1 //===- MergedLoadStoreMotion.cpp - merge and hoist/sink load/stores -------===// 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 //! \file 11 //! \brief This pass performs merges of loads and stores on both sides of a 12 // diamond (hammock). It hoists the loads and sinks the stores. 13 // 14 // The algorithm iteratively hoists two loads to the same address out of a 15 // diamond (hammock) and merges them into a single load in the header. Similar 16 // it sinks and merges two stores to the tail block (footer). The algorithm 17 // iterates over the instructions of one side of the diamond and attempts to 18 // find a matching load/store on the other side. It hoists / sinks when it 19 // thinks it safe to do so. This optimization helps with eg. hiding load 20 // latencies, triggering if-conversion, and reducing static code size. 21 // 22 //===----------------------------------------------------------------------===// 23 // 24 // 25 // Example: 26 // Diamond shaped code before merge: 27 // 28 // header: 29 // br %cond, label %if.then, label %if.else 30 // + + 31 // + + 32 // + + 33 // if.then: if.else: 34 // %lt = load %addr_l %le = load %addr_l 35 // <use %lt> <use %le> 36 // <...> <...> 37 // store %st, %addr_s store %se, %addr_s 38 // br label %if.end br label %if.end 39 // + + 40 // + + 41 // + + 42 // if.end ("footer"): 43 // <...> 44 // 45 // Diamond shaped code after merge: 46 // 47 // header: 48 // %l = load %addr_l 49 // br %cond, label %if.then, label %if.else 50 // + + 51 // + + 52 // + + 53 // if.then: if.else: 54 // <use %l> <use %l> 55 // <...> <...> 56 // br label %if.end br label %if.end 57 // + + 58 // + + 59 // + + 60 // if.end ("footer"): 61 // %s.sink = phi [%st, if.then], [%se, if.else] 62 // <...> 63 // store %s.sink, %addr_s 64 // <...> 65 // 66 // 67 //===----------------------- TODO -----------------------------------------===// 68 // 69 // 1) Generalize to regions other than diamonds 70 // 2) Be more aggressive merging memory operations 71 // Note that both changes require register pressure control 72 // 73 //===----------------------------------------------------------------------===// 74 75 #include "llvm/Transforms/Scalar.h" 76 #include "llvm/ADT/SetVector.h" 77 #include "llvm/ADT/SmallPtrSet.h" 78 #include "llvm/ADT/Statistic.h" 79 #include "llvm/Analysis/AliasAnalysis.h" 80 #include "llvm/Analysis/CFG.h" 81 #include "llvm/Analysis/Loads.h" 82 #include "llvm/Analysis/MemoryBuiltins.h" 83 #include "llvm/Analysis/MemoryDependenceAnalysis.h" 84 #include "llvm/Analysis/TargetLibraryInfo.h" 85 #include "llvm/IR/Metadata.h" 86 #include "llvm/IR/PatternMatch.h" 87 #include "llvm/Support/Allocator.h" 88 #include "llvm/Support/CommandLine.h" 89 #include "llvm/Support/Debug.h" 90 #include "llvm/Support/raw_ostream.h" 91 #include "llvm/Transforms/Utils/BasicBlockUtils.h" 92 #include "llvm/Transforms/Utils/SSAUpdater.h" 93 #include <vector> 94 using namespace llvm; 95 96 #define DEBUG_TYPE "mldst-motion" 97 98 //===----------------------------------------------------------------------===// 99 // MergedLoadStoreMotion Pass 100 //===----------------------------------------------------------------------===// 101 102 namespace { 103 class MergedLoadStoreMotion : public FunctionPass { 104 AliasAnalysis *AA; 105 MemoryDependenceAnalysis *MD; 106 107 public: 108 static char ID; // Pass identification, replacement for