EarlyCSE.cpp revision 10b883b13ff3ad44892758c7a57a435cdb379b45
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 // Don't value number anything that returns void. 149 if (Inst->getType()->isVoidTy()) 150 return false; 151 152 CallInst *CI = dyn_cast<CallInst>(Inst); 153 if (CI == 0 || !CI->onlyReadsMemory()) 154 return false; 155 return true; 156 } 157 }; 158} 159 160namespace llvm { 161 // CallValue is POD. 162 template<> struct isPodLike<CallValue> { 163 static const bool value = true; 164 }; 165 166 template<> struct DenseMapInfo<CallValue> { 167 static inline CallValue getEmptyKey() { 168 return DenseMapInfo<Instruction*>::getEmptyKey(); 169 } 170 static inline CallValue getTombstoneKey() { 171 return DenseMapInfo<Instruction*>::getTombstoneKey(); 172 } 173 static unsigned getHashValue(CallValue Val); 174 static bool isEqual(CallValue LHS, CallValue RHS); 175 }; 176} 177unsigned DenseMapInfo<CallValue>::getHashValue(CallValue Val) { 178 Instruction *Inst = Val.Inst; 179 // Hash in all of the operands as pointers. 180 unsigned Res = 0; 181 for (unsigned i = 0, e = Inst->getNumOperands(); i != e; ++i) { 182 assert(!Inst->getOperand(i)->getType()->isMetadataTy() && 183 "Cannot value number calls with metadata operands"); 184 Res ^= getHash(Inst->getOperand(i)) << i; 185 } 186 187 // Mix in the opcode. 188 return (Res << 1) ^ Inst->getOpcode(); 189} 190 191bool DenseMapInfo<CallValue>::isEqual(CallValue LHS, CallValue RHS) { 192 Instruction *LHSI = LHS.Inst, *RHSI = RHS.Inst; 193 if (LHS.isSentinel() || RHS.isSentinel()) 194 return LHSI == RHSI; 195 return LHSI->isIdenticalTo(RHSI); 196} 197 198 199//===----------------------------------------------------------------------===// 200// EarlyCSE pass. 201//===----------------------------------------------------------------------===// 202 203namespace { 204 205/// EarlyCSE - This pass does a simple depth-first walk over the dominator 206/// tree, eliminating trivially redundant instructions and using instsimplify 207/// to canonicalize things as it goes. It is intended to be fast and catch 208/// obvious cases so that instcombine and other passes are more effective. It 209/// is expected that a later pass of GVN will catch the interesting/hard 210/// cases. 211class EarlyCSE : public FunctionPass { 212public: 213 const TargetData *TD; 214 DominatorTree *DT; 215 typedef RecyclingAllocator<BumpPtrAllocator, 216 ScopedHashTableVal<SimpleValue, Value*> > AllocatorTy; 217 typedef ScopedHashTable<SimpleValue, Value*, DenseMapInfo<SimpleValue>, 218 AllocatorTy> ScopedHTType; 219 220 /// AvailableValues - This scoped hash table contains the current values of 221 /// all of our simple scalar expressions. As we walk down the domtree, we 222 /// look to see if instructions are in this: if so, we replace them with what 223 /// we find, otherwise we insert them so that dominated values can succeed in 224 /// their lookup. 225 ScopedHTType *AvailableValues; 226 227 /// AvailableLoads - This scoped hash table contains the current values 228 /// of loads. This allows us to get efficient access to dominating loads when 229 /// we have a fully redundant load. In addition to the most recent load, we 230 /// keep track of a generation count of the read, which is compared against 231 /// the current generation count. The current generation count is 232 /// incremented after every possibly writing memory operation, which ensures 233 /// that we only CSE loads with other loads that have no intervening store. 234 typedef RecyclingAllocator<BumpPtrAllocator, 235 ScopedHashTableVal<Value*, std::pair<Value*, unsigned> > > LoadMapAllocator; 236 typedef ScopedHashTable<Value*, std::pair<Value*, unsigned>, 237 DenseMapInfo<Value*>, LoadMapAllocator> LoadHTType; 238 LoadHTType *AvailableLoads; 239 240 /// AvailableCalls - This scoped hash table contains the current values 241 /// of read-only call values. It uses the same generation count as loads. 