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/IR/Metadata.h" 85 #include "llvm/IR/PatternMatch.h" 86 #include "llvm/Support/Allocator.h" 87 #include "llvm/Support/CommandLine.h" 88 #include "llvm/Support/Debug.h" 89 #include "llvm/Analysis/TargetLibraryInfo.h" 90 #include "llvm/Transforms/Utils/BasicBlockUtils.h" 91 #include "llvm/Transforms/Utils/SSAUpdater.h" 92 #include <vector> 93 using namespace llvm; 94 95 #define DEBUG_TYPE "mldst-motion" 96 97 //===----------------------------------------------------------------------===// 98 // MergedLoadStoreMotion Pass 99 //===----------------------------------------------------------------------===// 100 101 namespace { 102 class MergedLoadStoreMotion : public FunctionPass { 103 AliasAnalysis *AA; 104 MemoryDependenceAnalysis *MD; 105 106 public: 107 static char ID; // Pass identification, replacement for typeid 108 explicit MergedLoadStoreMotion(void) 109 : FunctionPass(ID), MD(nullptr), MagicCompileTimeControl(250) { 110 initializeMergedLoadStoreMotionPass(*PassRegistry::getPassRegistry()); 111 } 112 113 bool runOnFunction(Function &F) override; 114 115 private: 116 // This transformation requires dominator postdominator info 117 void getAnalysisUsage(AnalysisUsage &AU) const override { 118 AU.addRequired<TargetLibraryInfoWrapperPass>(); 119 AU.addRequired<MemoryDependenceAnalysis>(); 120 AU.addRequired<AliasAnalysis>(); 121 AU.addPreserved<AliasAnalysis>(); 122 } 123 124 // Helper routines 125 126 /// 127 /// \brief Remove instruction from parent and update memory dependence 128 /// analysis. 129 /// 130 void removeInstruction(Instruction *Inst); 131 BasicBlock *getDiamondTail(BasicBlock *BB); 132 bool isDiamondHead(BasicBlock *BB); 133 // Routines for hoisting loads 134 bool isLoadHoistBarrierInRange(const Instruction& Start, 135 const Instruction& End, 136 LoadInst* LI); 137 LoadInst *canHoistFromBlock(BasicBlock *BB, LoadInst *LI); 138 void hoistInstruction(BasicBlock *BB, Instruction *HoistCand, 139 Instruction *ElseInst); 140 bool isSafeToHoist(Instruction *I) const; 141 bool hoistLoad(BasicBlock *BB, LoadInst *HoistCand, LoadInst *ElseInst); 142 bool mergeLoads(BasicBlock *BB); 143 // Routines for sinking stores 144 StoreInst *canSinkFromBlock(BasicBlock *BB, StoreInst *SI); 145 PHINode *getPHIOperand(BasicBlock *BB, StoreInst *S0, StoreInst *S1); 146 bool isStoreSinkBarrierInRange(const Instruction& Start, 147 const Instruction& End, 148 AliasAnalysis::Location 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 } 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 AliasAnalysis::Location Loc = AA->getLocation(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 AliasAnalysis::Location Loc0 = AA->getLocation(Load0); 269 AliasAnalysis::Location Loc1 = AA->getLocation(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 AliasAnalysis::Location 405 Loc) { 406 return AA->canInstructionRangeModRef(Start, End, Loc, AliasAnalysis::ModRef); 407 } 408 409 /// 410 /// \brief Check if \p BB contains a store to the same address as \p SI 411 /// 412 /// \return The store in \p when it is safe to sink. Otherwise