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/IR/Module.h" 25 #include "llvm/Support/Debug.h" 26 #include "llvm/Support/raw_ostream.h" 27 using namespace llvm; 28 29 #define DEBUG_TYPE "sink" 30 31 STATISTIC(NumSunk, "Number of instructions sunk"); 32 STATISTIC(NumSinkIter, "Number of sinking iterations"); 33 34 namespace { 35 class Sinking : public FunctionPass { 36 DominatorTree *DT; 37 LoopInfo *LI; 38 AliasAnalysis *AA; 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<AAResultsWrapperPass>(); 52 AU.addRequired<DominatorTreeWrapperPass>(); 53 AU.addRequired<LoopInfoWrapperPass>(); 54 AU.addPreserved<DominatorTreeWrapperPass>(); 55 AU.addPreserved<LoopInfoWrapperPass>(); 56 } 57 private: 58 bool ProcessBlock(BasicBlock &BB); 59 bool SinkInstruction(Instruction *I, SmallPtrSetImpl<Instruction*> &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(LoopInfoWrapperPass) 68 INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass) 69 INITIALIZE_PASS_DEPENDENCY(AAResultsWrapperPass) 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<LoopInfoWrapperPass>().getLoopInfo(); 102 AA = &getAnalysis<AAResultsWrapperPass>().getAAResults(); 103 104 bool MadeChange, EverMadeChange = false; 105 106 do { 107 MadeChange = false; 108 DEBUG(dbgs() << "Sinking iteration " << NumSinkIter << "\n"); 109 // Process all basic blocks. 110 for (Function::iterator I = F.begin(), E = F.end(); 111 I != E; ++I) 112 MadeChange |= ProcessBlock(*I); 113 EverMadeChange |= MadeChange; 114 NumSinkIter++; 115 } while (MadeChange); 116 117 return EverMadeChange; 118 } 119 120 bool Sinking::ProcessBlock(BasicBlock &BB) { 121 // Can't sink anything out of a block that has less than two successors. 122 if (BB.getTerminator()->getNumSuccessors() <= 1) return false; 123 124 // Don't bother sinking code out of unreachable blocks. In addition to being 125 // unprofitable, it can also lead to infinite looping, because in an 126 // unreachable loop there may be nowhere to stop. 127 if (!DT->isReachableFromEntry(&BB)) return false; 128 129 bool MadeChange = false; 130 131 // Walk the basic block bottom-up. Remember if we saw a store. 132 BasicBlock::iterator I = BB.end(); 133 --I; 134 bool ProcessedBegin = false; 135 SmallPtrSet<Instruction *, 8> Stores; 136 do { 137 Instruction *Inst = &*I; // The instruction to sink. 138 139 // Predecrement I (if it's not begin) so that it isn't invalidated by 140 // sinking. 141 ProcessedBegin = I == BB.begin(); 142 if (!ProcessedBegin) 143 --I; 144 145 if (isa<DbgInfoIntrinsic>(Inst)) 146 continue; 147 148 if (SinkInstruction(Inst, Stores)) 149 ++NumSunk, MadeChange = true; 150 151 // If we just processed the first instruction in the block, we're done. 152 } while (!ProcessedBegin); 153 154 return MadeChange; 155 } 156 157 static bool isSafeToMove(Instruction *Inst, AliasAnalysis *AA, 158 SmallPtrSetImpl<Instruction *> &Stores) { 159 160 if (Inst->mayWriteToMemory()) { 161 Stores.insert(Inst); 162 return false; 163 } 164 165 if (LoadInst *L = dyn_cast<LoadInst>(Inst)) { 166 MemoryLocation Loc = MemoryLocation::get(L); 167 for (Instruction *S : Stores) 168 if (AA->getModRefInfo(S, Loc) & MRI_Mod) 169 return false; 170 } 171 172 if (isa<TerminatorInst>(Inst) || isa<PHINode>(Inst) || Inst->isEHPad() || 173 Inst->mayThrow()) 174 return false; 175 176 // Convergent operations cannot be made control-dependent on additional 177 // values. 178 if (auto CS = CallSite(Inst)) { 179 if (CS.hasFnAttr(Attribute::Convergent)) 180 return false; 181 } 182 183 return true; 184 } 185 186 /// IsAcceptableTarget - Return true if it is possible to sink the instruction 187 /// in the specified basic block. 188 bool Sinking::IsAcceptableTarget(Instruction *Inst, 189 BasicBlock *SuccToSinkTo) const { 190 assert(Inst && "Instruction to be sunk is null"); 191 assert(SuccToSinkTo && "Candidate sink target is null"); 192 193 // It is not possible to sink an instruction into its own block. This can 194 // happen with loops. 