242 typedef ScopedHashTable<CallValue, std::pair<Value*, unsigned> > CallHTType; 243 CallHTType *AvailableCalls; 244 245 /// CurrentGeneration - This is the current generation of the memory value. 246 unsigned CurrentGeneration; 247 248 static char ID; 249 explicit EarlyCSE() : FunctionPass(ID) { 250 initializeEarlyCSEPass(*PassRegistry::getPassRegistry()); 251 } 252 253 bool runOnFunction(Function &F); 254 255private: 256 257 bool processNode(DomTreeNode *Node); 258 259 // This transformation requires dominator postdominator info 260 virtual void getAnalysisUsage(AnalysisUsage &AU) const { 261 AU.addRequired<DominatorTree>(); 262 AU.setPreservesCFG(); 263 } 264}; 265} 266 267char EarlyCSE::ID = 0; 268 269// createEarlyCSEPass - The public interface to this file. 270FunctionPass *llvm::createEarlyCSEPass() { 271 return new EarlyCSE(); 272} 273 274INITIALIZE_PASS_BEGIN(EarlyCSE, "early-cse", "Early CSE", false, false) 275INITIALIZE_PASS_DEPENDENCY(DominatorTree) 276INITIALIZE_PASS_END(EarlyCSE, "early-cse", "Early CSE", false, false) 277 278bool EarlyCSE::processNode(DomTreeNode *Node) { 279 // Define a scope in the scoped hash table. When we are done processing this 280 // domtree node and recurse back up to our parent domtree node, this will pop 281 // off all the values we install. 282 ScopedHTType::ScopeTy Scope(*AvailableValues); 283 284 // Define a scope for the load values so that anything we add will get 285 // popped when we recurse back up to our parent domtree node. 286 LoadHTType::ScopeTy LoadScope(*AvailableLoads); 287 288 // Define a scope for the call values so that anything we add will get 289 // popped when we recurse back up to our parent domtree node. 290 CallHTType::ScopeTy CallScope(*AvailableCalls); 291 292 BasicBlock *BB = Node->getBlock(); 293 294 // If this block has a single predecessor, then the predecessor is the parent 295 // of the domtree node and all of the live out memory values are still current 296 // in this block. If this block has multiple predecessors, then they could 297 // have invalidated the live-out memory values of our parent value. For now, 298 // just be conservative and invalidate memory if this block has multiple 299 // predecessors. 300 if (BB->getSinglePredecessor() == 0) 301 ++CurrentGeneration; 302 303 /// LastStore - Keep track of the last non-volatile store that we saw... for 304 /// as long as there in no instruction that reads memory. If we see a store 305 /// to the same location, we delete the dead store. This zaps trivial dead 306 /// stores which can occur in bitfield code among other things. 307 StoreInst *LastStore = 0; 308 309 bool Changed = false; 310 311 // See if any instructions in the block can be eliminated. If so, do it. If 312 // not, add them to AvailableValues. 313 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ) { 314 Instruction *Inst = I++; 315 316 // Dead instructions should just be removed. 317 if (isInstructionTriviallyDead(Inst)) { 318 DEBUG(dbgs() << "EarlyCSE DCE: " << *Inst << '\n'); 319 Inst->eraseFromParent(); 320 Changed = true; 321 ++NumSimplify; 322 continue; 323 } 324 325 // If the instruction can be simplified (e.g. X+0 = X) then replace it with 326 // its simpler value. 327 if (Value *V = SimplifyInstruction(Inst, TD, DT)) { 328 DEBUG(dbgs() << "EarlyCSE Simplify: " << *Inst << " to: " << *V << '\n'); 329 Inst->replaceAllUsesWith(V); 330 Inst->eraseFromParent(); 331 Changed = true; 332 ++NumSimplify; 333 continue; 334 } 335 336 // If this is a simple instruction that we can value number, process it. 337 if (SimpleValue::canHandle(Inst)) { 338 // See if the instruction has an available value. If so, use it. 339 if (Value *V = AvailableValues->lookup(Inst)) { 340 DEBUG(dbgs() << "EarlyCSE CSE: " << *Inst << " to: " << *V << '\n'); 341 Inst->replaceAllUsesWith(V); 342 Inst->eraseFromParent(); 343 Changed = true; 344 ++NumCSE; 345 continue; 346 } 347 348 // Otherwise, just remember that this value is available. 