Sink.cpp revision 90c579de5a383cee278acc3f7e7b9d0a656e6a35
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" 26using namespace llvm; 27 28STATISTIC(NumSunk, "Number of instructions sunk"); 29 30namespace { 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 40 virtual bool runOnFunction(Function &F); 41 42 virtual void getAnalysisUsage(AnalysisUsage &AU) const { 43 AU.setPreservesCFG(); 44 FunctionPass::getAnalysisUsage(AU); 45 AU.addRequired<AliasAnalysis>(); 46 AU.addRequired<DominatorTree>(); 47 AU.addRequired<LoopInfo>(); 48 AU.addPreserved<DominatorTree>(); 49 AU.addPreserved<LoopInfo>(); 50 } 51 private: 52 bool ProcessBlock(BasicBlock &BB); 53 bool SinkInstruction(Instruction *I, SmallPtrSet<Instruction *, 8> &Stores); 54 bool AllUsesDominatedByBlock(Instruction *Inst, BasicBlock *BB) const; 55 }; 56} // end anonymous namespace 57 58char Sinking::ID = 0; 59INITIALIZE_PASS(Sinking, "sink", "Code sinking", false, false); 60 61FunctionPass *llvm::createSinkingPass() { return new Sinking(); } 62 63/// AllUsesDominatedByBlock - Return true if all uses of the specified value 64/// occur in blocks dominated by the specified block. 65bool Sinking::AllUsesDominatedByBlock(Instruction *Inst, 66 BasicBlock *BB) const { 67 // Ignoring debug uses is necessary so debug info doesn't affect the code. 68 // This may leave a referencing dbg_value in the original block, before 69 // the definition of the vreg. Dwarf generator handles this although the 70 // user might not get the right info at runtime. 71 for (Value::use_iterator I = Inst->use_begin(), 72 E = Inst->use_end(); I != E; ++I) { 73 // Determine the block of the use. 74 Instruction *UseInst = cast<Instruction>(*I); 75 BasicBlock *UseBlock = UseInst->getParent(); 76 if (PHINode *PN = dyn_cast<PHINode>(UseInst)) { 77 // PHI nodes use the operand in the predecessor block, not the block with 78 // the PHI. 79 unsigned Num = PHINode::getIncomingValueNumForOperand(I.getOperandNo()); 80 UseBlock = PN->getIncomingBlock(Num); 81 } 82 // Check that it dominates. 83 if (!DT->dominates(BB, UseBlock)) 84 return false; 85 } 86 return true; 87} 88 89bool Sinking::runOnFunction(Function &F) { 90 DT = &getAnalysis<DominatorTree>(); 91 LI = &getAnalysis<LoopInfo>(); 92 AA = &getAnalysis<AliasAnalysis>(); 93 94 bool EverMadeChange = false; 95 96 while (1) { 97 bool MadeChange = false; 98 99 // Process all basic blocks. 100 for (Function::iterator I = F.begin(), E = F.end(); 101 I != E; ++I) 102 MadeChange |= ProcessBlock(*I); 103 104 // If this iteration over the code changed anything, keep iterating. 105 if (!MadeChange) break; 106 EverMadeChange = true; 107 } 108 return EverMadeChange; 109} 110 111bool Sinking::ProcessBlock(BasicBlock &BB) { 112 // Can't sink anything out of a block that has less than two successors. 113 if (BB.getTerminator()->getNumSuccessors() <= 1 || BB.empty()) return false; 114 115 // Don't bother sinking code out of unreachable blocks. In addition to being 116 // unprofitable, it can also lead to infinite looping, because in an unreachable 117 // loop there may be nowhere to stop. 118 if (!DT->isReachableFromEntry(&BB)) return false; 119 120 bool MadeChange = false; 121 122 // Walk the basic block bottom-up. Remember if we saw a store. 123 BasicBlock::iterator I = BB.end(); 124 --I; 125 bool ProcessedBegin = false; 126 SmallPtrSet<Instruction *, 8> Stores; 127 do { 128 Instruction *Inst = I; // The instruction to sink. 129 130 // Predecrement I (if it's not begin) so that it isn't invalidated by 131 // sinking. 132 ProcessedBegin = I == BB.begin(); 133 if (!ProcessedBegin) 134 --I; 135 136 if (isa<DbgInfoIntrinsic>(Inst)) 137 continue; 138 139 if (SinkInstruction(Inst, Stores)) 140 ++NumSunk, MadeChange = true; 141 142 // If we just processed the first instruction in the block, we're done. 143 } while (!ProcessedBegin); 144 145 return MadeChange; 146} 147 148static bool isSafeToMove(Instruction *Inst, AliasAnalysis *AA, 149 SmallPtrSet<Instruction *, 8> &Stores) { 150 if (LoadInst *L = dyn_cast<LoadInst>(Inst)) { 151 if (L->isVolatile()) return false; 152 153 Value *Ptr = L->getPointerOperand(); 154 unsigned Size = AA->getTypeStoreSize(L->getType()); 155 for (SmallPtrSet<Instruction *, 8>::iterator I = Stores.begin(), 156 E = Stores.end(); I != E; ++I) 157 if (AA->getModRefInfo(*I, Ptr, Size) & AliasAnalysis::Mod) 158 return false; 159 } 160 161 if (Inst->mayWriteToMemory()) { 162 Stores.insert(Inst); 163 return false; 164 } 165 166 return Inst->isSafeToSpeculativelyExecute(); 167} 168 169/// SinkInstruction - Determine whether it is safe to sink the specified machine 170/// instruction out of its current block into a successor. 