Sink.cpp revision dce4a407a24b04eebc6a376f8e62b41aaa7b071f
15821806d5e7f356e8fa4b058a389a808ea183019Torne (Richard Coles)//===-- Sink.cpp - Code Sinking -------------------------------------------===// 25821806d5e7f356e8fa4b058a389a808ea183019Torne (Richard Coles)// 35821806d5e7f356e8fa4b058a389a808ea183019Torne (Richard Coles)// The LLVM Compiler Infrastructure 45821806d5e7f356e8fa4b058a389a808ea183019Torne (Richard Coles)// 55821806d5e7f356e8fa4b058a389a808ea183019Torne (Richard Coles)// This file is distributed under the University of Illinois Open Source 65821806d5e7f356e8fa4b058a389a808ea183019Torne (Richard Coles)// License. See LICENSE.TXT for details. 75821806d5e7f356e8fa4b058a389a808ea183019Torne (Richard Coles)// 85821806d5e7f356e8fa4b058a389a808ea183019Torne (Richard Coles)//===----------------------------------------------------------------------===// 95821806d5e7f356e8fa4b058a389a808ea183019Torne (Richard Coles)// 105821806d5e7f356e8fa4b058a389a808ea183019Torne (Richard Coles)// This pass moves instructions into successor blocks, when possible, so that 115821806d5e7f356e8fa4b058a389a808ea183019Torne (Richard Coles)// they aren't executed on paths where their results aren't needed. 125821806d5e7f356e8fa4b058a389a808ea183019Torne (Richard Coles)// 135821806d5e7f356e8fa4b058a389a808ea183019Torne (Richard Coles)//===----------------------------------------------------------------------===// 145821806d5e7f356e8fa4b058a389a808ea183019Torne (Richard Coles) 155821806d5e7f356e8fa4b058a389a808ea183019Torne (Richard Coles)#include "llvm/Transforms/Scalar.h" 165821806d5e7f356e8fa4b058a389a808ea183019Torne (Richard Coles)#include "llvm/ADT/Statistic.h" 175821806d5e7f356e8fa4b058a389a808ea183019Torne (Richard Coles)#include "llvm/Analysis/AliasAnalysis.h" 185821806d5e7f356e8fa4b058a389a808ea183019Torne (Richard Coles)#include "llvm/Analysis/LoopInfo.h" 195821806d5e7f356e8fa4b058a389a808ea183019Torne (Richard Coles)#include "llvm/Analysis/ValueTracking.h" 205821806d5e7f356e8fa4b058a389a808ea183019Torne (Richard Coles)#include "llvm/IR/CFG.h" 215821806d5e7f356e8fa4b058a389a808ea183019Torne (Richard Coles)#include "llvm/IR/Dominators.h" 225821806d5e7f356e8fa4b058a389a808ea183019Torne (Richard Coles)#include "llvm/IR/IntrinsicInst.h" 235821806d5e7f356e8fa4b058a389a808ea183019Torne (Richard Coles)#include "llvm/Support/Debug.h" 245821806d5e7f356e8fa4b058a389a808ea183019Torne (Richard Coles)#include "llvm/Support/raw_ostream.h" 255821806d5e7f356e8fa4b058a389a808ea183019Torne (Richard Coles)using namespace llvm; 265821806d5e7f356e8fa4b058a389a808ea183019Torne (Richard Coles) 27#define DEBUG_TYPE "sink" 28 29STATISTIC(NumSunk, "Number of instructions sunk"); 30STATISTIC(NumSinkIter, "Number of sinking iterations"); 31 32namespace { 33 class Sinking : public FunctionPass { 34 DominatorTree *DT; 35 LoopInfo *LI; 36 AliasAnalysis *AA; 37 38 public: 39 static char ID; // Pass identification 40 Sinking() : FunctionPass(ID) { 41 initializeSinkingPass(*PassRegistry::getPassRegistry()); 42 } 43 44 bool runOnFunction(Function &F) override; 45 46 void getAnalysisUsage(AnalysisUsage &AU) const override { 47 AU.setPreservesCFG(); 48 FunctionPass::getAnalysisUsage(AU); 49 AU.addRequired<AliasAnalysis>(); 50 AU.addRequired<DominatorTreeWrapperPass>(); 51 AU.addRequired<LoopInfo>(); 52 AU.addPreserved<DominatorTreeWrapperPass>(); 53 AU.addPreserved<LoopInfo>(); 54 } 55 private: 56 bool ProcessBlock(BasicBlock &BB); 57 bool SinkInstruction(Instruction *I, SmallPtrSet<Instruction *, 8> &Stores); 58 bool AllUsesDominatedByBlock(Instruction *Inst, BasicBlock *BB) const; 59 bool IsAcceptableTarget(Instruction *Inst, BasicBlock *SuccToSinkTo) const; 60 }; 61} // end anonymous namespace 62 63char Sinking::ID = 0; 64INITIALIZE_PASS_BEGIN(Sinking, "sink", "Code sinking", false, false) 65INITIALIZE_PASS_DEPENDENCY(LoopInfo) 66INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass) 67INITIALIZE_AG_DEPENDENCY(AliasAnalysis) 68INITIALIZE_PASS_END(Sinking, "sink", "Code sinking", false, false) 69 70FunctionPass *llvm::createSinkingPass() { return new Sinking(); } 71 72/// AllUsesDominatedByBlock - Return true if all uses of the specified value 73/// occur in blocks dominated by the specified block. 