, *I)) 264 SuccToSinkTo = *I; 265 } 266 267 // If we couldn't find a block to sink to, ignore this instruction. 268 if (!SuccToSinkTo) 269 return false; 270 271 DEBUG(dbgs() << "Sink" << *Inst << " ("; 272 Inst->getParent()->printAsOperand(dbgs(), false); 273 dbgs() << " -> "; 274 SuccToSinkTo->printAsOperand(dbgs(), false); 275 dbgs() << ")\n"); 276 277 // Move the instruction. 278 Inst->moveBefore(SuccToSinkTo->getFirstInsertionPt()); 279 return true; 280 } 281