EarlyCSE.cpp revision ef87fc2e0a19c3da55c596244e71e2ad03ae4b43
1//===- EarlyCSE.cpp - Simple and fast CSE pass ----------------------------===// 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 performs a simple dominator tree walk that eliminates trivially 11// redundant instructions. 12// 13//===----------------------------------------------------------------------===// 14 15#define DEBUG_TYPE "early-cse" 16#include "llvm/Transforms/Scalar.h" 17#include "llvm/Instructions.h" 18#include "llvm/Pass.h" 19#include "llvm/Analysis/Dominators.h" 20#include "llvm/Analysis/InstructionSimplify.h" 21#include "llvm/Target/TargetData.h" 22#include "llvm/Transforms/Utils/Local.h" 23#include "llvm/Support/Debug.h" 24#include "llvm/Support/RecyclingAllocator.h" 25#include "llvm/ADT/ScopedHashTable.h" 26#include "llvm/ADT/Statistic.h" 27using namespace llvm; 28 29STATISTIC(NumSimplify, "Number of instructions simplified or DCE'd"); 30STATISTIC(NumCSE, "Number of instructions CSE'd"); 31STATISTIC(NumCSELoad, "Number of load instructions CSE'd"); 32STATISTIC(NumCSECall, "Number of call instructions CSE'd"); 33 34static unsigned getHash(const void *V) { 35 return DenseMapInfo<const void*>::getHashValue(V); 36} 37 38//===----------------------------------------------------------------------===// 39// SimpleValue 40//===----------------------------------------------------------------------===// 41 42namespace { 43 /// SimpleValue - Instances of this struct represent available values in the 44 /// scoped hash table. 45 struct SimpleValue { 46 Instruction *Inst; 47 48 SimpleValue(Instruction *I) : Inst(I) { 49 assert((isSentinel() || canHandle(I)) && "Inst can't be handled!"); 50 } 51 52 bool isSentinel() const { 53 return Inst == DenseMapInfo<Instruction*>::getEmptyKey() || 54 Inst == DenseMapInfo<Instruction*>::getTombstoneKey(); 55 } 56 57 static bool canHandle(Instruction *Inst) { 58 return isa<CastInst>(Inst) || isa<BinaryOperator>(Inst) || 59 isa<GetElementPtrInst>(Inst) || isa<CmpInst>(Inst) || 60 isa<SelectInst>(Inst) || isa<ExtractElementInst>(Inst) || 61 isa<InsertElementInst>(Inst) || isa<ShuffleVectorInst>(Inst) || 62 isa<ExtractValueInst>(Inst) || isa<InsertValueInst>(Inst); 63 } 64 }; 65} 66 67namespace llvm { 68// SimpleValue is POD. 69template<> struct isPodLike<SimpleValue> { 70 static const bool value = true; 71}; 72 73template<> struct DenseMapInfo<SimpleValue> { 74 static inline SimpleValue getEmptyKey() { 75 return DenseMapInfo<Instruction*>::getEmptyKey(); 76 } 77 static inline SimpleValue getTombstoneKey() { 78 return DenseMapInfo<Instruction*>::getTombstoneKey(); 79 } 80 static unsigned getHashValue(SimpleValue Val); 81 static bool isEqual(SimpleValue LHS, SimpleValue RHS); 82}; 83} 84 85unsigned DenseMapInfo<SimpleValue>::getHashValue(SimpleValue Val) { 86 Instruction *Inst = Val.Inst; 87 88 // Hash in all of the operands as pointers. 89 unsigned Res = 0; 90 for (unsigned i = 0, e = Inst->getNumOperands(); i != e; ++i) 91 Res ^= getHash(Inst->getOperand(i)) << i; 92 93 if (CastInst *CI = dyn_cast<CastInst>(Inst)) 94 Res ^= getHash(CI->getType()); 95 else if (CmpInst *CI = dyn_cast<CmpInst>(Inst)) 96 Res ^= CI->getPredicate(); 97 else if (const ExtractValueInst *EVI = dyn_cast<ExtractValueInst>(Inst)) { 98 for (ExtractValueInst::idx_iterator I = EVI->idx_begin(), 99 E = EVI->idx_end(); I != E; ++I) 100 Res ^= *I; 101 } else if (const InsertValueInst *IVI = dyn_cast<InsertValueInst>(Inst)) { 102 for (InsertValueInst::idx_iterator I = IVI->idx_begin(), 103 E = IVI->idx_end(); I != E; ++I) 104 Res ^= *I; 105 } else { 106 // nothing extra to hash in. 107 assert((isa<BinaryOperator>(Inst) || isa<GetElementPtrInst>(Inst) || 108 isa<SelectInst>(Inst) || isa<ExtractElementInst>(Inst) || 109 isa<InsertElementInst>(Inst) || isa<ShuffleVectorInst>(Inst)) && 110 "Invalid/unknown instruction"); 111 } 112 113 // Mix in the opcode. 