EarlyCSE.cpp revision e508dd4c75705f325764e1197854c0e83266a7ea
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 // This can only handle non-void readnone functions. 60 if (CallInst *CI = dyn_cast<CallInst>(Inst)) 61 return CI->doesNotAccessMemory() && !CI->getType()->isVoidTy(); 62 return isa<CastInst>(Inst) || isa<BinaryOperator>(Inst) || 63 isa<GetElementPtrInst>(Inst) || isa<CmpInst>(Inst) || 64 isa<SelectInst>(Inst) || isa<ExtractElementInst>(Inst) || 65 isa<InsertElementInst>(Inst) || isa<ShuffleVectorInst>(Inst) || 66 isa<ExtractValueInst>(Inst) || isa<InsertValueInst>(Inst); 67 } 68 }; 69} 70 71namespace llvm { 72// SimpleValue is POD. 73template<> struct isPodLike<SimpleValue> { 74 static const bool value = true; 75}; 76 77template<> struct DenseMapInfo<SimpleValue> { 78 static inline SimpleValue getEmptyKey() { 79 return DenseMapInfo<Instruction*>::getEmptyKey(); 80 } 81 static inline SimpleValue getTombstoneKey() { 82 return DenseMapInfo<Instruction*>::getTombstoneKey(); 83 } 84 static unsigned getHashValue(SimpleValue Val); 85 static bool isEqual(SimpleValue LHS, SimpleValue RHS); 86}; 87} 88 89unsigned DenseMapInfo<SimpleValue>::getHashValue(SimpleValue Val) { 90 Instruction *Inst = Val.Inst; 91 92 // Hash in all of the operands as pointers. 93 unsigned Res = 0; 94 for (unsigned i = 0, e = Inst->getNumOperands(); i != e; ++i) 95 Res ^= getHash(Inst->getOperand(i)) << i; 96 97 if (CastInst *CI = dyn_cast<CastInst>(Inst)) 98 Res ^= getHash(CI->getType()); 99 else if (CmpInst *CI = dyn_cast<CmpInst>(Inst)) 100 Res ^= CI->getPredicate(); 101 else if (const ExtractValueInst *EVI = dyn_cast<ExtractValueInst>(Inst)) { 102 for (ExtractValueInst::idx_iterator I = EVI->idx_begin(), 103 E = EVI->idx_end(); I != E; ++I) 104 Res ^= *I; 105 } else if (const InsertValueInst *IVI = dyn_cast<InsertValueInst>(Inst)) { 106 for (InsertValueInst::idx_iterator I = IVI->idx_begin(), 107 E = IVI->idx_end(); I != E; ++I) 108 Res ^= *I; 109 } else { 110 // nothing extra to hash in. 111 assert((isa<CallInst>(Inst) || 112 isa<BinaryOperator>(Inst) || isa<GetElementPtrInst>(Inst) || 113 isa<SelectInst>(Inst) || isa<ExtractElementInst>(Inst) || 114 isa<InsertElementInst>(Inst) || isa<ShuffleVectorInst>(Inst)) && 115 "Invalid/unknown instruction"); 116 } 117 118 // Mix in the opcode. 119 return (Res << 1) ^ Inst->getOpcode(); 120} 121 122bool DenseMapInfo<SimpleValue>::isEqual(SimpleValue LHS, SimpleValue RHS) { 123 Instruction *LHSI = LHS.Inst, *RHSI = RHS.Inst; 124 125 if (LHS.isSentinel() || RHS.isSentinel()) 126 return LHSI == RHSI; 127 128 if (LHSI->getOpcode() != RHSI->getOpcode()) return false; 129 return LHSI->isIdenticalTo(RHSI); 130} 131 132//===----------------------------------------------------------------------===// 133// CallValue 134//===----------------------------------------------------------------------===// 135 136namespace { 137 /// CallValue - Instances of this struct represent available call values in 138 /// the scoped hash table. 139 struct CallValue { 140 Instruction *Inst; 141 142 CallValue(Instruction *I) : Inst(I) { 143 assert((isSentinel() || canHandle(I)) && "Inst can't be handled!"); 144 } 145 146 bool isSentinel() const { 147 return Inst == DenseMapInfo<Instruction*>::getEmptyKey() || 148 Inst == DenseMapInfo<Instruction*>::getTombstoneKey(); 149 } 150 151 static bool canHandle(Instruction *Inst) { 152 // Don't value number anything that returns void. 153 if (Inst->getType()->isVoidTy()) 154 return false; 155 156 CallInst *CI = dyn_cast<CallInst>(Inst); 157 if (CI == 0 || !CI->onlyReadsMemory()) 158 return false; 159 return true; 160 } 161 }; 162} 163 164namespace llvm { 165 // CallValue is POD. 166 template<> struct isPodLike<CallValue> { 167 static const bool value = true; 168 }; 169 170 template<> struct DenseMapInfo<CallValue> { 171 static inline CallValue getEmptyKey() { 172 return DenseMapInfo<Instruction*>::getEmptyKey(); 173 } 174 static inline CallValue getTombstoneKey() { 175 return DenseMapInfo<Instruction*>::getTombstoneKey(); 176 } 177 static unsigned getHashValue(CallValue Val); 178 static bool isEqual(CallValue LHS, CallValue RHS); 179 }; 180} 181unsigned DenseMapInfo<CallValue>::getHashValue(CallValue Val) { 182 Instruction *Inst = Val.Inst; 183 // Hash in all of the operands as pointers. 