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/Target/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<TargetLibraryInfo>(); 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 isLoadHoistBarrier(Instruction *Inst); 135 LoadInst *canHoistFromBlock(BasicBlock *BB, LoadInst *LI); 136 void hoistInstruction(BasicBlock *BB, Instruction *HoistCand, 137 Instruction *ElseInst); 138 bool isSafeToHoist(Instruction *I) const; 139 bool hoistLoad(BasicBlock *BB, LoadInst *HoistCand, LoadInst *ElseInst); 140 bool mergeLoads(BasicBlock *BB); 141 // Routines for sinking stores 142 StoreInst *canSinkFromBlock(BasicBlock *BB, StoreInst *SI); 143 PHINode *getPHIOperand(BasicBlock *BB, StoreInst *S0, StoreInst *S1); 144 bool isStoreSinkBarrier(Instruction *Inst); 145 bool sinkStore(BasicBlock *BB, StoreInst *SinkCand, StoreInst *ElseInst); 146 bool mergeStores(BasicBlock *BB); 147 // The mergeLoad/Store algorithms could have Size0 * Size1 complexity, 148 // where Size0 and Size1 are the #instructions on the two sides of 149 // the diamond. The constant chosen here is arbitrary. Compiler Time 150 // Control is enforced by the check Size0 * Size1 < MagicCompileTimeControl. 151 const int MagicCompileTimeControl; 152 }; 153 154 char MergedLoadStoreMotion::ID = 0; 155 } 156 157 /// 158 /// \brief createMergedLoadStoreMotionPass - The public interface to this file. 159 /// 160 FunctionPass *llvm::createMergedLoadStoreMotionPass() { 161 return new MergedLoadStoreMotion(); 162 } 163 164 INITIALIZE_PASS_BEGIN(MergedLoadStoreMotion, "mldst-motion", 165 "MergedLoadStoreMotion", false, false) 166 INITIALIZE_PASS_DEPENDENCY(MemoryDependenceAnalysis) 167 INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfo) 168 INITIALIZE_AG_DEPENDENCY(AliasAnalysis) 169 INITIALIZE_PASS_END(MergedLoadStoreMotion, "mldst-motion", 170 "MergedLoadStoreMotion", false, false) 171 172 /// 173 /// \brief Remove instruction from parent and update memory dependence analysis. 174 /// 175 void MergedLoadStoreMotion::removeInstruction(Instruction *Inst) { 176 // Notify the memory dependence analysis. 177 if (MD) { 178 MD->removeInstruction(Inst); 179 if (LoadInst *LI = dyn_cast<LoadInst>(Inst)) 180 MD->invalidateCachedPointerInfo(LI->getPointerOperand()); 181 if (Inst->getType()->getScalarType()->isPointerTy()) { 182 MD->invalidateCachedPointerInfo(Inst); 183 } 184 } 185 Inst->eraseFromParent(); 186 } 187 188 /// 189 /// \brief Return tail block of a diamond. 190 /// 191 BasicBlock *MergedLoadStoreMotion::getDiamondTail(BasicBlock *BB) { 192 assert(isDiamondHead(BB) && "Basic block is not head of a diamond"); 193 BranchInst *BI = (BranchInst *)(BB->getTerminator()); 194 BasicBlock *Succ0 = BI->getSuccessor(0); 195 BasicBlock *Tail = Succ0->getTerminator()->getSuccessor(0); 196 return Tail; 197 } 198 199 /// 200 /// \brief True when BB is the head of a diamond (hammock) 201 /// 202 bool MergedLoadStoreMotion::isDiamondHead(BasicBlock *BB) { 203 if (!BB) 204 return false; 205 if (!isa<BranchInst>(BB->getTerminator())) 206 return false; 207 if (BB->getTerminator()->getNumSuccessors() != 2) 208 return false; 209 210 BranchInst *BI = (BranchInst *)(BB->getTerminator()); 211 BasicBlock *Succ0 = BI->getSuccessor(0); 212 BasicBlock *Succ1 = BI->getSuccessor(1); 213 214 if (!Succ0->getSinglePredecessor() || 215 Succ0->getTerminator()->getNumSuccessors() != 1) 216 return false; 217 if (!Succ1->getSinglePredecessor() || 218 Succ1->getTerminator()->getNumSuccessors() != 1) 219 return false; 220 221 BasicBlock *Tail = Succ0->getTerminator()->getSuccessor(0); 222 // Ignore triangles. 