74bool Sinking::AllUsesDominatedByBlock(Instruction *Inst, 75 BasicBlock *BB) const { 76 // Ignoring debug uses is necessary so debug info doesn't affect the code. 77 // This may leave a referencing dbg_value in the original block, before 78 // the definition of the vreg. Dwarf generator handles this although the 79 // user might not get the right info at runtime. 80 for (Use &U : Inst->uses()) { 81 // Determine the block of the use. 82 Instruction *UseInst = cast<Instruction>(U.getUser()); 83 BasicBlock *UseBlock = UseInst->getParent(); 84 if (PHINode *PN = dyn_cast<PHINode>(UseInst)) { 85 // PHI nodes use the operand in the predecessor block, not the block with 86 // the PHI. 87 unsigned Num = PHINode::getIncomingValueNumForOperand(U.getOperandNo()); 88 UseBlock = PN->getIncomingBlock(Num); 89 } 90 // Check that it dominates. 91 if (!DT->dominates(BB, UseBlock)) 92 return false; 93 } 94 return true; 95} 96 97bool Sinking::runOnFunction(Function &F) { 98 DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree(); 99 LI = &getAnalysis<LoopInfo>(); 100 AA = &getAnalysis<AliasAnalysis>(); 101 102 bool MadeChange, EverMadeChange = false; 103 104 do { 105 MadeChange = false; 106 DEBUG(dbgs() << "Sinking iteration " << NumSinkIter << "\n"); 107 // Process all basic blocks. 108 for (Function::iterator I = F.begin(), E = F.end(); 109 I != E; ++I) 110 MadeChange |= ProcessBlock(*I); 111 EverMadeChange |= MadeChange; 112 NumSinkIter++; 113 } while (MadeChange); 114 115 return EverMadeChange; 116} 117 118bool Sinking::ProcessBlock(BasicBlock &BB) { 119 // Can't sink anything out of a block that has less than two successors. 120 if (BB.getTerminator()->getNumSuccessors() <= 1 || BB.empty()) return false; 121 122 // Don't bother sinking code out of unreachable blocks. In addition to being 123 // unprofitable, it can also lead to infinite looping, because in an 124 // unreachable loop there may be nowhere to stop. 125 if (!DT->isReachableFromEntry(&BB)) return false; 126 127 bool MadeChange = false; 128 129 // Walk the basic block bottom-up. Remember if we saw a store. 130 BasicBlock::iterator I = BB.end(); 131 --I; 132 bool ProcessedBegin = false; 133 SmallPtrSet<Instruction *, 8> Stores; 134 do { 135 Instruction *Inst = I; // The instruction to sink. 136 137 // Predecrement I (if it's not begin) so that it isn't invalidated by 138 // sinking. 139 ProcessedBegin = I == BB.begin(); 140 if (!ProcessedBegin) 141 --I; 142 143 if (isa<DbgInfoIntrinsic>(Inst)) 144 continue; 145 146 if (SinkInstruction(Inst, Stores)) 147 ++NumSunk, MadeChange = true; 148 149 // If we just processed the first instruction in the block, we're done. 150 } while (!ProcessedBegin); 151 152 return MadeChange; 153} 154 155static bool isSafeToMove(Instruction *Inst, AliasAnalysis *AA, 156 SmallPtrSet<Instruction *, 8> &Stores) { 157 158 if (Inst->mayWriteToMemory()) { 159 Stores.insert(Inst); 160 return false; 161 } 162 163 if (LoadInst *L = dyn_cast<LoadInst>(Inst)) { 164 AliasAnalysis::Location Loc = AA->getLocation(L); 165 for (SmallPtrSet<Instruction *, 8>::iterator I = Stores.begin(), 166 E = Stores.end(); I != E; ++I) 167 if (AA->getModRefInfo(*I, Loc) & AliasAnalysis::Mod) 168 return false; 169 } 170 171 if (isa<TerminatorInst>(Inst) || isa<PHINode>(Inst)) 172 return false; 173 174 return true; 175} 176 177/// IsAcceptableTarget - Return true if it is possible to sink the instruction 178/// in the specified basic block. 