114 return (Res << 1) ^ Inst->getOpcode(); 115} 116 117bool DenseMapInfo<SimpleValue>::isEqual(SimpleValue LHS, SimpleValue RHS) { 118 Instruction *LHSI = LHS.Inst, *RHSI = RHS.Inst; 119 120 if (LHS.isSentinel() || RHS.isSentinel()) 121 return LHSI == RHSI; 122 123 if (LHSI->getOpcode() != RHSI->getOpcode()) return false; 124 return LHSI->isIdenticalTo(RHSI); 125} 126 127//===----------------------------------------------------------------------===// 128// CallValue 129//===----------------------------------------------------------------------===// 130 131namespace { 132 /// CallValue - Instances of this struct represent available call values in 133 /// the scoped hash table. 134 struct CallValue { 135 Instruction *Inst; 136 137 CallValue(Instruction *I) : Inst(I) { 138 assert((isSentinel() || canHandle(I)) && "Inst can't be handled!"); 139 } 140 141 bool isSentinel() const { 142 return Inst == DenseMapInfo<Instruction*>::getEmptyKey() || 143 Inst == DenseMapInfo<Instruction*>::getTombstoneKey(); 144 } 145 146 static bool canHandle(Instruction *Inst) { 147 if (CallInst *CI = dyn_cast<CallInst>(Inst)) 148 return CI->onlyReadsMemory(); 149 return false; 150 } 151 }; 152} 153 154namespace llvm { 155 // CallValue is POD. 156 template<> struct isPodLike<CallValue> { 157 static const bool value = true; 158 }; 159 160 template<> struct DenseMapInfo<CallValue> { 161 static inline CallValue getEmptyKey() { 162 return DenseMapInfo<Instruction*>::getEmptyKey(); 163 } 164 static inline CallValue getTombstoneKey() { 165 return DenseMapInfo<Instruction*>::getTombstoneKey(); 166 } 167 static unsigned getHashValue(CallValue Val); 168 static bool isEqual(CallValue LHS, CallValue RHS); 169 }; 170} 171unsigned DenseMapInfo<CallValue>::getHashValue(CallValue Val) { 172 Instruction *Inst = Val.Inst; 173 // Hash in all of the operands as pointers. 174 unsigned Res = 0; 175 for (unsigned i = 0, e = Inst->getNumOperands(); i != e; ++i) 176 Res ^= getHash(Inst->getOperand(i)) << i; 177 // Mix in the opcode. 178 return (Res << 1) ^ Inst->getOpcode(); 179} 180 181bool DenseMapInfo<CallValue>::isEqual(CallValue LHS, CallValue RHS) { 182 Instruction *LHSI = LHS.Inst, *RHSI = RHS.Inst; 183 if (LHS.isSentinel() || RHS.isSentinel()) 184 return LHSI == RHSI; 185 return LHSI->isIdenticalTo(RHSI); 186} 187 188 189//===----------------------------------------------------------------------===// 190// EarlyCSE pass. 191//===----------------------------------------------------------------------===// 192 193namespace { 194 195/// EarlyCSE - This pass does a simple depth-first walk over the dominator 196/// tree, eliminating trivially redundant instructions and using instsimplify 197/// to canonicalize things as it goes. It is intended to be fast and catch 198/// obvious cases so that instcombine and other passes are more effective. It 199/// is expected that a later pass of GVN will catch the interesting/hard 200/// cases. 201class EarlyCSE : public FunctionPass { 202public: 203 const TargetData *TD; 204 DominatorTree *DT; 205 typedef RecyclingAllocator<BumpPtrAllocator, 206 ScopedHashTableVal<SimpleValue, Value*> > AllocatorTy; 207 typedef ScopedHashTable<SimpleValue, Value*, DenseMapInfo<SimpleValue>, 208 AllocatorTy> ScopedHTType; 209 210 /// AvailableValues - This scoped hash table contains the current values of 211 /// all of our simple scalar expressions. As we walk down the domtree, we 212 /// look to see if instructions are in this: if so, we replace them with what 213 /// we find, otherwise we insert them so that dominated values can succeed in 214 /// their lookup. 