184 unsigned Res = 0; 185 for (unsigned i = 0, e = Inst->getNumOperands(); i != e; ++i) { 186 assert(!Inst->getOperand(i)->getType()->isMetadataTy() && 187 "Cannot value number calls with metadata operands"); 188 Res ^= getHash(Inst->getOperand(i)) << i; 189 } 190 191 // Mix in the opcode. 192 return (Res << 1) ^ Inst->getOpcode(); 193} 194 195bool DenseMapInfo<CallValue>::isEqual(CallValue LHS, CallValue RHS) { 196 Instruction *LHSI = LHS.Inst, *RHSI = RHS.Inst; 197 if (LHS.isSentinel() || RHS.isSentinel()) 198 return LHSI == RHSI; 199 return LHSI->isIdenticalTo(RHSI); 200} 201 202 203//===----------------------------------------------------------------------===// 204// EarlyCSE pass. 205//===----------------------------------------------------------------------===// 206 207namespace { 208 209/// EarlyCSE - This pass does a simple depth-first walk over the dominator 210/// tree, eliminating trivially redundant instructions and using instsimplify 211/// to canonicalize things as it goes. It is intended to be fast and catch 212/// obvious cases so that instcombine and other passes are more effective. It 213/// is expected that a later pass of GVN will catch the interesting/hard 214/// cases. 215class EarlyCSE : public FunctionPass { 216public: 217 const TargetData *TD; 218 DominatorTree *DT; 219 typedef RecyclingAllocator<BumpPtrAllocator, 220 ScopedHashTableVal<SimpleValue, Value*> > AllocatorTy; 221 typedef ScopedHashTable<SimpleValue, Value*, DenseMapInfo<SimpleValue>, 222 AllocatorTy> ScopedHTType; 223 224 /// AvailableValues - This scoped hash table contains the current values of 225 /// all of our simple scalar expressions. As we walk down the domtree, we 226 /// look to see if instructions are in this: if so, we replace them with what 227 /// we find, otherwise we insert them so that dominated values can succeed in 228 /// their lookup. 229 ScopedHTType *AvailableValues; 230 231 /// AvailableLoads - This scoped hash table contains the current values 232 /// of loads. This allows us to get efficient access to dominating loads when 233 /// we have a fully redundant load. In addition to the most recent load, we 234 /// keep track of a generation count of the read, which is compared against 235 /// the current generation count. The current generation count is 236 /// incremented after every possibly writing memory operation, which ensures 237 /// that we only CSE loads with other loads that have no intervening store. 238 typedef RecyclingAllocator<BumpPtrAllocator, 239 ScopedHashTableVal<Value*, std::pair<Value*, unsigned> > > LoadMapAllocator; 240 typedef ScopedHashTable<Value*, std::pair<Value*, unsigned>, 241 DenseMapInfo<Value*>, LoadMapAllocator> LoadHTType; 242 LoadHTType *AvailableLoads; 243 244 /// AvailableCalls - This scoped hash table contains the current values 245 /// of read-only call values. It uses the same generation count as loads. 246 typedef ScopedHashTable<CallValue, std::pair<Value*, unsigned> > CallHTType; 247 CallHTType *AvailableCalls; 248 249 /// CurrentGeneration - This is the current generation of the memory value. 