return Null. 413 /// 414 StoreInst *MergedLoadStoreMotion::canSinkFromBlock(BasicBlock *BB1, 415 StoreInst *Store0) { 416 DEBUG(dbgs() << "can Sink? : "; Store0->dump(); dbgs() << "\n"); 417 BasicBlock *BB0 = Store0->getParent(); 418 for (BasicBlock::reverse_iterator RBI = BB1->rbegin(), RBE = BB1->rend(); 419 RBI != RBE; ++RBI) { 420 Instruction *Inst = &*RBI; 421 422 if (!isa<StoreInst>(Inst)) 423 continue; 424 425 StoreInst *Store1 = cast<StoreInst>(Inst); 426 427 AliasAnalysis::Location Loc0 = AA->getLocation(Store0); 428 AliasAnalysis::Location Loc1 = AA->getLocation(Store1); 429 if (AA->isMustAlias(Loc0, Loc1) && Store0->isSameOperationAs(Store1) && 430 !isStoreSinkBarrierInRange(*(std::next(BasicBlock::iterator(Store1))), 431 BB1->back(), Loc1) && 432 !isStoreSinkBarrierInRange(*(std::next(BasicBlock::iterator(Store0))), 433 BB0->back(), Loc0)) { 434 return Store1; 435 } 436 } 437 return nullptr; 438 } 439 440 /// 441 /// \brief Create a PHI node in BB for the operands of S0 and S1 442 /// 443 PHINode *MergedLoadStoreMotion::getPHIOperand(BasicBlock *BB, StoreInst *S0, 444 StoreInst *S1) { 445 // Create a phi if the values mismatch. 446 PHINode *NewPN = 0; 447 Value *Opd1 = S0->getValueOperand(); 448 Value *Opd2 = S1->getValueOperand(); 449 if (Opd1 != Opd2) { 450 NewPN = PHINode::Create(Opd1->getType(), 2, Opd2->getName() + ".sink", 451 BB->begin()); 452 NewPN->addIncoming(Opd1, S0->getParent()); 453 NewPN->addIncoming(Opd2, S1->getParent()); 454 if (NewPN->getType()->getScalarType()->isPointerTy()) { 455 // Notify AA of the new value. 456 AA->copyValue(Opd1, NewPN); 457 AA->copyValue(Opd2, NewPN); 458 // AA needs to be informed when a PHI-use of the pointer value is added 459 for (unsigned I = 0, E = NewPN->getNumIncomingValues(); I != E; ++I) { 460 unsigned J = PHINode::getOperandNumForIncomingValue(I); 461 AA->addEscapingUse(NewPN->getOperandUse(J)); 462 } 463 if (MD) 464 MD->invalidateCachedPointerInfo(NewPN); 465 } 466 } 467 return NewPN; 468 } 469 470 /// 471 /// \brief Merge two stores to same address and sink into \p BB 472 /// 473 /// Also sinks GEP instruction computing the store address 474 /// 475 bool MergedLoadStoreMotion::sinkStore(BasicBlock *BB, StoreInst *S0, 476 StoreInst *S1) { 477 // Only one definition? 478 Instruction *A0 = dyn_cast<Instruction>(S0->getPointerOperand()); 479 Instruction *A1 = dyn_cast<Instruction>(S1->getPointerOperand()); 480 if (A0 && A1 && A0->isIdenticalTo(A1) && A0->hasOneUse() && 481 (A0->getParent() == S0->getParent()) && A1->hasOneUse() && 482 (A1->getParent() == S1->getParent()) && isa<GetElementPtrInst>(A0)) { 483 DEBUG(dbgs() << "Sink Instruction into BB \n"; BB->dump(); 484 dbgs() << "Instruction Left\n"; S0->dump(); dbgs() << "\n"; 485 dbgs() << "Instruction Right\n"; S1->dump(); dbgs() << "\n"); 486 // Hoist the instruction. 487 BasicBlock::iterator InsertPt = BB->getFirstInsertionPt(); 488 // Intersect optional metadata. 489 S0->intersectOptionalDataWith(S1); 490 S0->dropUnknownMetadata(); 491 492 // Create the new store to be inserted at the join point. 