EarlyCSE.cpp revision a12ba39a1d4d22c8c2d348f1397d52be58391177
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"); 33STATISTIC(NumDSE, "Number of trivial dead stores removed"); 34 35static unsigned getHash(const void *V) { 36 return DenseMapInfo<const void*>::getHashValue(V); 37} 38 39//===----------------------------------------------------------------------===// 40// SimpleValue 41//===----------------------------------------------------------------------===// 42 43namespace { 44 /// SimpleValue - Instances of this struct represent available values in the 45 /// scoped hash table. 46 struct SimpleValue { 47 Instruction *Inst; 48 49 SimpleValue(Instruction *I) : Inst(I) { 50 assert((isSentinel() || canHandle(I)) && "Inst can't be handled!"); 51 } 52 53 bool isSentinel() const { 54 return Inst == DenseMapInfo<Instruction*>::getEmptyKey() || 55 Inst == DenseMapInfo<Instruction*>::getTombstoneKey(); 56 } 57 58 static bool canHandle(Instruction *Inst) { 59 return isa<CastInst>(Inst) || isa<BinaryOperator>(Inst) || 60 isa<GetElementPtrInst>(Inst) || isa<CmpInst>(Inst) || 61 isa<SelectInst>(Inst) || isa<ExtractElementInst>(Inst) || 62 isa<InsertElementInst>(Inst) || isa<ShuffleVectorInst>(Inst) || 63 isa<ExtractValueInst>(Inst) || isa<InsertValueInst>(Inst); 64 } 65 }; 66} 67 68namespace llvm { 69// SimpleValue is POD. 70template<> struct isPodLike<SimpleValue> { 71 static const bool value = true; 72}; 73 74template<> struct DenseMapInfo<SimpleValue> { 75 static inline SimpleValue getEmptyKey() { 76 return DenseMapInfo<Instruction*>::getEmptyKey(); 77 } 78 static inline SimpleValue getTombstoneKey() { 79 return DenseMapInfo<Instruction*>::getTombstoneKey(); 80 } 81 static unsigned getHashValue(SimpleValue Val); 82 static bool isEqual(SimpleValue LHS, SimpleValue RHS); 83}; 84} 85 86unsigned DenseMapInfo<SimpleValue>::getHashValue(SimpleValue Val) { 87 Instruction *Inst = Val.Inst; 88 89 // Hash in all of the operands as pointers. 90 unsigned Res = 0; 91 for (unsigned i = 0, e = Inst->getNumOperands(); i != e; ++i) 92 Res ^= getHash(Inst->getOperand(i)) << i; 93 94 if (CastInst *CI = dyn_cast<CastInst>(Inst)) 95 Res ^= getHash(CI->getType()); 96 else if (CmpInst *CI = dyn_cast<CmpInst>(Inst)) 97 Res ^= CI->getPredicate(); 98 else if (const ExtractValueInst *EVI = dyn_cast<ExtractValueInst>(Inst)) { 99 for (ExtractValueInst::idx_iterator I = EVI->idx_begin(), 100 E = EVI->idx_end(); I != E; ++I) 101 Res ^= *I; 102 } else if (const InsertValueInst *IVI = dyn_cast<InsertValueInst>(Inst)) { 103 for (InsertValueInst::idx_iterator I = IVI->idx_begin(), 104 E = IVI->idx_end(); I != E; ++I) 105 Res ^= *I; 106 } else { 107 // nothing extra to hash in. 108 assert((isa<BinaryOperator>(Inst) || isa<GetElementPtrInst>(Inst) || 109 isa<SelectInst>(Inst) || isa<ExtractElementInst>(Inst) || 110 isa<InsertElementInst>(Inst) || isa<ShuffleVectorInst>(Inst)) && 111 "Invalid/unknown instruction"); 112 } 113 114 // Mix in the opcode. 115 return (Res << 1) ^ Inst->getOpcode(); 116} 117 118bool DenseMapInfo<SimpleValue>::isEqual(SimpleValue LHS, SimpleValue RHS) { 119 Instruction *LHSI = LHS.Inst, *RHSI = RHS.Inst; 120 121 if (LHS.isSentinel() || RHS.isSentinel()) 122 return LHSI == RHSI; 123 124 if (LHSI->getOpcode() != RHSI->getOpcode()) return false; 125 return LHSI->isIdenticalTo(RHSI); 126} 127 128//===----------------------------------------------------------------------===// 129// CallValue 130//===----------------------------------------------------------------------===// 131 132namespace { 133 /// CallValue - Instances of this struct represent available call values in 134 /// the scoped hash table. 135 struct CallValue { 136 Instruction *Inst; 137 138 CallValue(Instruction *I) : Inst(I) { 139 assert((isSentinel() || canHandle(I)) && "Inst can't be handled!"); 140 } 141 142 bool isSentinel() const { 143 return Inst == DenseMapInfo<Instruction*>::getEmptyKey() || 144 Inst == DenseMapInfo<Instruction*>::getTombstoneKey(); 145 } 146 147 static bool canHandle(Instruction *Inst) { 148 CallInst *CI = dyn_cast<CallInst>(Inst); 149 if (CI == 0 || !CI->onlyReadsMemory()) 150 return false; 151 152 // Check that there are no metadata operands. 