250 unsigned CurrentGeneration; 251 252 static char ID; 253 explicit EarlyCSE() : FunctionPass(ID) { 254 initializeEarlyCSEPass(*PassRegistry::getPassRegistry()); 255 } 256 257 bool runOnFunction(Function &F); 258 259private: 260 261 bool processNode(DomTreeNode *Node); 262 263 // This transformation requires dominator postdominator info 264 virtual void getAnalysisUsage(AnalysisUsage &AU) const { 265 AU.addRequired<DominatorTree>(); 266 AU.setPreservesCFG(); 267 } 268}; 269} 270 271char EarlyCSE::ID = 0; 272 273// createEarlyCSEPass - The public interface to this file. 274FunctionPass *llvm::createEarlyCSEPass() { 275 return new EarlyCSE(); 276} 277 278INITIALIZE_PASS_BEGIN(EarlyCSE, "early-cse", "Early CSE", false, false) 279INITIALIZE_PASS_DEPENDENCY(DominatorTree) 280INITIALIZE_PASS_END(EarlyCSE, "early-cse", "Early CSE", false, false) 281 282bool EarlyCSE::processNode(DomTreeNode *Node) { 283 // Define a scope in the scoped hash table. When we are done processing this 284 // domtree node and recurse back up to our parent domtree node, this will pop 285 // off all the values we install. 286 ScopedHTType::ScopeTy Scope(*AvailableValues); 287 288 // Define a scope for the load values so that anything we add will get 289 // popped when we recurse back up to our parent domtree node. 290 LoadHTType::ScopeTy LoadScope(*AvailableLoads); 291 292 // Define a scope for the call values so that anything we add will get 293 // popped when we recurse back up to our parent domtree node. 294 CallHTType::ScopeTy CallScope(*AvailableCalls); 295 296 BasicBlock *BB = Node->getBlock(); 297 298 // If this block has a single predecessor, then the predecessor is the parent 299 // of the domtree node and all of the live out memory values are still current 300 // in this block. If this block has multiple predecessors, then they could 301 // have invalidated the live-out memory values of our parent value. For now, 302 // just be conservative and invalidate memory if this block has multiple 303 // predecessors. 304 if (BB->getSinglePredecessor() == 0) 305 ++CurrentGeneration; 306 307 /// LastStore - Keep track of the last non-volatile store that we saw... for 308 /// as long as there in no instruction that reads memory. If we see a store 309 /// to the same location, we delete the dead store. This zaps trivial dead 310 /// stores which can occur in bitfield code among other things. 311 StoreInst *LastStore = 0; 312 313 bool Changed = false; 314 315 // See if any instructions in the block can be eliminated. If so, do it. If 316 // not, add them to AvailableValues. 317 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ) { 318 Instruction *Inst = I++; 319 320 // Dead instructions should just be removed. 321 if (isInstructionTriviallyDead(Inst)) { 322 DEBUG(dbgs() << "EarlyCSE DCE: " << *Inst << '\n'); 323 Inst->eraseFromParent(); 324 Changed = true; 325 ++NumSimplify; 326 continue; 327 } 328 329 // If the instruction can be simplified (e.g. X+0 = X) then replace it with 330 // its simpler value. 331 if (Value *V = SimplifyInstruction(Inst, TD, DT)) { 332 DEBUG(dbgs() << "EarlyCSE Simplify: " << *Inst << " to: " << *V << '\n'); 333 Inst->replaceAllUsesWith(V); 334 Inst->eraseFromParent(); 335 Changed = true; 336 ++NumSimplify; 337 continue; 338 } 339 340 // If this is a simple instruction that we can value number, process it. 341 if (SimpleValue::canHandle(Inst)) { 342 // See if the instruction has an available value. If so, use it. 343 if (Value *V = AvailableValues->lookup(Inst)) { 344 DEBUG(dbgs() << "EarlyCSE CSE: " << *Inst << " to: " << *V << '\n'); 345 Inst->replaceAllUsesWith(V); 346 Inst->eraseFromParent(); 347 Changed = true; 348 ++NumCSE; 349 continue; 350 } 351 352 // Otherwise, just remember that this value is available. 353 AvailableValues->insert(Inst, Inst); 354 continue; 355 } 356 357 // If this is a non-volatile load, process it. 358 if (LoadInst *LI = dyn_cast<LoadInst>(Inst)) { 359 // Ignore volatile loads. 