215 ScopedHTType *AvailableValues; 216 217 /// AvailableLoads - This scoped hash table contains the current values 218 /// of loads. This allows us to get efficient access to dominating loads when 219 /// we have a fully redundant load. In addition to the most recent load, we 220 /// keep track of a generation count of the read, which is compared against 221 /// the current generation count. The current generation count is 222 /// incremented after every possibly writing memory operation, which ensures 223 /// that we only CSE loads with other loads that have no intervening store. 224 typedef ScopedHashTable<Value*, std::pair<Value*, unsigned> > LoadHTType; 225 LoadHTType *AvailableLoads; 226 227 /// AvailableCalls - This scoped hash table contains the current values 228 /// of read-only call values. It uses the same generation count as loads. 229 typedef ScopedHashTable<CallValue, std::pair<Value*, unsigned> > CallHTType; 230 CallHTType *AvailableCalls; 231 232 /// CurrentGeneration - This is the current generation of the memory value. 233 unsigned CurrentGeneration; 234 235 static char ID; 236 explicit EarlyCSE() : FunctionPass(ID) { 237 initializeEarlyCSEPass(*PassRegistry::getPassRegistry()); 238 } 239 240 bool runOnFunction(Function &F); 241 242private: 243 244 bool processNode(DomTreeNode *Node); 245 246 // This transformation requires dominator postdominator info 247 virtual void getAnalysisUsage(AnalysisUsage &AU) const { 248 AU.addRequired<DominatorTree>(); 249 AU.setPreservesCFG(); 250 } 251}; 252} 253 254char EarlyCSE::ID = 0; 255 256// createEarlyCSEPass - The public interface to this file. 257FunctionPass *llvm::createEarlyCSEPass() { 258 return new EarlyCSE(); 259} 260 261INITIALIZE_PASS_BEGIN(EarlyCSE, "early-cse", "Early CSE", false, false) 262INITIALIZE_PASS_DEPENDENCY(DominatorTree) 263INITIALIZE_PASS_END(EarlyCSE, "early-cse", "Early CSE", false, false) 264 265bool EarlyCSE::processNode(DomTreeNode *Node) { 266 // Define a scope in the scoped hash table. When we are done processing this 267 // domtree node and recurse back up to our parent domtree node, this will pop 268 // off all the values we install. 269 ScopedHTType::ScopeTy Scope(*AvailableValues); 270 271 // Define a scope for the load values so that anything we add will get 272 // popped when we recurse back up to our parent domtree node. 273 LoadHTType::ScopeTy LoadScope(*AvailableLoads); 274 275 // Define a scope for the call values so that anything we add will get 276 // popped when we recurse back up to our parent domtree node. 277 CallHTType::ScopeTy CallScope(*AvailableCalls); 278 279 BasicBlock *BB = Node->getBlock(); 280 281 // If this block has a single predecessor, then the predecessor is the parent 282 // of the domtree node and all of the live out memory values are still current 283 // in this block. If this block has multiple predecessors, then they could 284 // have invalidated the live-out memory values of our parent value. For now, 285 // just be conservative and invalidate memory if this block has multiple 286 // predecessors. 287 if (BB->getSinglePredecessor() == 0) 288 ++CurrentGeneration; 289 290 bool Changed = false; 291 292 // See if any instructions in the block can be eliminated. If so, do it. If 293 // not, add them to AvailableValues. 294 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ) { 295 Instruction *Inst = I++; 296 297 // Dead instructions should just be removed. 298 if (isInstructionTriviallyDead(Inst)) { 299 DEBUG(dbgs() << "EarlyCSE DCE: " << *Inst << '\n'); 300 Inst->eraseFromParent(); 301 Changed = true; 302 ++NumSimplify; 303 continue; 304 } 305 306 // If the instruction can be simplified (e.g. X+0 = X) then replace it with 307 // its simpler value. 