223 if (Succ1->getTerminator()->getSuccessor(0) != Tail) 224 return false; 225 return true; 226 } 227 228 /// 229 /// \brief True when instruction is a hoist barrier for a load 230 /// 231 /// Whenever an instruction could possibly modify the value 232 /// being loaded or protect against the load from happening 233 /// it is considered a hoist barrier. 234 /// 235 bool MergedLoadStoreMotion::isLoadHoistBarrier(Instruction *Inst) { 236 // FIXME: A call with no side effects should not be a barrier. 237 // Aren't all such calls covered by mayHaveSideEffects() below? 238 // Then this check can be removed. 239 if (isa<CallInst>(Inst)) 240 return true; 241 if (isa<TerminatorInst>(Inst)) 242 return true; 243 // FIXME: Conservatively let a store instruction block the load. 244 // Use alias analysis instead. 245 if (isa<StoreInst>(Inst)) 246 return true; 247 // Note: mayHaveSideEffects covers all instructions that could 248 // trigger a change to state. Eg. in-flight stores have to be executed 249 // before ordered loads or fences, calls could invoke functions that store 250 // data to memory etc. 251 if (Inst->mayHaveSideEffects()) { 252 return true; 253 } 254 DEBUG(dbgs() << "No Hoist Barrier\n"); 255 return false; 256 } 257 258 /// 259 /// \brief Decide if a load can be hoisted 260 /// 261 /// When there is a load in \p BB to the same address as \p LI 262 /// and it can be hoisted from \p BB, return that load. 263 /// Otherwise return Null. 264 /// 265 LoadInst *MergedLoadStoreMotion::canHoistFromBlock(BasicBlock *BB, 266 LoadInst *LI) { 267 LoadInst *I = nullptr; 268 assert(isa<LoadInst>(LI)); 269 if (LI->isUsedOutsideOfBlock(LI->getParent())) 270 return nullptr; 271 272 for (BasicBlock::iterator BBI = BB->begin(), BBE = BB->end(); BBI != BBE; 273 ++BBI) { 274 Instruction *Inst = BBI; 275 276 // Only merge and hoist loads when their result in used only in BB 277 if (isLoadHoistBarrier(Inst)) 278 break; 279 if (!isa<LoadInst>(Inst)) 280 continue; 281 if (Inst->isUsedOutsideOfBlock(Inst->getParent())) 282 continue; 283 284 AliasAnalysis::Location LocLI = AA->getLocation(LI); 285 AliasAnalysis::Location LocInst = AA->getLocation((LoadInst *)Inst); 286 if (AA->isMustAlias(LocLI, LocInst) && LI->getType() == Inst->getType()) { 287 I = (LoadInst *)Inst; 288 break; 289 } 290 } 291 return I; 292 } 293 294 /// 295 /// \brief Merge two equivalent instructions \p HoistCand and \p ElseInst into 296 /// \p BB 297 /// 298 /// BB is the head of a diamond 299 /// 300 void MergedLoadStoreMotion::hoistInstruction(BasicBlock *BB, 301 Instruction *HoistCand, 302 Instruction *ElseInst) { 303 DEBUG(dbgs() << " Hoist Instruction into BB \n"; BB->dump(); 304 dbgs() << "Instruction Left\n"; HoistCand->dump(); dbgs() << "\n"; 305 dbgs() << "Instruction Right\n"; ElseInst->dump(); dbgs() << "\n"); 306 // Hoist the instruction. 307 assert(HoistCand->getParent() != BB); 308 309 // Intersect optional metadata. 310 HoistCand->intersectOptionalDataWith(ElseInst); 311 HoistCand->dropUnknownMetadata(); 312 313 // Prepend point for instruction insert 314 Instruction *HoistPt = BB->getTerminator(); 315 316 // Merged instruction 317 Instruction *HoistedInst = HoistCand->clone(); 318 319 // Notify AA of the new value. 