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 #define DEBUG_TYPE "sink" 16 #include "llvm/Transforms/Scalar.h" 17 #include "llvm/IntrinsicInst.h" 18 #include "llvm/Analysis/Dominators.h" 19 #include "llvm/Analysis/LoopInfo.h" 20 #include "llvm/Analysis/AliasAnalysis.h" 21 #include "llvm/Assembly/Writer.h" 22 #include "llvm/ADT/Statistic.h" 23 #include "llvm/Support/CFG.h" 24 #include "llvm/Support/Debug.h" 25 #include "llvm/Support/raw_ostream.h" 26 using namespace llvm; 27 28 STATISTIC(NumSunk, "Number of instructions sunk"); 29 30 namespace { 31 class Sinking : public FunctionPass { 32 DominatorTree *DT; 33 LoopInfo *LI; 34 AliasAnalysis *AA; 35 36 public: 37 static char ID; // Pass identification 38 Sinking() : FunctionPass(ID) { 39 initializeSinkingPass(*PassRegistry::getPassRegistry()); 40 } 41 42 virtual bool runOnFunction(Function &F); 43 44 virtual void getAnalysisUsage(AnalysisUsage &AU) const { 45 AU.setPreservesCFG(); 46 FunctionPass::getAnalysisUsage(AU); 47 AU.addRequired<AliasAnalysis>(); 48 AU.addRequired<DominatorTree>(); 49 AU.addRequired<LoopInfo>(); 50 AU.addPreserved<DominatorTree>(); 51 AU.addPreserved<LoopInfo>(); 52 } 53 private: 54 bool ProcessBlock(BasicBlock &BB); 55 bool SinkInstruction(Instruction *I, SmallPtrSet<Instruction *, 8> &Stores); 56 bool AllUsesDominatedByBlock(Instruction *Inst, BasicBlock *BB) const; 57 }; 58 } // end anonymous namespace 59 60 char Sinking::ID = 0; 61 INITIALIZE_PASS_BEGIN(Sinking, "sink", "Code sinking", false, false) 62 INITIALIZE_PASS_DEPENDENCY(LoopInfo) 63 INITIALIZE_PASS_DEPENDENCY(DominatorTree) 64 INITIALIZE_AG_DEPENDENCY(AliasAnalysis) 65 INITIALIZE_PASS_END(Sinking, "sink", "Code sinking", false, false) 66 67 FunctionPass *llvm::createSinkingPass() { return new Sinking(); } 68 69 /// AllUsesDominatedByBlock - Return true if all uses of the specified value 70 /// occur in blocks dominated by the specified block. 71 bool Sinking::AllUsesDominatedByBlock(Instruction *Inst, 72 BasicBlock *BB) const { 73 // Ignoring debug uses is necessary so debug info doesn't affect the code. 74 // This may leave a referencing dbg_value in the original block, before 75 // the definition of the vreg. Dwarf generator handles this although the 76 // user might not get the right info at runtime. 77 for (Value::use_iterator I = Inst->use_begin(), 78 E = Inst->use_end(); I != E; ++I) { 79 // Determine the block of the use. 80 Instruction *UseInst = cast<Instruction>(*I); 81 BasicBlock *UseBlock = UseInst->getParent(); 82 if (PHINode *PN = dyn_cast<PHINode>(UseInst)) { 83 // PHI nodes use the operand in the predecessor block, not the block with 84 // the PHI. 85 unsigned Num = PHINode::getIncomingValueNumForOperand(I.getOperandNo()); 86 UseBlock = PN->getIncomingBlock(Num); 87 } 88 // Check that it dominates. 89 if (!DT->dominates(BB, UseBlock)) 90 return false; 91 } 92 return true; 93 } 94 95 bool Sinking::runOnFunction(Function &F) { 96 DT = &getAnalysis<DominatorTree>(); 97 LI = &getAnalysis<LoopInfo>(); 98 AA = &getAnalysis<AliasAnalysis>(); 99 100 bool EverMadeChange = false; 101 102 while (1) { 103 bool MadeChange = false; 104 105 // Process all basic blocks. 106 for (Function::iterator I = F.begin(), E = F.end(); 107 I != E; ++I) 108 MadeChange |= ProcessBlock(*I); 109 110 // If this iteration over the code changed anything, keep iterating. 111 if (!MadeChange) break; 112 EverMadeChange = true; 113 } 114 return EverMadeChange; 115 } 116 117 bool Sinking::ProcessBlock(BasicBlock &BB) { 118 // Can't sink anything out of a block that has less than two successors. 119 if (BB.getTerminator()->getNumSuccessors() <= 1 || BB.empty()) return false; 120 121 // Don't bother sinking code out of unreachable blocks. In addition to being 122 // unprofitable, it can also lead to infinite looping, because in an unreachable 123 // loop there may be nowhere to stop. 124 if (!DT->isReachableFromEntry(&BB)) return false; 125 126 bool MadeChange = false; 127 128 // Walk the basic block bottom-up. Remember if we saw a store. 129 BasicBlock::iterator I = BB.end(); 130 --I; 131 bool ProcessedBegin = false; 132 SmallPtrSet<Instruction *, 8> Stores; 133 do { 134 Instruction *Inst = I; // The instruction to sink. 135 136 // Predecrement I (if it's not begin) so that it isn't invalidated by 137 // sinking. 