1 //===-- Sink.cpp - Code Sinking -------------------------------------------===// 2 // 3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions. 4 // See https://llvm.org/LICENSE.txt for license information. 5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception 6 // 7 //===----------------------------------------------------------------------===// 8 // 9 // This pass moves instructions into successor blocks, when possible, so that 10 // they aren't executed on paths where their results aren't needed. 11 // 12 //===----------------------------------------------------------------------===// 13 14 #include "llvm/Transforms/Scalar/Sink.h" 15 #include "llvm/ADT/Statistic.h" 16 #include "llvm/Analysis/AliasAnalysis.h" 17 #include "llvm/Analysis/LoopInfo.h" 18 #include "llvm/IR/Dominators.h" 19 #include "llvm/InitializePasses.h" 20 #include "llvm/Support/Debug.h" 21 #include "llvm/Support/raw_ostream.h" 22 #include "llvm/Transforms/Scalar.h" 23 using namespace llvm; 24 25 #define DEBUG_TYPE "sink" 26 27 STATISTIC(NumSunk, "Number of instructions sunk"); 28 STATISTIC(NumSinkIter, "Number of sinking iterations"); 29 30 static bool isSafeToMove(Instruction *Inst, AliasAnalysis &AA, 31 SmallPtrSetImpl<Instruction *> &Stores) { 32 33 if (Inst->mayWriteToMemory()) { 34 Stores.insert(Inst); 35 return false; 36 } 37 38 if (LoadInst *L = dyn_cast<LoadInst>(Inst)) { 39 MemoryLocation Loc = MemoryLocation::get(L); 40 for (Instruction *S : Stores) 41 if (isModSet(AA.getModRefInfo(S, Loc))) 42 return false; 43 } 44 45 if (Inst->isTerminator() || isa<PHINode>(Inst) || Inst->isEHPad() || 46 Inst->mayThrow() || !Inst->willReturn()) 47 return false; 48 49 if (auto *Call = dyn_cast<CallBase>(Inst)) { 50 // Convergent operations cannot be made control-dependent on additional 51 // values. 52 if (Call->isConvergent()) 53 return false; 54 55 for (Instruction *S : Stores) 56 if (isModSet(AA.getModRefInfo(S, Call))) 57 return false; 58 } 59 60 return true; 61 } 62 63 /// IsAcceptableTarget - Return true if it is possible to sink the instruction 64 /// in the specified basic block. 65 static bool IsAcceptableTarget(Instruction *Inst, BasicBlock *SuccToSinkTo, 66 DominatorTree &DT, LoopInfo &LI) { 67 assert(Inst && "Instruction to be sunk is null"); 68 assert(SuccToSinkTo && "Candidate sink target is null"); 69 70 // It's never legal to sink an instruction into an EH-pad block. 71 if (SuccToSinkTo->isEHPad()) 72 return false; 73 74 // If the block has multiple predecessors, this would introduce computation 75 // on different code paths. We could split the critical edge, but for now we 76 // just punt. 77 // FIXME: Split critical edges if not backedges. 78 if (SuccToSinkTo->getUniquePredecessor() != Inst->getParent()) { 79 // We cannot sink a load across a critical edge - there may be stores in 80 // other code paths. 81 if (Inst->mayReadFromMemory() && 82 !Inst->hasMetadata(LLVMContext::MD_invariant_load)) 83 return false; 84 85 // We don't want to sink across a critical edge if we don't dominate the 86 // successor. We could be introducing calculations to new code paths. 