typeid 109 explicit MergedLoadStoreMotion(void) 110 : FunctionPass(ID), MD(nullptr), MagicCompileTimeControl(250) { 111 initializeMergedLoadStoreMotionPass(*PassRegistry::getPassRegistry()); 112 } 113 114 bool runOnFunction(Function &F) override; 115 116 private: 117 // This transformation requires dominator postdominator info 118 void getAnalysisUsage(AnalysisUsage &AU) const override { 119 AU.addRequired<TargetLibraryInfoWrapperPass>(); 120 AU.addRequired<AliasAnalysis>(); 121 AU.addPreserved<MemoryDependenceAnalysis>(); 122 AU.addPreserved<AliasAnalysis>(); 123 } 124 125 // Helper routines 126 127 /// 128 /// \brief Remove instruction from parent and update memory dependence 129 /// analysis. 130 /// 131 void removeInstruction(Instruction *Inst); 132 BasicBlock *getDiamondTail(BasicBlock *BB); 133 bool isDiamondHead(BasicBlock *BB); 134 // Routines for hoisting loads 135 bool isLoadHoistBarrierInRange(const Instruction& Start, 136 const Instruction& End, 137 LoadInst* LI); 138 LoadInst *canHoistFromBlock(BasicBlock *BB, LoadInst *LI); 139 void hoistInstruction(BasicBlock *BB, Instruction *HoistCand, 140 Instruction *ElseInst); 141 bool isSafeToHoist(Instruction *I) const; 142 bool hoistLoad(BasicBlock *BB, LoadInst *HoistCand, LoadInst *ElseInst); 143 bool mergeLoads(BasicBlock *BB); 144 // Routines for sinking stores 145 StoreInst *canSinkFromBlock(BasicBlock *BB, StoreInst *SI); 146 PHINode *getPHIOperand(BasicBlock *BB, StoreInst *S0, StoreInst *S1); 147 bool isStoreSinkBarrierInRange(const Instruction &Start, 148 const Instruction &End, MemoryLocation Loc); 149 bool sinkStore(BasicBlock *BB, StoreInst *SinkCand, StoreInst *ElseInst); 150 bool mergeStores(BasicBlock *BB); 151 // The mergeLoad/Store algorithms could have Size0 * Size1 complexity, 152 // where Size0 and Size1 are the #instructions on the two sides of 153 // the diamond. The constant chosen here is arbitrary. Compiler Time 154 // Control is enforced by the check Size0 * Size1 < MagicCompileTimeControl. 155 const int MagicCompileTimeControl; 156 }; 157 158 char MergedLoadStoreMotion::ID = 0; 159 } // namespace 160 161 /// 162 /// \brief createMergedLoadStoreMotionPass - The public interface to this file. 163 /// 164 FunctionPass *llvm::createMergedLoadStoreMotionPass() { 165 return new MergedLoadStoreMotion(); 166 } 167 168 INITIALIZE_PASS_BEGIN(MergedLoadStoreMotion, "mldst-motion", 169 "MergedLoadStoreMotion", false, false) 170 INITIALIZE_PASS_DEPENDENCY(MemoryDependenceAnalysis) 171 INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfoWrapperPass) 172 INITIALIZE_AG_DEPENDENCY(AliasAnalysis) 173 INITIALIZE_PASS_END(MergedLoadStoreMotion, "mldst-motion", 174 "MergedLoadStoreMotion", false, false) 175 176 /// 177 /// \brief Remove instruction from parent and update memory dependence analysis. 178 /// 179 void MergedLoadStoreMotion::removeInstruction(Instruction *Inst) { 180 // Notify the memory dependence analysis. 181 if (MD) { 182 MD->removeInstruction(Inst); 183 if (LoadInst *LI = dyn_cast<LoadInst>(Inst)) 184 MD->invalidateCachedPointerInfo(LI->getPointerOperand()); 185 if (Inst->getType()->getScalarType()->isPointerTy()) { 186 MD->invalidateCachedPointerInfo(Inst); 187 } 188 } 189 Inst->eraseFromParent(); 190 } 191 192 /// 193 /// \brief Return tail block of a diamond. 