195 if (Inst->getParent() == SuccToSinkTo) 196 return false; 197 198 // It's never legal to sink an instruction into a block which terminates in an 199 // EH-pad. 200 if (SuccToSinkTo->getTerminator()->isExceptional()) 201 return false; 202 203 // If the block has multiple predecessors, this would introduce computation 204 // on different code paths. We could split the critical edge, but for now we 205 // just punt. 206 // FIXME: Split critical edges if not backedges. 207 if (SuccToSinkTo->getUniquePredecessor() != Inst->getParent()) { 208 // We cannot sink a load across a critical edge - there may be stores in 209 // other code paths. 210 if (!isSafeToSpeculativelyExecute(Inst)) 211 return false; 212 213 // We don't want to sink across a critical edge if we don't dominate the 214 // successor. We could be introducing calculations to new code paths. 215 if (!DT->dominates(Inst->getParent(), SuccToSinkTo)) 216 return false; 217 218 // Don't sink instructions into a loop. 219 Loop *succ = LI->getLoopFor(SuccToSinkTo); 220 Loop *cur = LI->getLoopFor(Inst->getParent()); 221 if (succ != nullptr && succ != cur) 222 return false; 223 } 224 225 // Finally, check that all the uses of the instruction are actually 226 // dominated by the candidate 227 return AllUsesDominatedByBlock(Inst, SuccToSinkTo); 228 } 229 230 /// SinkInstruction - Determine whether it is safe to sink the specified machine 231 /// instruction out of its current block into a successor. 232 bool Sinking::SinkInstruction(Instruction *Inst, 233 SmallPtrSetImpl<Instruction *> &Stores) { 234 235 // Don't sink static alloca instructions. CodeGen assumes allocas outside the 236 // entry block are dynamically sized stack objects. 237 if (AllocaInst *AI = dyn_cast<AllocaInst>(Inst)) 238 if (AI->isStaticAlloca()) 239 return false; 240 241 // Check if it's safe to move the instruction. 242 if (!isSafeToMove(Inst, AA, Stores)) 243 return false; 244 245 // FIXME: This should include support for sinking instructions within the 246 // block they are currently in to shorten the live ranges. We often get 247 // instructions sunk into the top of a large block, but it would be better to 248 // also sink them down before their first use in the block. This xform has to 249 // be careful not to *increase* register pressure though, e.g. sinking 250 // "x = y + z" down if it kills y and z would increase the live ranges of y 251 // and z and only shrink the live range of x. 252 253 // SuccToSinkTo - This is the successor to sink this instruction to, once we 254 // decide. 255 BasicBlock *SuccToSinkTo = nullptr; 256 257 // Instructions can only be sunk if all their uses are in blocks 258 // dominated by one of the successors. 259 // Look at all the postdominators and see if we can sink it in one. 260 DomTreeNode *DTN = DT->getNode(Inst->getParent()); 261 for (DomTreeNode::iterator I = DTN->begin(), E = DTN->end(); 262 I != E && SuccToSinkTo == nullptr; ++I) { 263 BasicBlock *Candidate = (*I)->getBlock(); 264 if ((*I)->getIDom()->getBlock() == Inst->getParent() && 265 IsAcceptableTarget(Inst, Candidate)) 266 SuccToSinkTo = Candidate; 267 } 268 269 // If no suitable postdominator was found, look at all the successors and 270 // decide which one we should sink to, if any. 271 for (succ_iterator I = succ_begin(Inst->getParent()), 272 E = succ_end(Inst->getParent()); I != E && !SuccToSinkTo; ++I) { 273 if (IsAcceptableTarget(Inst, *I)) 274 SuccToSinkTo = *I; 275 } 276 277 // If we couldn't find a block to sink to, ignore this instruction. 278 if (!SuccToSinkTo) 279 return false; 280 281 DEBUG(dbgs() << "Sink" << *Inst << " ("; 282 Inst->getParent()->printAsOperand(dbgs(), false); 283 dbgs() << " -> "; 284 SuccToSinkTo->printAsOperand(dbgs(), false); 285 dbgs() << ")\n"); 286 287 // Move the instruction. 288 Inst->moveBefore(&*SuccToSinkTo->getFirstInsertionPt()); 289 return true; 290 } 291