360 if (LI->isVolatile()) { 361 LastStore = 0; 362 continue; 363 } 364 365 // If we have an available version of this load, and if it is the right 366 // generation, replace this instruction. 367 std::pair<Value*, unsigned> InVal = 368 AvailableLoads->lookup(Inst->getOperand(0)); 369 if (InVal.first != 0 && InVal.second == CurrentGeneration) { 370 DEBUG(dbgs() << "EarlyCSE CSE LOAD: " << *Inst << " to: " 371 << *InVal.first << '\n'); 372 if (!Inst->use_empty()) Inst->replaceAllUsesWith(InVal.first); 373 Inst->eraseFromParent(); 374 Changed = true; 375 ++NumCSELoad; 376 continue; 377 } 378 379 // Otherwise, remember that we have this instruction. 380 AvailableLoads->insert(Inst->getOperand(0), 381 std::pair<Value*, unsigned>(Inst, CurrentGeneration)); 382 LastStore = 0; 383 continue; 384 } 385 386 // If this instruction may read from memory, forget LastStore. 387 if (Inst->mayReadFromMemory()) 388 LastStore = 0; 389 390 // If this is a read-only call, process it. 391 if (CallValue::canHandle(Inst)) { 392 // If we have an available version of this call, and if it is the right 393 // generation, replace this instruction. 394 std::pair<Value*, unsigned> InVal = AvailableCalls->lookup(Inst); 395 if (InVal.first != 0 && InVal.second == CurrentGeneration) { 396 DEBUG(dbgs() << "EarlyCSE CSE CALL: " << *Inst << " to: " 397 << *InVal.first << '\n'); 398 if (!Inst->use_empty()) Inst->replaceAllUsesWith(InVal.first); 399 Inst->eraseFromParent(); 400 Changed = true; 401 ++NumCSECall; 402 continue; 403 } 404 405 // Otherwise, remember that we have this instruction. 406 AvailableCalls->insert(Inst, 407 std::pair<Value*, unsigned>(Inst, CurrentGeneration)); 408 continue; 409 } 410 411 // Okay, this isn't something we can CSE at all. Check to see if it is 412 // something that could modify memory. If so, our available memory values 413 // cannot be used so bump the generation count. 414 if (Inst->mayWriteToMemory()) { 415 ++CurrentGeneration; 416 417 if (StoreInst *SI = dyn_cast<StoreInst>(Inst)) { 418 // We do a trivial form of DSE if there are two stores to the same 419 // location with no intervening loads. Delete the earlier store. 420 if (LastStore && 421 LastStore->getPointerOperand() == SI->getPointerOperand()) { 422 DEBUG(dbgs() << "EarlyCSE DEAD STORE: " << *LastStore << " due to: " 423 << *Inst << '\n'); 424 LastStore->eraseFromParent(); 425 Changed = true; 426 ++NumDSE; 427 LastStore = 0; 428 continue; 429 } 430 431 // Okay, we just invalidated anything we knew about loaded values. Try 432 // to salvage *something* by remembering that the stored value is a live 433 // version of the pointer. It is safe to forward from volatile stores 434 // to non-volatile loads, so we don't have to check for volatility of 435 // the store. 436 AvailableLoads->insert(SI->getPointerOperand(), 437 std::pair<Value*, unsigned>(SI->getValueOperand(), CurrentGeneration)); 438 439 // Remember that this was the last store we saw for DSE. 440 if (!SI->isVolatile()) 441 LastStore = SI; 442 } 443 } 444 } 445 446 unsigned LiveOutGeneration = CurrentGeneration; 447 for (DomTreeNode::iterator I = Node->begin(), E = Node->end(); I != E; ++I) { 448 Changed |= processNode(*I); 449 // Pop any generation changes off the stack from the recursive walk. 450 CurrentGeneration = LiveOutGeneration; 451 } 452 return Changed; 453} 454 455 456bool EarlyCSE::runOnFunction(Function &F) { 457 TD = getAnalysisIfAvailable<TargetData>(); 458 DT = &getAnalysis<DominatorTree>(); 459 460 // Tables that the pass uses when walking the domtree. 461 ScopedHTType AVTable; 462 AvailableValues = &AVTable; 463 LoadHTType LoadTable; 464 AvailableLoads = &LoadTable; 465 CallHTType CallTable; 466 AvailableCalls = &CallTable; 467 468 CurrentGeneration = 0; 469 return processNode(DT->getRootNode()); 470} 471