493 StoreInst *SNew = (StoreInst *)(S0->clone()); 494 Instruction *ANew = A0->clone(); 495 AA->copyValue(S0, SNew); 496 SNew->insertBefore(InsertPt); 497 ANew->insertBefore(SNew); 498 499 assert(S0->getParent() == A0->getParent()); 500 assert(S1->getParent() == A1->getParent()); 501 502 PHINode *NewPN = getPHIOperand(BB, S0, S1); 503 // New PHI operand? Use it. 504 if (NewPN) 505 SNew->setOperand(0, NewPN); 506 removeInstruction(S0); 507 removeInstruction(S1); 508 A0->replaceAllUsesWith(ANew); 509 removeInstruction(A0); 510 A1->replaceAllUsesWith(ANew); 511 removeInstruction(A1); 512 return true; 513 } 514 return false; 515 } 516 517 /// 518 /// \brief True when two stores are equivalent and can sink into the footer 519 /// 520 /// Starting from a diamond tail block, iterate over the instructions in one 521 /// predecessor block and try to match a store in the second predecessor. 522 /// 523 bool MergedLoadStoreMotion::mergeStores(BasicBlock *T) { 524 525 bool MergedStores = false; 526 assert(T && "Footer of a diamond cannot be empty"); 527 528 pred_iterator PI = pred_begin(T), E = pred_end(T); 529 assert(PI != E); 530 BasicBlock *Pred0 = *PI; 531 ++PI; 532 BasicBlock *Pred1 = *PI; 533 ++PI; 534 // tail block of a diamond/hammock? 535 if (Pred0 == Pred1) 536 return false; // No. 537 if (PI != E) 538 return false; // No. More than 2 predecessors. 539 540 // #Instructions in Succ1 for Compile Time Control 541 int Size1 = Pred1->size(); 542 int NStores = 0; 543 544 for (BasicBlock::reverse_iterator RBI = Pred0->rbegin(), RBE = Pred0->rend(); 545 RBI != RBE;) { 546 547 Instruction *I = &*RBI; 548 ++RBI; 549 550 // Sink move non-simple (atomic, volatile) stores 551 if (!isa<StoreInst>(I)) 552 continue; 553 StoreInst *S0 = (StoreInst *)I; 554 if (!S0->isSimple()) 555 continue; 556 557 ++NStores; 558 if (NStores * Size1 >= MagicCompileTimeControl) 559 break; 560 if (StoreInst *S1 = canSinkFromBlock(Pred1, S0)) { 561 bool Res = sinkStore(T, S0, S1); 562 MergedStores |= Res; 563 // Don't attempt to sink below stores that had to stick around 564 // But after removal of a store and some of its feeding 565 // instruction search again from the beginning since the iterator 566 // is likely stale at this point. 567 if (!Res) 568 break; 569 else { 570 RBI = Pred0->rbegin(); 571 RBE = Pred0->rend(); 572 DEBUG(dbgs() << "Search again\n"; Instruction *I = &*RBI; I->dump()); 573 } 574 } 575 } 576 return MergedStores; 577 } 578 /// 579 /// \brief Run the transformation for each function 580 /// 581 bool MergedLoadStoreMotion::runOnFunction(Function &F) { 582 MD = &getAnalysis<MemoryDependenceAnalysis>(); 583 AA = &getAnalysis<AliasAnalysis>(); 584 585 bool Changed = false; 586 DEBUG(dbgs() << "Instruction Merger\n"); 587 588 // Merge unconditional branches, allowing PRE to catch more 589 // optimization opportunities. 590 for (Function::iterator FI = F.begin(), FE = F.end(); FI != FE;) { 591 BasicBlock *BB = FI++; 592 593 // Hoist equivalent loads and sink stores 594 // outside diamonds when possible 595 if (isDiamondHead(BB)) { 596 Changed |= mergeLoads(BB); 597 Changed |= mergeStores(getDiamondTail(BB)); 598 } 599 } 600 return Changed; 601 } 602