153 for (unsigned i = 0, e = CI->getNumOperands(); i != e; ++i) 154 if (CI->getOperand(i)->getType()->isMetadataTy()) 155 return false; 156 return true; 157 } 158 }; 159} 160 161namespace llvm { 162 // CallValue is POD. 163 template<> struct isPodLike<CallValue> { 164 static const bool value = true; 165 }; 166 167 template<> struct DenseMapInfo<CallValue> { 168 static inline CallValue getEmptyKey() { 169 return DenseMapInfo<Instruction*>::getEmptyKey(); 170 } 171 static inline CallValue getTombstoneKey() { 172 return DenseMapInfo<Instruction*>::getTombstoneKey(); 173 } 174 static unsigned getHashValue(CallValue Val); 175 static bool isEqual(CallValue LHS, CallValue RHS); 176 }; 177} 178unsigned DenseMapInfo<CallValue>::getHashValue(CallValue Val) { 179 Instruction *Inst = Val.Inst; 180 // Hash in all of the operands as pointers. 181 unsigned Res = 0; 182 for (unsigned i = 0, e = Inst->getNumOperands(); i != e; ++i) 183 Res ^= getHash(Inst->getOperand(i)) << i; 184 // Mix in the opcode. 185 return (Res << 1) ^ Inst->getOpcode(); 186} 187 188bool DenseMapInfo<CallValue>::isEqual(CallValue LHS, CallValue RHS) { 189 Instruction *LHSI = LHS.Inst, *RHSI = RHS.Inst; 190 if (LHS.isSentinel() || RHS.isSentinel()) 191 return LHSI == RHSI; 192 return LHSI->isIdenticalTo(RHSI); 193} 194 195 196//===----------------------------------------------------------------------===// 197// EarlyCSE pass. 198//===----------------------------------------------------------------------===// 199 200namespace { 201 202/// EarlyCSE - This pass does a simple depth-first walk over the dominator 203/// tree, eliminating trivially redundant instructions and using instsimplify 204/// to canonicalize things as it goes. It is intended to be fast and catch 205/// obvious cases so that instcombine and other passes are more effective. It 206/// is expected that a later pass of GVN will catch the interesting/hard 207/// cases. 208class EarlyCSE : public FunctionPass { 209public: 210 const TargetData *TD; 211 DominatorTree *DT; 212 typedef RecyclingAllocator<BumpPtrAllocator, 213 ScopedHashTableVal<SimpleValue, Value*> > AllocatorTy; 214 typedef ScopedHashTable<SimpleValue, Value*, DenseMapInfo<SimpleValue>, 215 AllocatorTy> ScopedHTType; 216 217 /// AvailableValues - This scoped hash table contains the current values of 218 /// all of our simple scalar expressions. As we walk down the domtree, we 219 /// look to see if instructions are in this: if so, we replace them with what 220 /// we find, otherwise we insert them so that dominated values can succeed in 221 /// their lookup. 222 ScopedHTType *AvailableValues; 223 224 /// AvailableLoads - This scoped hash table contains the current values 225 /// of loads. This allows us to get efficient access to dominating loads when 226 /// we have a fully redundant load. In addition to the most recent load, we 227 /// keep track of a generation count of the read, which is compared against 228 /// the current generation count. The current generation count is 229 /// incremented after every possibly writing memory operation, which ensures 230 /// that we only CSE loads with other loads that have no intervening store. 231 typedef RecyclingAllocator<BumpPtrAllocator, 232 ScopedHashTableVal<Value*, std::pair<Value*, unsigned> > > LoadMapAllocator; 233 typedef ScopedHashTable<Value*, std::pair<Value*, unsigned>, 234 DenseMapInfo<Value*>, LoadMapAllocator> LoadHTType; 235 LoadHTType *AvailableLoads; 236 237 /// AvailableCalls - This scoped hash table contains the current values 238 /// of read-only call values. It uses the same generation count as loads. 