194 /// 195 BasicBlock *MergedLoadStoreMotion::getDiamondTail(BasicBlock *BB) { 196 assert(isDiamondHead(BB) && "Basic block is not head of a diamond"); 197 BranchInst *BI = (BranchInst *)(BB->getTerminator()); 198 BasicBlock *Succ0 = BI->getSuccessor(0); 199 BasicBlock *Tail = Succ0->getTerminator()->getSuccessor(0); 200 return Tail; 201 } 202 203 /// 204 /// \brief True when BB is the head of a diamond (hammock) 205 /// 206 bool MergedLoadStoreMotion::isDiamondHead(BasicBlock *BB) { 207 if (!BB) 208 return false; 209 if (!isa<BranchInst>(BB->getTerminator())) 210 return false; 211 if (BB->getTerminator()->getNumSuccessors() != 2) 212 return false; 213 214 BranchInst *BI = (BranchInst *)(BB->getTerminator()); 215 BasicBlock *Succ0 = BI->getSuccessor(0); 216 BasicBlock *Succ1 = BI->getSuccessor(1); 217 218 if (!Succ0->getSinglePredecessor() || 219 Succ0->getTerminator()->getNumSuccessors() != 1) 220 return false; 221 if (!Succ1->getSinglePredecessor() || 222 Succ1->getTerminator()->getNumSuccessors() != 1) 223 return false; 224 225 BasicBlock *Tail = Succ0->getTerminator()->getSuccessor(0); 226 // Ignore triangles. 227 if (Succ1->getTerminator()->getSuccessor(0) != Tail) 228 return false; 229 return true; 230 } 231 232 /// 233 /// \brief True when instruction is a hoist barrier for a load 234 /// 235 /// Whenever an instruction could possibly modify the value 236 /// being loaded or protect against the load from happening 237 /// it is considered a hoist barrier. 238 /// 239 240 bool MergedLoadStoreMotion::isLoadHoistBarrierInRange(const Instruction& Start, 241 const Instruction& End, 242 LoadInst* LI) { 243 MemoryLocation Loc = MemoryLocation::get(LI); 244 return AA->canInstructionRangeModRef(Start, End, Loc, AliasAnalysis::Mod); 245 } 246 247 /// 248 /// \brief Decide if a load can be hoisted 249 /// 250 /// When there is a load in \p BB to the same address as \p LI 251 /// and it can be hoisted from \p BB, return that load. 252 /// Otherwise return Null. 253 /// 254 LoadInst *MergedLoadStoreMotion::canHoistFromBlock(BasicBlock *BB1, 255 LoadInst *Load0) { 256 257 for (BasicBlock::iterator BBI = BB1->begin(), BBE = BB1->end(); BBI != BBE; 258 ++BBI) { 259 Instruction *Inst = BBI; 260 261 // Only merge and hoist loads when their result in used only in BB 262 if (!isa<LoadInst>(Inst) || Inst->isUsedOutsideOfBlock(BB1)) 263 continue; 264 265 LoadInst *Load1 = dyn_cast<LoadInst>(Inst); 266 BasicBlock *BB0 = Load0->getParent(); 267 268 MemoryLocation Loc0 = MemoryLocation::get(Load0); 269 MemoryLocation Loc1 = MemoryLocation::get(Load1); 270 if (AA->isMustAlias(Loc0, Loc1) && Load0->isSameOperationAs(Load1) && 271 !isLoadHoistBarrierInRange(BB1->front(), *Load1, Load1) && 272 !isLoadHoistBarrierInRange(BB0->front(), *Load0, Load0)) { 273 return Load1; 274 } 275 } 276 return nullptr; 277 } 278 279 /// 280 /// \brief Merge two equivalent instructions \p HoistCand and \p ElseInst into 281 /// \p BB 282 /// 283 /// BB is the head of a diamond 284 /// 285 void MergedLoadStoreMotion::hoistInstruction(BasicBlock *BB, 286 Instruction *HoistCand, 287 Instruction *ElseInst) { 288 DEBUG(dbgs() << " Hoist Instruction into BB \n"; BB->dump(); 289 dbgs() << "Instruction Left\n"; HoistCand->dump(); dbgs() << "\n"; 290 dbgs() << "Instruction Right\n"; ElseInst->dump(); dbgs() << "\n"); 291 // Hoist the instruction. 292 assert(HoistCand->getParent() != BB); 293 294 // Intersect optional metadata. 