349 AvailableValues->insert(Inst, Inst); 350 continue; 351 } 352 353 // If this is a non-volatile load, process it. 354 if (LoadInst *LI = dyn_cast<LoadInst>(Inst)) { 355 // Ignore volatile loads. 356 if (LI->isVolatile()) { 357 LastStore = 0; 358 continue; 359 } 360 361 // If we have an available version of this load, and if it is the right 362 // generation, replace this instruction. 363 std::pair<Value*, unsigned> InVal = 364 AvailableLoads->lookup(Inst->getOperand(0)); 365 if (InVal.first != 0 && InVal.second == CurrentGeneration) { 366 DEBUG(dbgs() << "EarlyCSE CSE LOAD: " << *Inst << " to: " 367 << *InVal.first << '\n'); 368 if (!Inst->use_empty()) Inst->replaceAllUsesWith(InVal.first); 369 Inst->eraseFromParent(); 370 Changed = true; 371 ++NumCSELoad; 372 continue; 373 } 374 375 // Otherwise, remember that we have this instruction. 376 AvailableLoads->insert(Inst->getOperand(0), 377 std::pair<Value*, unsigned>(Inst, CurrentGeneration)); 378 LastStore = 0; 379 continue; 380 } 381 382 // If this instruction may read from memory, forget LastStore. 383 if (Inst->mayReadFromMemory()) 384 LastStore = 0; 385 386 // If this is a read-only call, process it. 387 if (CallValue::canHandle(Inst)) { 388 // If we have an available version of this call, and if it is the right 389 // generation, replace this instruction. 390 std::pair<Value*, unsigned> InVal = AvailableCalls->lookup(Inst); 391 if (InVal.first != 0 && InVal.second == CurrentGeneration) { 392 DEBUG(dbgs() << "EarlyCSE CSE CALL: " << *Inst << " to: " 393 << *InVal.first << '\n'); 394 if (!Inst->use_empty()) Inst->replaceAllUsesWith(InVal.first); 395 Inst->eraseFromParent(); 396 Changed = true; 397 ++NumCSECall; 398 continue; 399 } 400 401 // Otherwise, remember that we have this instruction. 402 AvailableCalls->insert(Inst, 403 std::pair<Value*, unsigned>(Inst, CurrentGeneration)); 404 continue; 405 } 406 407 // Okay, this isn't something we can CSE at all. Check to see if it is 408 // something that could modify memory. If so, our available memory values 409 // cannot be used so bump the generation count. 410 if (Inst->mayWriteToMemory()) { 411 ++CurrentGeneration; 412 413 if (StoreInst *SI = dyn_cast<StoreInst>(Inst)) { 414 // We do a trivial form of DSE if there are two stores to the same 415 // location with no intervening loads. Delete the earlier store. 416 if (LastStore && 417 LastStore->getPointerOperand() == SI->getPointerOperand()) { 418 DEBUG(dbgs() << "EarlyCSE DEAD STORE: " << *LastStore << " due to: " 419 << *Inst << '\n'); 420 LastStore->eraseFromParent(); 421 Changed = true; 422 ++NumDSE; 423 LastStore = 0; 424 continue; 425 } 426 427 // Okay, we just invalidated anything we knew about loaded values. Try 428 // to salvage *something* by remembering that the stored value is a live 429 // version of the pointer. It is safe to forward from volatile stores 430 // to non-volatile loads, so we don't have to check for volatility of 431 // the store. 432 AvailableLoads->insert(SI->getPointerOperand(), 433 std::pair<Value*, unsigned>(SI->getValueOperand(), CurrentGeneration)); 434 435 // Remember that this was the last store we saw for DSE. 436 if (!SI->isVolatile()) 437 LastStore = SI; 438 } 439 } 440 } 441 442 unsigned LiveOutGeneration = CurrentGeneration; 443 for (DomTreeNode::iterator I = Node->begin(), E = Node->end(); I != E; ++I) { 444 Changed |= processNode(*I); 445 // Pop any generation changes off the stack from the recursive walk. 446 CurrentGeneration = LiveOutGeneration; 447 } 448 return Changed; 449} 450 451 452bool EarlyCSE::runOnFunction(Function &F) { 453 TD = getAnalysisIfAvailable<TargetData>(); 454 DT = &getAnalysis<DominatorTree>(); 455 456 // Tables that the pass uses when walking the domtree. 457 ScopedHTType AVTable; 458 AvailableValues = &AVTable; 459 LoadHTType LoadTable; 460 AvailableLoads = &LoadTable; 461 CallHTType CallTable; 462 AvailableCalls = &CallTable; 463 464 CurrentGeneration = 0; 465 return processNode(DT->getRootNode()); 466} 467