308 if (Value *V = SimplifyInstruction(Inst, TD, DT)) { 309 DEBUG(dbgs() << "EarlyCSE Simplify: " << *Inst << " to: " << *V << '\n'); 310 Inst->replaceAllUsesWith(V); 311 Inst->eraseFromParent(); 312 Changed = true; 313 ++NumSimplify; 314 continue; 315 } 316 317 // If this is a simple instruction that we can value number, process it. 318 if (SimpleValue::canHandle(Inst)) { 319 // See if the instruction has an available value. If so, use it. 320 if (Value *V = AvailableValues->lookup(Inst)) { 321 DEBUG(dbgs() << "EarlyCSE CSE: " << *Inst << " to: " << *V << '\n'); 322 Inst->replaceAllUsesWith(V); 323 Inst->eraseFromParent(); 324 Changed = true; 325 ++NumCSE; 326 continue; 327 } 328 329 // Otherwise, just remember that this value is available. 330 AvailableValues->insert(Inst, Inst); 331 continue; 332 } 333 334 // If this is a non-volatile load, process it. 335 if (LoadInst *LI = dyn_cast<LoadInst>(Inst)) { 336 // Ignore volatile loads. 337 if (LI->isVolatile()) continue; 338 339 // If we have an available version of this load, and if it is the right 340 // generation, replace this instruction. 341 std::pair<Value*, unsigned> InVal = 342 AvailableLoads->lookup(Inst->getOperand(0)); 343 if (InVal.first != 0 && InVal.second == CurrentGeneration) { 344 DEBUG(dbgs() << "EarlyCSE CSE LOAD: " << *Inst << " to: " 345 << *InVal.first << '\n'); 346 if (!Inst->use_empty()) Inst->replaceAllUsesWith(InVal.first); 347 Inst->eraseFromParent(); 348 Changed = true; 349 ++NumCSELoad; 350 continue; 351 } 352 353 // Otherwise, remember that we have this instruction. 354 AvailableLoads->insert(Inst->getOperand(0), 355 std::pair<Value*, unsigned>(Inst, CurrentGeneration)); 356 continue; 357 } 358 359 // If this is a read-only call, process it. 360 if (CallValue::canHandle(Inst)) { 361 // If we have an available version of this call, and if it is the right 362 // generation, replace this instruction. 363 std::pair<Value*, unsigned> InVal = AvailableCalls->lookup(Inst); 364 if (InVal.first != 0 && InVal.second == CurrentGeneration) { 365 DEBUG(dbgs() << "EarlyCSE CSE CALL: " << *Inst << " to: " 366 << *InVal.first << '\n'); 367 if (!Inst->use_empty()) Inst->replaceAllUsesWith(InVal.first); 368 Inst->eraseFromParent(); 369 Changed = true; 370 ++NumCSECall; 371 continue; 372 } 373 374 // Otherwise, remember that we have this instruction. 375 AvailableCalls->insert(Inst, 376 std::pair<Value*, unsigned>(Inst, CurrentGeneration)); 377 continue; 378 } 379 380 // Okay, this isn't something we can CSE at all. Check to see if it is 381 // something that could modify memory. If so, our available memory values 382 // cannot be used so bump the generation count. 383 if (Inst->mayWriteToMemory()) { 384 ++CurrentGeneration; 385 386 // Okay, we just invalidated anything we knew about loaded values. Try to 387 // salvage *something* by remembering that the stored value is a live 388 // version of the pointer. It is safe to forward from volatile stores to 389 // non-volatile loads, so we don't have to check for volatility of the 390 // store. 391 if (StoreInst *SI = dyn_cast<StoreInst>(Inst)) { 392 AvailableLoads->insert(SI->getPointerOperand(), 393 std::pair<Value*, unsigned>(SI->getValueOperand(), CurrentGeneration)); 394 } 395 } 396 } 397 398 unsigned LiveOutGeneration = CurrentGeneration; 399 for (DomTreeNode::iterator I = Node->begin(), E = Node->end(); I != E; ++I) { 400 Changed |= processNode(*I); 401 // Pop any generation changes off the stack from the recursive walk. 402 CurrentGeneration = LiveOutGeneration; 403 } 404 return Changed; 405} 406 407 408bool EarlyCSE::runOnFunction(Function &F) { 409 TD = getAnalysisIfAvailable<TargetData>(); 410 DT = &getAnalysis<DominatorTree>(); 411 412 // Tables that the pass uses when walking the domtree. 413 ScopedHTType AVTable; 414 AvailableValues = &AVTable; 415 LoadHTType LoadTable; 416 AvailableLoads = &LoadTable; 417 CallHTType CallTable; 418 AvailableCalls = &CallTable; 419 420 CurrentGeneration = 0; 421 return processNode(DT->getRootNode()); 422} 423