138 ProcessedBegin = I == BB.begin(); 139 if (!ProcessedBegin) 140 --I; 141 142 if (isa<DbgInfoIntrinsic>(Inst)) 143 continue; 144 145 if (SinkInstruction(Inst, Stores)) 146 ++NumSunk, MadeChange = true; 147 148 // If we just processed the first instruction in the block, we're done. 149 } while (!ProcessedBegin); 150 151 return MadeChange; 152 } 153 154 static bool isSafeToMove(Instruction *Inst, AliasAnalysis *AA, 155 SmallPtrSet<Instruction *, 8> &Stores) { 156 157 if (Inst->mayWriteToMemory()) { 158 Stores.insert(Inst); 159 return false; 160 } 161 162 if (LoadInst *L = dyn_cast<LoadInst>(Inst)) { 163 AliasAnalysis::Location Loc = AA->getLocation(L); 164 for (SmallPtrSet<Instruction *, 8>::iterator I = Stores.begin(), 165 E = Stores.end(); I != E; ++I) 166 if (AA->getModRefInfo(*I, Loc) & AliasAnalysis::Mod) 167 return false; 168 } 169 170 if (isa<TerminatorInst>(Inst) || isa<PHINode>(Inst)) 171 return false; 172 173 return true; 174 } 175 176 /// SinkInstruction - Determine whether it is safe to sink the specified machine 177 /// instruction out of its current block into a successor. 178 bool Sinking::SinkInstruction(Instruction *Inst, 179 SmallPtrSet<Instruction *, 8> &Stores) { 180 // Check if it's safe to move the instruction. 181 if (!isSafeToMove(Inst, AA, Stores)) 182 return false; 183 184 // FIXME: This should include support for sinking instructions within the 185 // block they are currently in to shorten the live ranges. We often get 186 // instructions sunk into the top of a large block, but it would be better to 187 // also sink them down before their first use in the block. This xform has to 188 // be careful not to *increase* register pressure though, e.g. sinking 189 // "x = y + z" down if it kills y and z would increase the live ranges of y 190 // and z and only shrink the live range of x. 191 192 // Loop over all the operands of the specified instruction. If there is 193 // anything we can't handle, bail out. 194 BasicBlock *ParentBlock = Inst->getParent(); 195 196 // SuccToSinkTo - This is the successor to sink this instruction to, once we 197 // decide. 198 BasicBlock *SuccToSinkTo = 0; 199 200 // FIXME: This picks a successor to sink into based on having one 201 // successor that dominates all the uses. However, there are cases where 202 // sinking can happen but where the sink point isn't a successor. For 203 // example: 204 // x = computation 205 // if () {} else {} 206 // use x 207 // the instruction could be sunk over the whole diamond for the 208 // if/then/else (or loop, etc), allowing it to be sunk into other blocks 209 // after that. 210 211 // Instructions can only be sunk if all their uses are in blocks 212 // dominated by one of the successors. 213 // Look at all the successors and decide which one 214 // we should sink to. 215 for (succ_iterator SI = succ_begin(ParentBlock), 216 E = succ_end(ParentBlock); SI != E; ++SI) { 217 if (AllUsesDominatedByBlock(Inst, *SI)) { 218 SuccToSinkTo = *SI; 219 break; 220 } 221 } 222 223 // If we couldn't find a block to sink to, ignore this instruction. 224 if (SuccToSinkTo == 0) 225 return false; 226 227 // It is not possible to sink an instruction into its own block. This can 228 // happen with loops. 229 if (Inst->getParent() == SuccToSinkTo) 230 return false; 231 232 DEBUG(dbgs() << "Sink instr " << *Inst); 233 DEBUG(dbgs() << "to block "; 234 WriteAsOperand(dbgs(), SuccToSinkTo, false)); 235 236 // If the block has multiple predecessors, this would introduce computation on 237 // a path that it doesn't already exist. We could split the critical edge, 238 // but for now we just punt. 239 // FIXME: Split critical edges if not backedges. 240 if (SuccToSinkTo->getUniquePredecessor() != ParentBlock) { 241 // We cannot sink a load across a critical edge - there may be stores in 242 // other code paths. 243 if (!Inst->isSafeToSpeculativelyExecute()) { 244 DEBUG(dbgs() << " *** PUNTING: Wont sink load along critical edge.\n"); 245 return false; 246 } 247 248 // We don't want to sink across a critical edge if we don't dominate the 249 // successor. We could be introducing calculations to new code paths. 250 if (!DT->dominates(ParentBlock, SuccToSinkTo)) { 251 DEBUG(dbgs() << " *** PUNTING: Critical edge found\n"); 252 return false; 253 } 254 255 // Don't sink instructions into a loop. 256 if (LI->isLoopHeader(SuccToSinkTo)) { 257 DEBUG(dbgs() << " *** PUNTING: Loop header found\n"); 258 return false; 259 } 260 261 // Otherwise we are OK with sinking along a critical edge. 262 DEBUG(dbgs() << "Sinking along critical edge.\n"); 263 } 264 265 // Determine where to insert into. Skip phi nodes. 266 BasicBlock::iterator InsertPos = SuccToSinkTo->begin(); 267 while (InsertPos != SuccToSinkTo->end() && isa<PHINode>(InsertPos)) 268 ++InsertPos; 269 270 // Move the instruction. 271 Inst->moveBefore(InsertPos); 272 return true; 273 } 274
