src/llvm/lib/Transforms/Scalar/MergedLoadStoreMotion.cpp

   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 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(TargetLibraryInfo)
 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 }