239 typedef ScopedHashTable<CallValue, std::pair<Value*, unsigned> > CallHTType; 240 CallHTType *AvailableCalls; 241 242 /// CurrentGeneration - This is the current generation of the memory value. 243 unsigned CurrentGeneration; 244 245 static char ID; 246 explicit EarlyCSE() : FunctionPass(ID) { 247 initializeEarlyCSEPass(*PassRegistry::getPassRegistry()); 248 } 249 250 bool runOnFunction(Function &F); 251 252private: 253 254 bool processNode(DomTreeNode *Node); 255 256 // This transformation requires dominator postdominator info 257 virtual void getAnalysisUsage(AnalysisUsage &AU) const { 258 AU.addRequired<DominatorTree>(); 259 AU.setPreservesCFG(); 260 } 261}; 262} 263 264char EarlyCSE::ID = 0; 265 266// createEarlyCSEPass - The public interface to this file. 267FunctionPass *llvm::createEarlyCSEPass() { 268 return new EarlyCSE(); 269} 270 271INITIALIZE_PASS_BEGIN(EarlyCSE, "early-cse", "Early CSE", false, false) 272INITIALIZE_PASS_DEPENDENCY(DominatorTree) 273INITIALIZE_PASS_END(EarlyCSE, "early-cse", "Early CSE", false, false) 274 275bool EarlyCSE::processNode(DomTreeNode *Node) { 276 // Define a scope in the scoped hash table. When we are done processing this 277 // domtree node and recurse back up to our parent domtree node, this will pop 278 // off all the values we install. 279 ScopedHTType::ScopeTy Scope(*AvailableValues); 280 281 // Define a scope for the load values so that anything we add will get 282 // popped when we recurse back up to our parent domtree node. 283 LoadHTType::ScopeTy LoadScope(*AvailableLoads); 284 285 // Define a scope for the call values so that anything we add will get 286 // popped when we recurse back up to our parent domtree node. 287 CallHTType::ScopeTy CallScope(*AvailableCalls); 288 289 BasicBlock *BB = Node->getBlock(); 290 291 // If this block has a single predecessor, then the predecessor is the parent 292 // of the domtree node and all of the live out memory values are still current 293 // in this block. If this block has multiple predecessors, then they could 294 // have invalidated the live-out memory values of our parent value. For now, 295 // just be conservative and invalidate memory if this block has multiple 296 // predecessors. 297 if (BB->getSinglePredecessor() == 0) 298 ++CurrentGeneration; 299 300 /// LastStore - Keep track of the last non-volatile store that we saw... for 301 /// as long as there in no instruction that reads memory. If we see a store 302 /// to the same location, we delete the dead store. This zaps trivial dead 303 /// stores which can occur in bitfield code among other things. 304 StoreInst *LastStore = 0; 305 306 bool Changed = false; 307 308 // See if any instructions in the block can be eliminated. If so, do it. If 309 // not, add them to AvailableValues. 310 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ) { 311 Instruction *Inst = I++; 312 313 // Dead instructions should just be removed. 314 if (isInstructionTriviallyDead(Inst)) { 315 DEBUG(dbgs() << "EarlyCSE DCE: " << *Inst << '\n'); 316 Inst->eraseFromParent(); 317 Changed = true; 318 ++NumSimplify; 319 continue; 320 } 321 322 // If the instruction can be simplified (e.g. X+0 = X) then replace it with 323 // its simpler value. 324 if (Value *V = SimplifyInstruction(Inst, TD, DT)) { 325 DEBUG(dbgs() << "EarlyCSE Simplify: " << *Inst << " to: " << *V << '\n'); 326 Inst->replaceAllUsesWith(V); 327 Inst->eraseFromParent(); 328 Changed = true; 329 ++NumSimplify; 330 continue; 331 } 332 333 // If this is a simple instruction that we can value number, process it. 334 if (SimpleValue::canHandle(Inst)) { 335 // See if the instruction has an available value. If so, use it. 336 if (Value *V = AvailableValues->lookup(Inst)) { 337 DEBUG(dbgs() << "EarlyCSE CSE: " << *Inst << " to: " << *V << '\n'); 338 Inst->replaceAllUsesWith(V); 339 Inst->eraseFromParent(); 340 Changed = true; 341 ++NumCSE; 342 continue; 343 } 344 345 // Otherwise, just remember that this value is available. 