171bool Sinking::SinkInstruction(Instruction *Inst, 172 SmallPtrSet<Instruction *, 8> &Stores) { 173 // Check if it's safe to move the instruction. 174 if (!isSafeToMove(Inst, AA, Stores)) 175 return false; 176 177 // FIXME: This should include support for sinking instructions within the 178 // block they are currently in to shorten the live ranges. We often get 179 // instructions sunk into the top of a large block, but it would be better to 180 // also sink them down before their first use in the block. This xform has to 181 // be careful not to *increase* register pressure though, e.g. sinking 182 // "x = y + z" down if it kills y and z would increase the live ranges of y 183 // and z and only shrink the live range of x. 184 185 // Loop over all the operands of the specified instruction. If there is 186 // anything we can't handle, bail out. 187 BasicBlock *ParentBlock = Inst->getParent(); 188 189 // SuccToSinkTo - This is the successor to sink this instruction to, once we 190 // decide. 191 BasicBlock *SuccToSinkTo = 0; 192 193 // FIXME: This picks a successor to sink into based on having one 194 // successor that dominates all the uses. However, there are cases where 195 // sinking can happen but where the sink point isn't a successor. For 196 // example: 197 // x = computation 198 // if () {} else {} 199 // use x 200 // the instruction could be sunk over the whole diamond for the 201 // if/then/else (or loop, etc), allowing it to be sunk into other blocks 202 // after that. 203 204 // Instructions can only be sunk if all their uses are in blocks 205 // dominated by one of the successors. 206 // Look at all the successors and decide which one 207 // we should sink to. 208 for (succ_iterator SI = succ_begin(ParentBlock), 209 E = succ_end(ParentBlock); SI != E; ++SI) { 210 if (AllUsesDominatedByBlock(Inst, *SI)) { 211 SuccToSinkTo = *SI; 212 break; 213 } 214 } 215 216 // If we couldn't find a block to sink to, ignore this instruction. 217 if (SuccToSinkTo == 0) 218 return false; 219 220 // It is not possible to sink an instruction into its own block. This can 221 // happen with loops. 222 if (Inst->getParent() == SuccToSinkTo) 223 return false; 224 225 DEBUG(dbgs() << "Sink instr " << *Inst); 226 DEBUG(dbgs() << "to block "; 227 WriteAsOperand(dbgs(), SuccToSinkTo, false)); 228 229 // If the block has multiple predecessors, this would introduce computation on 230 // a path that it doesn't already exist. We could split the critical edge, 231 // but for now we just punt. 232 // FIXME: Split critical edges if not backedges. 233 if (SuccToSinkTo->getUniquePredecessor() != ParentBlock) { 234 // We cannot sink a load across a critical edge - there may be stores in 235 // other code paths. 236 if (!Inst->isSafeToSpeculativelyExecute()) { 237 DEBUG(dbgs() << " *** PUNTING: Wont sink load along critical edge.\n"); 238 return false; 239 } 240 241 // We don't want to sink across a critical edge if we don't dominate the 242 // successor. We could be introducing calculations to new code paths. 243 if (!DT->dominates(ParentBlock, SuccToSinkTo)) { 244 DEBUG(dbgs() << " *** PUNTING: Critical edge found\n"); 245 return false; 246 } 247 248 // Don't sink instructions into a loop. 249 if (LI->isLoopHeader(SuccToSinkTo)) { 250 DEBUG(dbgs() << " *** PUNTING: Loop header found\n"); 251 return false; 252 } 253 254 // Otherwise we are OK with sinking along a critical edge. 255 DEBUG(dbgs() << "Sinking along critical edge.\n"); 256 } 257 258 // Determine where to insert into. Skip phi nodes. 259 BasicBlock::iterator InsertPos = SuccToSinkTo->begin(); 260 while (InsertPos != SuccToSinkTo->end() && isa<PHINode>(InsertPos)) 261 ++InsertPos; 262 263 // Move the instruction. 264 Inst->moveBefore(InsertPos); 265 return true; 266} 267