179bool Sinking::IsAcceptableTarget(Instruction *Inst, 180 BasicBlock *SuccToSinkTo) const { 181 assert(Inst && "Instruction to be sunk is null"); 182 assert(SuccToSinkTo && "Candidate sink target is null"); 183 184 // It is not possible to sink an instruction into its own block. This can 185 // happen with loops. 186 if (Inst->getParent() == SuccToSinkTo) 187 return false; 188 189 // If the block has multiple predecessors, this would introduce computation 190 // on different code paths. We could split the critical edge, but for now we 191 // just punt. 192 // FIXME: Split critical edges if not backedges. 193 if (SuccToSinkTo->getUniquePredecessor() != Inst->getParent()) { 194 // We cannot sink a load across a critical edge - there may be stores in 195 // other code paths. 196 if (!isSafeToSpeculativelyExecute(Inst)) 197 return false; 198 199 // We don't want to sink across a critical edge if we don't dominate the 200 // successor. We could be introducing calculations to new code paths. 201 if (!DT->dominates(Inst->getParent(), SuccToSinkTo)) 202 return false; 203 204 // Don't sink instructions into a loop. 205 Loop *succ = LI->getLoopFor(SuccToSinkTo); 206 Loop *cur = LI->getLoopFor(Inst->getParent()); 207 if (succ != nullptr && succ != cur) 208 return false; 209 } 210 211 // Finally, check that all the uses of the instruction are actually 212 // dominated by the candidate 213 return AllUsesDominatedByBlock(Inst, SuccToSinkTo); 214} 215 216/// SinkInstruction - Determine whether it is safe to sink the specified machine 217/// instruction out of its current block into a successor. 218bool Sinking::SinkInstruction(Instruction *Inst, 219 SmallPtrSet<Instruction *, 8> &Stores) { 220 221 // Don't sink static alloca instructions. CodeGen assumes allocas outside the 222 // entry block are dynamically sized stack objects. 223 if (AllocaInst *AI = dyn_cast<AllocaInst>(Inst)) 224 if (AI->isStaticAlloca()) 225 return false; 226 227 // Check if it's safe to move the instruction. 228 if (!isSafeToMove(Inst, AA, Stores)) 229 return false; 230 231 // FIXME: This should include support for sinking instructions within the 232 // block they are currently in to shorten the live ranges. We often get 233 // instructions sunk into the top of a large block, but it would be better to 234 // also sink them down before their first use in the block. This xform has to 235 // be careful not to *increase* register pressure though, e.g. sinking 236 // "x = y + z" down if it kills y and z would increase the live ranges of y 237 // and z and only shrink the live range of x. 238 239 // SuccToSinkTo - This is the successor to sink this instruction to, once we 240 // decide. 241 BasicBlock *SuccToSinkTo = nullptr; 242 243 // Instructions can only be sunk if all their uses are in blocks 244 // dominated by one of the successors. 245 // Look at all the postdominators and see if we can sink it in one. 246 DomTreeNode *DTN = DT->getNode(Inst->getParent()); 247 for (DomTreeNode::iterator I = DTN->begin(), E = DTN->end(); 248 I != E && SuccToSinkTo == nullptr; ++I) { 249 BasicBlock *Candidate = (*I)->getBlock(); 250 if ((*I)->getIDom()->getBlock() == Inst->getParent() && 251 IsAcceptableTarget(Inst, Candidate)) 252 SuccToSinkTo = Candidate; 253 } 254 255 // If no suitable postdominator was found, look at all the successors and 256 // decide which one we should sink to, if any. 257 for (succ_iterator I = succ_begin(Inst->getParent()), 258 E = succ_end(Inst->getParent()); I != E && !SuccToSinkTo; ++I) { 259 if (IsAcceptableTarget(Inst, *I)) 260 SuccToSinkTo = *I; 261 } 262 263 // If we couldn't find a block to sink to, ignore this instruction. 264 if (!SuccToSinkTo) 265 return false; 266 267 DEBUG(dbgs() << "Sink" << *Inst << " ("; 268 Inst->getParent()->printAsOperand(dbgs(), false); 269 dbgs() << " -> "; 270 SuccToSinkTo->printAsOperand(dbgs(), false); 271 dbgs() << ")\n"); 272 273 // Move the instruction. 274 Inst->moveBefore(SuccToSinkTo->getFirstInsertionPt()); 275 return true; 276} 277