320 if (isa<LoadInst>(HoistCand)) 321 AA->copyValue(HoistCand, HoistedInst); 322 323 // Hoist instruction. 324 HoistedInst->insertBefore(HoistPt); 325 326 HoistCand->replaceAllUsesWith(HoistedInst); 327 removeInstruction(HoistCand); 328 // Replace the else block instruction. 329 ElseInst->replaceAllUsesWith(HoistedInst); 330 removeInstruction(ElseInst); 331 } 332 333 /// 334 /// \brief Return true if no operand of \p I is defined in I's parent block 335 /// 336 bool MergedLoadStoreMotion::isSafeToHoist(Instruction *I) const { 337 BasicBlock *Parent = I->getParent(); 338 for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i) { 339 Instruction *Instr = dyn_cast<Instruction>(I->getOperand(i)); 340 if (Instr && Instr->getParent() == Parent) 341 return false; 342 } 343 return true; 344 } 345 346 /// 347 /// \brief Merge two equivalent loads and GEPs and hoist into diamond head 348 /// 349 bool MergedLoadStoreMotion::hoistLoad(BasicBlock *BB, LoadInst *L0, 350 LoadInst *L1) { 351 // Only one definition? 352 Instruction *A0 = dyn_cast<Instruction>(L0->getPointerOperand()); 353 Instruction *A1 = dyn_cast<Instruction>(L1->getPointerOperand()); 354 if (A0 && A1 && A0->isIdenticalTo(A1) && isSafeToHoist(A0) && 355 A0->hasOneUse() && (A0->getParent() == L0->getParent()) && 356 A1->hasOneUse() && (A1->getParent() == L1->getParent()) && 357 isa<GetElementPtrInst>(A0)) { 358 DEBUG(dbgs() << "Hoist Instruction into BB \n"; BB->dump(); 359 dbgs() << "Instruction Left\n"; L0->dump(); dbgs() << "\n"; 360 dbgs() << "Instruction Right\n"; L1->dump(); dbgs() << "\n"); 361 hoistInstruction(BB, A0, A1); 362 hoistInstruction(BB, L0, L1); 363 return true; 364 } else 365 return false; 366 } 367 368 /// 369 /// \brief Try to hoist two loads to same address into diamond header 370 /// 371 /// Starting from a diamond head block, iterate over the instructions in one 372 /// successor block and try to match a load in the second successor. 373 /// 374 bool MergedLoadStoreMotion::mergeLoads(BasicBlock *BB) { 375 bool MergedLoads = false; 376 assert(isDiamondHead(BB)); 377 BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator()); 378 BasicBlock *Succ0 = BI->getSuccessor(0); 379 BasicBlock *Succ1 = BI->getSuccessor(1); 380 // #Instructions in Succ1 for Compile Time Control 381 int Size1 = Succ1->size(); 382 int NLoads = 0; 383 for (BasicBlock::iterator BBI = Succ0->begin(), BBE = Succ0->end(); 384 BBI != BBE;) { 385 386 Instruction *I = BBI; 387 ++BBI; 388 if (isLoadHoistBarrier(I)) 389 break; 390 391 // Only move non-simple (atomic, volatile) loads. 392 if (!isa<LoadInst>(I)) 393 continue; 394 395 LoadInst *L0 = (LoadInst *)I; 396 if (!L0->isSimple()) 397 continue; 398 399 ++NLoads; 400 if (NLoads * Size1 >= MagicCompileTimeControl) 401 break; 402 if (LoadInst *L1 = canHoistFromBlock(Succ1, L0)) { 403 bool Res = hoistLoad(BB, L0, L1); 404 MergedLoads |= Res; 405 // Don't attempt to hoist above loads that had not been hoisted. 406 if (!Res) 407 break; 408 } 409 } 410 return MergedLoads; 411 } 412 413 /// 414 /// \brief True when instruction is sink barrier for a store 415 /// 416 bool MergedLoadStoreMotion::isStoreSinkBarrier(Instruction *Inst) { 417 // FIXME: Conservatively let a load instruction block the store. 418 // Use alias analysis instead. 