295 HoistCand->intersectOptionalDataWith(ElseInst); 296 HoistCand->dropUnknownMetadata(); 297 298 // Prepend point for instruction insert 299 Instruction *HoistPt = BB->getTerminator(); 300 301 // Merged instruction 302 Instruction *HoistedInst = HoistCand->clone(); 303 304 // Notify AA of the new value. 305 if (isa<LoadInst>(HoistCand)) 306 AA->copyValue(HoistCand, HoistedInst); 307 308 // Hoist instruction. 309 HoistedInst->insertBefore(HoistPt); 310 311 HoistCand->replaceAllUsesWith(HoistedInst); 312 removeInstruction(HoistCand); 313 // Replace the else block instruction. 314 ElseInst->replaceAllUsesWith(HoistedInst); 315 removeInstruction(ElseInst); 316 } 317 318 /// 319 /// \brief Return true if no operand of \p I is defined in I's parent block 320 /// 321 bool MergedLoadStoreMotion::isSafeToHoist(Instruction *I) const { 322 BasicBlock *Parent = I->getParent(); 323 for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i) { 324 Instruction *Instr = dyn_cast<Instruction>(I->getOperand(i)); 325 if (Instr && Instr->getParent() == Parent) 326 return false; 327 } 328 return true; 329 } 330 331 /// 332 /// \brief Merge two equivalent loads and GEPs and hoist into diamond head 333 /// 334 bool MergedLoadStoreMotion::hoistLoad(BasicBlock *BB, LoadInst *L0, 335 LoadInst *L1) { 336 // Only one definition? 337 Instruction *A0 = dyn_cast<Instruction>(L0->getPointerOperand()); 338 Instruction *A1 = dyn_cast<Instruction>(L1->getPointerOperand()); 339 if (A0 && A1 && A0->isIdenticalTo(A1) && isSafeToHoist(A0) && 340 A0->hasOneUse() && (A0->getParent() == L0->getParent()) && 341 A1->hasOneUse() && (A1->getParent() == L1->getParent()) && 342 isa<GetElementPtrInst>(A0)) { 343 DEBUG(dbgs() << "Hoist Instruction into BB \n"; BB->dump(); 344 dbgs() << "Instruction Left\n"; L0->dump(); dbgs() << "\n"; 345 dbgs() << "Instruction Right\n"; L1->dump(); dbgs() << "\n"); 346 hoistInstruction(BB, A0, A1); 347 hoistInstruction(BB, L0, L1); 348 return true; 349 } else 350 return false; 351 } 352 353 /// 354 /// \brief Try to hoist two loads to same address into diamond header 355 /// 356 /// Starting from a diamond head block, iterate over the instructions in one 357 /// successor block and try to match a load in the second successor. 358 /// 359 bool MergedLoadStoreMotion::mergeLoads(BasicBlock *BB) { 360 bool MergedLoads = false; 361 assert(isDiamondHead(BB)); 362 BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator()); 363 BasicBlock *Succ0 = BI->getSuccessor(0); 364 BasicBlock *Succ1 = BI->getSuccessor(1); 365 // #Instructions in Succ1 for Compile Time Control 366 int Size1 = Succ1->size(); 367 int NLoads = 0; 368 for (BasicBlock::iterator BBI = Succ0->begin(), BBE = Succ0->end(); 369 BBI != BBE;) { 370 371 Instruction *I = BBI; 372 ++BBI; 373 374 // Only move non-simple (atomic, volatile) loads. 375 LoadInst *L0 = dyn_cast<LoadInst>(I); 376 if (!L0 || !L0->isSimple() || L0->isUsedOutsideOfBlock(Succ0)) 377 continue; 378 379 ++NLoads; 380 if (NLoads * Size1 >= MagicCompileTimeControl) 381 break; 382 if (LoadInst *L1 = canHoistFromBlock(Succ1, L0)) { 383 bool Res = hoistLoad(BB, L0, L1); 384 MergedLoads |= Res; 385 // Don't attempt to hoist above loads that had not been hoisted. 386 if (!Res) 387 break; 388 } 389 } 390 return MergedLoads; 391 } 392 393 /// 394 /// \brief True when instruction is a sink barrier for a store 395 /// located in Loc 396 /// 397 /// Whenever an instruction could possibly read or modify the 398 /// value being stored or protect against the store from 399 /// happening it is considered a sink barrier. 