87 if (!DT.dominates(Inst->getParent(), SuccToSinkTo)) 88 return false; 89 90 // Don't sink instructions into a loop. 91 Loop *succ = LI.getLoopFor(SuccToSinkTo); 92 Loop *cur = LI.getLoopFor(Inst->getParent()); 93 if (succ != nullptr && succ != cur) 94 return false; 95 } 96 97 return true; 98 } 99 100 /// SinkInstruction - Determine whether it is safe to sink the specified machine 101 /// instruction out of its current block into a successor. 102 static bool SinkInstruction(Instruction *Inst, 103 SmallPtrSetImpl<Instruction *> &Stores, 104 DominatorTree &DT, LoopInfo &LI, AAResults &AA) { 105 106 // Don't sink static alloca instructions. CodeGen assumes allocas outside the 107 // entry block are dynamically sized stack objects. 108 if (AllocaInst *AI = dyn_cast<AllocaInst>(Inst)) 109 if (AI->isStaticAlloca()) 110 return false; 111 112 // Check if it's safe to move the instruction. 113 if (!isSafeToMove(Inst, AA, Stores)) 114 return false; 115 116 // FIXME: This should include support for sinking instructions within the 117 // block they are currently in to shorten the live ranges. We often get 118 // instructions sunk into the top of a large block, but it would be better to 119 // also sink them down before their first use in the block. This xform has to 120 // be careful not to *increase* register pressure though, e.g. sinking 121 // "x = y + z" down if it kills y and z would increase the live ranges of y 122 // and z and only shrink the live range of x. 123 124 // SuccToSinkTo - This is the successor to sink this instruction to, once we 125 // decide. 126 BasicBlock *SuccToSinkTo = nullptr; 127 128 // Find the nearest common dominator of all users as the candidate. 129 BasicBlock *BB = Inst->getParent(); 130 for (Use &U : Inst->uses()) { 131 Instruction *UseInst = cast<Instruction>(U.getUser()); 132 BasicBlock *UseBlock = UseInst->getParent(); 133 if (PHINode *PN = dyn_cast<PHINode>(UseInst)) { 134 // PHI nodes use the operand in the predecessor block, not the block with 135 // the PHI. 136 unsigned Num = PHINode::getIncomingValueNumForOperand(U.getOperandNo()); 137 UseBlock = PN->getIncomingBlock(Num); 138 } 139 // Don't worry about dead users. 140 if (!DT.isReachableFromEntry(UseBlock)) 141 continue; 142 143 if (SuccToSinkTo) 144 SuccToSinkTo = DT.findNearestCommonDominator(SuccToSinkTo, UseBlock); 145 else 146 SuccToSinkTo = UseBlock; 147 // The current basic block needs to dominate the candidate. 148 if (!DT.dominates(BB, SuccToSinkTo)) 149 return false; 150 } 151 152 if (SuccToSinkTo) { 153 // The nearest common dominator may be in a parent loop of BB, which may not 154 // be beneficial. Find an ancestor. 155 while (SuccToSinkTo != BB && 156 !IsAcceptableTarget(Inst, SuccToSinkTo, DT, LI)) 157 SuccToSinkTo = DT.getNode(SuccToSinkTo)->getIDom()->getBlock(); 158 if (SuccToSinkTo == BB) 159 SuccToSinkTo = nullptr; 160 } 161 162 // If we couldn't find a block to sink to, ignore this instruction. 163 if (!SuccToSinkTo) 164 return false; 165 166 LLVM_DEBUG(dbgs() << "Sink" << *Inst << " ("; 167 Inst->getParent()->printAsOperand(dbgs(), false); dbgs() << " -> "; 168 SuccToSinkTo->printAsOperand(dbgs(), false); dbgs() << ")\n"); 169 170 // Move the instruction. 