419 if (isa<LoadInst>(Inst)) 420 return true; 421 if (isa<CallInst>(Inst)) 422 return true; 423 if (isa<TerminatorInst>(Inst) && !isa<BranchInst>(Inst)) 424 return true; 425 // Note: mayHaveSideEffects covers all instructions that could 426 // trigger a change to state. Eg. in-flight stores have to be executed 427 // before ordered loads or fences, calls could invoke functions that store 428 // data to memory etc. 429 if (!isa<StoreInst>(Inst) && Inst->mayHaveSideEffects()) { 430 return true; 431 } 432 DEBUG(dbgs() << "No Sink Barrier\n"); 433 return false; 434 } 435 436 /// 437 /// \brief Check if \p BB contains a store to the same address as \p SI 438 /// 439 /// \return The store in \p when it is safe to sink. Otherwise return Null. 440 /// 441 StoreInst *MergedLoadStoreMotion::canSinkFromBlock(BasicBlock *BB, 442 StoreInst *SI) { 443 StoreInst *I = 0; 444 DEBUG(dbgs() << "can Sink? : "; SI->dump(); dbgs() << "\n"); 445 for (BasicBlock::reverse_iterator RBI = BB->rbegin(), RBE = BB->rend(); 446 RBI != RBE; ++RBI) { 447 Instruction *Inst = &*RBI; 448 449 // Only move loads if they are used in the block. 450 if (isStoreSinkBarrier(Inst)) 451 break; 452 if (isa<StoreInst>(Inst)) { 453 AliasAnalysis::Location LocSI = AA->getLocation(SI); 454 AliasAnalysis::Location LocInst = AA->getLocation((StoreInst *)Inst); 455 if (AA->isMustAlias(LocSI, LocInst)) { 456 I = (StoreInst *)Inst; 457 break; 458 } 459 } 460 } 461 return I; 462 } 463 464 /// 465 /// \brief Create a PHI node in BB for the operands of S0 and S1 466 /// 467 PHINode *MergedLoadStoreMotion::getPHIOperand(BasicBlock *BB, StoreInst *S0, 468 StoreInst *S1) { 469 // Create a phi if the values mismatch. 470 PHINode *NewPN = 0; 471 Value *Opd1 = S0->getValueOperand(); 472 Value *Opd2 = S1->getValueOperand(); 473 if (Opd1 != Opd2) { 474 NewPN = PHINode::Create(Opd1->getType(), 2, Opd2->getName() + ".sink", 475 BB->begin()); 476 NewPN->addIncoming(Opd1, S0->getParent()); 477 NewPN->addIncoming(Opd2, S1->getParent()); 478 if (NewPN->getType()->getScalarType()->isPointerTy()) { 479 // Notify AA of the new value. 480 AA->copyValue(Opd1, NewPN); 481 AA->copyValue(Opd2, NewPN); 482 // AA needs to be informed when a PHI-use of the pointer value is added 483 for (unsigned I = 0, E = NewPN->getNumIncomingValues(); I != E; ++I) { 484 unsigned J = PHINode::getOperandNumForIncomingValue(I); 485 AA->addEscapingUse(NewPN->getOperandUse(J)); 486 } 487 if (MD) 488 MD->invalidateCachedPointerInfo(NewPN); 489 } 490 } 491 return NewPN; 492 } 493 494 /// 495 /// \brief Merge two stores to same address and sink into \p BB 496 /// 497 /// Also sinks GEP instruction computing the store address 498 /// 499 bool MergedLoadStoreMotion::sinkStore(BasicBlock *BB, StoreInst *S0, 500 StoreInst *S1) { 501 // Only one definition? 502 Instruction *A0 = dyn_cast<Instruction>(S0->getPointerOperand()); 503 Instruction *A1 = dyn_cast<Instruction>(S1->getPointerOperand()); 504 if (A0 && A1 && A0->isIdenticalTo(A1) && A0->hasOneUse() && 505 (A0->getParent() == S0->getParent()) && A1->hasOneUse() && 506 (A1->getParent() == S1->getParent()) && isa<GetElementPtrInst>(A0)) { 507 DEBUG(dbgs() << "Sink Instruction into BB \n"; BB->dump(); 508 dbgs() << "Instruction Left\n"; S0->dump(); dbgs() << "\n"; 509 dbgs() << "Instruction Right\n"; S1->dump(); dbgs() << "\n"); 510 // Hoist the instruction. 511 BasicBlock::iterator InsertPt = BB->getFirstInsertionPt(); 512 // Intersect optional metadata. 