400 /// 401 402 bool MergedLoadStoreMotion::isStoreSinkBarrierInRange(const Instruction &Start, 403 const Instruction &End, 404 MemoryLocation Loc) { 405 return AA->canInstructionRangeModRef(Start, End, Loc, AliasAnalysis::ModRef); 406 } 407 408 /// 409 /// \brief Check if \p BB contains a store to the same address as \p SI 410 /// 411 /// \return The store in \p when it is safe to sink. Otherwise return Null. 412 /// 413 StoreInst *MergedLoadStoreMotion::canSinkFromBlock(BasicBlock *BB1, 414 StoreInst *Store0) { 415 DEBUG(dbgs() << "can Sink? : "; Store0->dump(); dbgs() << "\n"); 416 BasicBlock *BB0 = Store0->getParent(); 417 for (BasicBlock::reverse_iterator RBI = BB1->rbegin(), RBE = BB1->rend(); 418 RBI != RBE; ++RBI) { 419 Instruction *Inst = &*RBI; 420 421 if (!isa<StoreInst>(Inst)) 422 continue; 423 424 StoreInst *Store1 = cast<StoreInst>(Inst); 425 426 MemoryLocation Loc0 = MemoryLocation::get(Store0); 427 MemoryLocation Loc1 = MemoryLocation::get(Store1); 428 if (AA->isMustAlias(Loc0, Loc1) && Store0->isSameOperationAs(Store1) && 429 !isStoreSinkBarrierInRange(*(std::next(BasicBlock::iterator(Store1))), 430 BB1->back(), Loc1) && 431 !isStoreSinkBarrierInRange(*(std::next(BasicBlock::iterator(Store0))), 432 BB0->back(), Loc0)) { 433 return Store1; 434 } 435 } 436 return nullptr; 437 } 438 439 /// 440 /// \brief Create a PHI node in BB for the operands of S0 and S1 441 /// 442 PHINode *MergedLoadStoreMotion::getPHIOperand(BasicBlock *BB, StoreInst *S0, 443 StoreInst *S1) { 444 // Create a phi if the values mismatch. 445 PHINode *NewPN = 0; 446 Value *Opd1 = S0->getValueOperand(); 447 Value *Opd2 = S1->getValueOperand(); 448 if (Opd1 != Opd2) { 449 NewPN = PHINode::Create(Opd1->getType(), 2, Opd2->getName() + ".sink", 450 BB->begin()); 451 NewPN->addIncoming(Opd1, S0->getParent()); 452 NewPN->addIncoming(Opd2, S1->getParent()); 453 if (NewPN->getType()->getScalarType()->isPointerTy()) { 454 // Notify AA of the new value. 455 AA->copyValue(Opd1, NewPN); 456 AA->copyValue(Opd2, NewPN); 457 // AA needs to be informed when a PHI-use of the pointer value is added 458 for (unsigned I = 0, E = NewPN->getNumIncomingValues(); I != E; ++I) { 459 unsigned J = PHINode::getOperandNumForIncomingValue(I); 460 AA->addEscapingUse(NewPN->getOperandUse(J)); 461 } 462 if (MD) 463 MD->invalidateCachedPointerInfo(NewPN); 464 } 465 } 466 return NewPN; 467 } 468 469 /// 470 /// \brief Merge two stores to same address and sink into \p BB 471 /// 472 /// Also sinks GEP instruction computing the store address 473 /// 474 bool MergedLoadStoreMotion::sinkStore(BasicBlock *BB, StoreInst *S0, 475 StoreInst *S1) { 476 // Only one definition? 477 Instruction *A0 = dyn_cast<Instruction>(S0->getPointerOperand()); 478 Instruction *A1 = dyn_cast<Instruction>(S1->getPointerOperand()); 479 if (A0 && A1 && A0->isIdenticalTo(A1) && A0->hasOneUse() && 480 (A0->getParent() == S0->getParent()) && A1->hasOneUse() && 481 (A1->getParent() == S1->getParent()) && isa<GetElementPtrInst>(A0)) { 482 DEBUG(dbgs() << "Sink Instruction into BB \n"; BB->dump(); 483 dbgs() << "Instruction Left\n"; S0->dump(); dbgs() << "\n"; 484 dbgs() << "Instruction Right\n"; S1->dump(); dbgs() << "\n"); 485 // Hoist the instruction. 