346 AvailableValues->insert(Inst, Inst); 347 continue; 348 } 349 350 // If this is a non-volatile load, process it. 351 if (LoadInst *LI = dyn_cast<LoadInst>(Inst)) { 352 // Ignore volatile loads. 353 if (LI->isVolatile()) { 354 LastStore = 0; 355 continue; 356 } 357 358 // If we have an available version of this load, and if it is the right 359 // generation, replace this instruction. 360 std::pair<Value*, unsigned> InVal = 361 AvailableLoads->lookup(Inst->getOperand(0)); 362 if (InVal.first != 0 && InVal.second == CurrentGeneration) { 363 DEBUG(dbgs() << "EarlyCSE CSE LOAD: " << *Inst << " to: " 364 << *InVal.first << '\n'); 365 if (!Inst->use_empty()) Inst->replaceAllUsesWith(InVal.first); 366 Inst->eraseFromParent(); 367 Changed = true; 368 ++NumCSELoad; 369 continue; 370 } 371 372 // Otherwise, remember that we have this instruction. 373 AvailableLoads->insert(Inst->getOperand(0), 374 std::pair<Value*, unsigned>(Inst, CurrentGeneration)); 375 LastStore = 0; 376 continue; 377 } 378 379 // If this instruction may read from memory, forget LastStore. 380 if (Inst->mayReadFromMemory()) 381 LastStore = 0; 382 383 // If this is a read-only call, process it. 384 if (CallValue::canHandle(Inst)) { 385 // If we have an available version of this call, and if it is the right 386 // generation, replace this instruction. 387 std::pair<Value*, unsigned> InVal = AvailableCalls->lookup(Inst); 388 if (InVal.first != 0 && InVal.second == CurrentGeneration) { 389 DEBUG(dbgs() << "EarlyCSE CSE CALL: " << *Inst << " to: " 390 << *InVal.first << '\n'); 391 if (!Inst->use_empty()) Inst->replaceAllUsesWith(InVal.first); 392 Inst->eraseFromParent(); 393 Changed = true; 394 ++NumCSECall; 395 continue; 396 } 397 398 // Otherwise, remember that we have this instruction. 399 AvailableCalls->insert(Inst, 400 std::pair<Value*, unsigned>(Inst, CurrentGeneration)); 401 continue; 402 } 403 404 // Okay, this isn't something we can CSE at all. Check to see if it is 405 // something that could modify memory. If so, our available memory values 406 // cannot be used so bump the generation count. 407 if (Inst->mayWriteToMemory()) { 408 ++CurrentGeneration; 409 410 if (StoreInst *SI = dyn_cast<StoreInst>(Inst)) { 411 // We do a trivial form of DSE if there are two stores to the same 412 // location with no intervening loads. Delete the earlier store. 413 if (LastStore && 414 LastStore->getPointerOperand() == SI->getPointerOperand()) { 415 DEBUG(dbgs() << "EarlyCSE DEAD STORE: " << *LastStore << " due to: " 416 << *Inst << '\n'); 417 LastStore->eraseFromParent(); 418 Changed = true; 419 ++NumDSE; 420 LastStore = 0; 421 continue; 422 } 423 424 // Okay, we just invalidated anything we knew about loaded values. Try 425 // to salvage *something* by remembering that the stored value is a live 426 // version of the pointer. It is safe to forward from volatile stores 427 // to non-volatile loads, so we don't have to check for volatility of 428 // the store. 429 AvailableLoads->insert(SI->getPointerOperand(), 430 std::pair<Value*, unsigned>(SI->getValueOperand(), CurrentGeneration)); 431 432 // Remember that this was the last store we saw for DSE. 433 if (!SI->isVolatile()) 434 LastStore = SI; 435 } 436 } 437 } 438 439 unsigned LiveOutGeneration = CurrentGeneration; 440 for (DomTreeNode::iterator I = Node->begin(), E = Node->end(); I != E; ++I) { 441 Changed |= processNode(*I); 442 // Pop any generation changes off the stack from the recursive walk. 443 CurrentGeneration = LiveOutGeneration; 444 } 445 return Changed; 446} 447 448 449bool EarlyCSE::runOnFunction(Function &F) { 450 TD = getAnalysisIfAvailable<TargetData>(); 451 DT = &getAnalysis<DominatorTree>(); 452 453 // Tables that the pass uses when walking the domtree. 454 ScopedHTType AVTable; 455 AvailableValues = &AVTable; 456 LoadHTType LoadTable; 457 AvailableLoads = &LoadTable; 458 CallHTType CallTable; 459 AvailableCalls = &CallTable; 460 461 CurrentGeneration = 0; 462 return processNode(DT->getRootNode()); 463} 464