513 S0->intersectOptionalDataWith(S1); 514 S0->dropUnknownMetadata(); 515 516 // Create the new store to be inserted at the join point. 517 StoreInst *SNew = (StoreInst *)(S0->clone()); 518 Instruction *ANew = A0->clone(); 519 AA->copyValue(S0, SNew); 520 SNew->insertBefore(InsertPt); 521 ANew->insertBefore(SNew); 522 523 assert(S0->getParent() == A0->getParent()); 524 assert(S1->getParent() == A1->getParent()); 525 526 PHINode *NewPN = getPHIOperand(BB, S0, S1); 527 // New PHI operand? Use it. 528 if (NewPN) 529 SNew->setOperand(0, NewPN); 530 removeInstruction(S0); 531 removeInstruction(S1); 532 A0->replaceAllUsesWith(ANew); 533 removeInstruction(A0); 534 A1->replaceAllUsesWith(ANew); 535 removeInstruction(A1); 536 return true; 537 } 538 return false; 539 } 540 541 /// 542 /// \brief True when two stores are equivalent and can sink into the footer 543 /// 544 /// Starting from a diamond tail block, iterate over the instructions in one 545 /// predecessor block and try to match a store in the second predecessor. 546 /// 547 bool MergedLoadStoreMotion::mergeStores(BasicBlock *T) { 548 549 bool MergedStores = false; 550 assert(T && "Footer of a diamond cannot be empty"); 551 552 pred_iterator PI = pred_begin(T), E = pred_end(T); 553 assert(PI != E); 554 BasicBlock *Pred0 = *PI; 555 ++PI; 556 BasicBlock *Pred1 = *PI; 557 ++PI; 558 // tail block of a diamond/hammock? 559 if (Pred0 == Pred1) 560 return false; // No. 561 if (PI != E) 562 return false; // No. More than 2 predecessors. 563 564 // #Instructions in Succ1 for Compile Time Control 565 int Size1 = Pred1->size(); 566 int NStores = 0; 567 568 for (BasicBlock::reverse_iterator RBI = Pred0->rbegin(), RBE = Pred0->rend(); 569 RBI != RBE;) { 570 571 Instruction *I = &*RBI; 572 ++RBI; 573 if (isStoreSinkBarrier(I)) 574 break; 575 // Sink move non-simple (atomic, volatile) stores 576 if (!isa<StoreInst>(I)) 577 continue; 578 StoreInst *S0 = (StoreInst *)I; 579 if (!S0->isSimple()) 580 continue; 581 582 ++NStores; 583 if (NStores * Size1 >= MagicCompileTimeControl) 584 break; 585 if (StoreInst *S1 = canSinkFromBlock(Pred1, S0)) { 586 bool Res = sinkStore(T, S0, S1); 587 MergedStores |= Res; 588 // Don't attempt to sink below stores that had to stick around 589 // But after removal of a store and some of its feeding 590 // instruction search again from the beginning since the iterator 591 // is likely stale at this point. 592 if (!Res) 593 break; 594 else { 595 RBI = Pred0->rbegin(); 596 RBE = Pred0->rend(); 597 DEBUG(dbgs() << "Search again\n"; Instruction *I = &*RBI; I->dump()); 598 } 599 } 600 } 601 return MergedStores; 602 } 603 /// 604 /// \brief Run the transformation for each function 605 /// 606 bool MergedLoadStoreMotion::runOnFunction(Function &F) { 607 MD = &getAnalysis<MemoryDependenceAnalysis>(); 608 AA = &getAnalysis<AliasAnalysis>(); 609 610 bool Changed = false; 611 DEBUG(dbgs() << "Instruction Merger\n"); 612 613 // Merge unconditional branches, allowing PRE to catch more 614 // optimization opportunities. 615 for (Function::iterator FI = F.begin(), FE = F.end(); FI != FE;) { 616 BasicBlock *BB = FI++; 617 618 // Hoist equivalent loads and sink stores 619 // outside diamonds when possible 620 if (isDiamondHead(BB)) { 621 Changed |= mergeLoads(BB); 622 Changed |= mergeStores(getDiamondTail(BB)); 623 } 624 } 625 return Changed; 626 } 627