486 BasicBlock::iterator InsertPt = BB->getFirstInsertionPt(); 487 // Intersect optional metadata. 488 S0->intersectOptionalDataWith(S1); 489 S0->dropUnknownMetadata(); 490 491 // Create the new store to be inserted at the join point. 492 StoreInst *SNew = (StoreInst *)(S0->clone()); 493 Instruction *ANew = A0->clone(); 494 AA->copyValue(S0, SNew); 495 SNew->insertBefore(InsertPt); 496 ANew->insertBefore(SNew); 497 498 assert(S0->getParent() == A0->getParent()); 499 assert(S1->getParent() == A1->getParent()); 500 501 PHINode *NewPN = getPHIOperand(BB, S0, S1); 502 // New PHI operand? Use it. 503 if (NewPN) 504 SNew->setOperand(0, NewPN); 505 removeInstruction(S0); 506 removeInstruction(S1); 507 A0->replaceAllUsesWith(ANew); 508 removeInstruction(A0); 509 A1->replaceAllUsesWith(ANew); 510 removeInstruction(A1); 511 return true; 512 } 513 return false; 514 } 515 516 /// 517 /// \brief True when two stores are equivalent and can sink into the footer 518 /// 519 /// Starting from a diamond tail block, iterate over the instructions in one 520 /// predecessor block and try to match a store in the second predecessor. 521 /// 522 bool MergedLoadStoreMotion::mergeStores(BasicBlock *T) { 523 524 bool MergedStores = false; 525 assert(T && "Footer of a diamond cannot be empty"); 526 527 pred_iterator PI = pred_begin(T), E = pred_end(T); 528 assert(PI != E); 529 BasicBlock *Pred0 = *PI; 530 ++PI; 531 BasicBlock *Pred1 = *PI; 532 ++PI; 533 // tail block of a diamond/hammock? 534 if (Pred0 == Pred1) 535 return false; // No. 536 if (PI != E) 537 return false; // No. More than 2 predecessors. 538 539 // #Instructions in Succ1 for Compile Time Control 540 int Size1 = Pred1->size(); 541 int NStores = 0; 542 543 for (BasicBlock::reverse_iterator RBI = Pred0->rbegin(), RBE = Pred0->rend(); 544 RBI != RBE;) { 545 546 Instruction *I = &*RBI; 547 ++RBI; 548 549 // Sink move non-simple (atomic, volatile) stores 550 if (!isa<StoreInst>(I)) 551 continue; 552 StoreInst *S0 = (StoreInst *)I; 553 if (!S0->isSimple()) 554 continue; 555 556 ++NStores; 557 if (NStores * Size1 >= MagicCompileTimeControl) 558 break; 559 if (StoreInst *S1 = canSinkFromBlock(Pred1, S0)) { 560 bool Res = sinkStore(T, S0, S1); 561 MergedStores |= Res; 562 // Don't attempt to sink below stores that had to stick around 563 // But after removal of a store and some of its feeding 564 // instruction search again from the beginning since the iterator 565 // is likely stale at this point. 566 if (!Res) 567 break; 568 else { 569 RBI = Pred0->rbegin(); 570 RBE = Pred0->rend(); 571 DEBUG(dbgs() << "Search again\n"; Instruction *I = &*RBI; I->dump()); 572 } 573 } 574 } 575 return MergedStores; 576 } 577 /// 578 /// \brief Run the transformation for each function 579 /// 580 bool MergedLoadStoreMotion::runOnFunction(Function &F) { 581 MD = getAnalysisIfAvailable<MemoryDependenceAnalysis>(); 582 AA = &getAnalysis<AliasAnalysis>(); 583 584 bool Changed = false; 585 DEBUG(dbgs() << "Instruction Merger\n"); 586 587 // Merge unconditional branches, allowing PRE to catch more 588 // optimization opportunities. 589 for (Function::iterator FI = F.begin(), FE = F.end(); FI != FE;) { 590 BasicBlock *BB = FI++; 591 592 // Hoist equivalent loads and sink stores 593 // outside diamonds when possible 594 if (isDiamondHead(BB)) { 595 Changed |= mergeLoads(BB); 596 Changed |= mergeStores(getDiamondTail(BB)); 597 } 598 } 599 return Changed; 600 } 601