171 Inst->moveBefore(&*SuccToSinkTo->getFirstInsertionPt()); 172 return true; 173 } 174 175 static bool ProcessBlock(BasicBlock &BB, DominatorTree &DT, LoopInfo &LI, 176 AAResults &AA) { 177 // Don't bother sinking code out of unreachable blocks. In addition to being 178 // unprofitable, it can also lead to infinite looping, because in an 179 // unreachable loop there may be nowhere to stop. 180 if (!DT.isReachableFromEntry(&BB)) return false; 181 182 bool MadeChange = false; 183 184 // Walk the basic block bottom-up. Remember if we saw a store. 185 BasicBlock::iterator I = BB.end(); 186 --I; 187 bool ProcessedBegin = false; 188 SmallPtrSet<Instruction *, 8> Stores; 189 do { 190 Instruction *Inst = &*I; // The instruction to sink. 191 192 // Predecrement I (if it's not begin) so that it isn't invalidated by 193 // sinking. 194 ProcessedBegin = I == BB.begin(); 195 if (!ProcessedBegin) 196 --I; 197 198 if (Inst->isDebugOrPseudoInst()) 199 continue; 200 201 if (SinkInstruction(Inst, Stores, DT, LI, AA)) { 202 ++NumSunk; 203 MadeChange = true; 204 } 205 206 // If we just processed the first instruction in the block, we're done. 207 } while (!ProcessedBegin); 208 209 return MadeChange; 210 } 211 212 static bool iterativelySinkInstructions(Function &F, DominatorTree &DT, 213 LoopInfo &LI, AAResults &AA) { 214 bool MadeChange, EverMadeChange = false; 215 216 do { 217 MadeChange = false; 218 LLVM_DEBUG(dbgs() << "Sinking iteration " << NumSinkIter << "\n"); 219 // Process all basic blocks. 220 for (BasicBlock &I : F) 221 MadeChange |= ProcessBlock(I, DT, LI, AA); 222 EverMadeChange |= MadeChange; 223 NumSinkIter++; 224 } while (MadeChange); 225 226 return EverMadeChange; 227 } 228 229 PreservedAnalyses SinkingPass::run(Function &F, FunctionAnalysisManager &AM) { 230 auto &DT = AM.getResult<DominatorTreeAnalysis>(F); 231 auto &LI = AM.getResult<LoopAnalysis>(F); 232 auto &AA = AM.getResult<AAManager>(F); 233 234 if (!iterativelySinkInstructions(F, DT, LI, AA)) 235 return PreservedAnalyses::all(); 236 237 PreservedAnalyses PA; 238 PA.preserveSet<CFGAnalyses>(); 239 return PA; 240 } 241 242 namespace { 243 class SinkingLegacyPass : public FunctionPass { 244 public: 245 static char ID; // Pass identification 246 SinkingLegacyPass() : FunctionPass(ID) { 247 initializeSinkingLegacyPassPass(*PassRegistry::getPassRegistry()); 248 } 249 250 bool runOnFunction(Function &F) override { 251 auto &DT = getAnalysis<DominatorTreeWrapperPass>().getDomTree(); 252 auto &LI = getAnalysis<LoopInfoWrapperPass>().getLoopInfo(); 253 auto &AA = getAnalysis<AAResultsWrapperPass>().getAAResults(); 254 255 return iterativelySinkInstructions(F, DT, LI, AA); 256 } 257 258 void getAnalysisUsage(AnalysisUsage &AU) const override { 259 AU.setPreservesCFG(); 260 FunctionPass::getAnalysisUsage(AU); 261 AU.addRequired<AAResultsWrapperPass>(); 262 AU.addRequired<DominatorTreeWrapperPass>(); 263 AU.addRequired<LoopInfoWrapperPass>(); 264 AU.addPreserved<DominatorTreeWrapperPass>(); 265 AU.addPreserved<LoopInfoWrapperPass>(); 266 } 267 }; 268 } // end anonymous namespace 269 270 char SinkingLegacyPass::ID = 0; 271 INITIALIZE_PASS_BEGIN(SinkingLegacyPass, "sink", "Code sinking", false, false) 272 INITIALIZE_PASS_DEPENDENCY(LoopInfoWrapperPass) 273 INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass) 274 INITIALIZE_PASS_DEPENDENCY(AAResultsWrapperPass) 275 INITIALIZE_PASS_END(SinkingLegacyPass, "sink", "Code sinking", false, false) 276 277 FunctionPass *llvm::createSinkingPass() { return new SinkingLegacyPass(); } 278