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      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/MergedLoadStoreMotion.h"
     76 #include "llvm/ADT/Statistic.h"
     77 #include "llvm/Analysis/AliasAnalysis.h"
     78 #include "llvm/Analysis/CFG.h"
     79 #include "llvm/Analysis/GlobalsModRef.h"
     80 #include "llvm/Analysis/Loads.h"
     81 #include "llvm/Analysis/MemoryBuiltins.h"
     82 #include "llvm/Analysis/MemoryDependenceAnalysis.h"
     83 #include "llvm/Analysis/ValueTracking.h"
     84 #include "llvm/IR/Metadata.h"
     85 #include "llvm/IR/PatternMatch.h"
     86 #include "llvm/Support/Debug.h"
     87 #include "llvm/Support/raw_ostream.h"
     88 #include "llvm/Transforms/Scalar.h"
     89 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
     90 #include "llvm/Transforms/Utils/SSAUpdater.h"
     91 
     92 using namespace llvm;
     93 
     94 #define DEBUG_TYPE "mldst-motion"
     95 
     96 namespace {
     97 //===----------------------------------------------------------------------===//
     98 //                         MergedLoadStoreMotion Pass
     99 //===----------------------------------------------------------------------===//
    100 class MergedLoadStoreMotion {
    101   MemoryDependenceResults *MD = nullptr;
    102   AliasAnalysis *AA = nullptr;
    103 
    104   // The mergeLoad/Store algorithms could have Size0 * Size1 complexity,
    105   // where Size0 and Size1 are the #instructions on the two sides of
    106   // the diamond. The constant chosen here is arbitrary. Compiler Time
    107   // Control is enforced by the check Size0 * Size1 < MagicCompileTimeControl.
    108   const int MagicCompileTimeControl = 250;
    109 
    110 public:
    111   bool run(Function &F, MemoryDependenceResults *MD, AliasAnalysis &AA);
    112 
    113 private:
    114   ///
    115   /// \brief Remove instruction from parent and update memory dependence
    116   /// analysis.
    117   ///
    118   void removeInstruction(Instruction *Inst);
    119   BasicBlock *getDiamondTail(BasicBlock *BB);
    120   bool isDiamondHead(BasicBlock *BB);
    121   // Routines for hoisting loads
    122   bool isLoadHoistBarrierInRange(const Instruction &Start,
    123                                  const Instruction &End, LoadInst *LI,
    124                                  bool SafeToLoadUnconditionally);
    125   LoadInst *canHoistFromBlock(BasicBlock *BB, LoadInst *LI);
    126   void hoistInstruction(BasicBlock *BB, Instruction *HoistCand,
    127                         Instruction *ElseInst);
    128   bool isSafeToHoist(Instruction *I) const;
    129   bool hoistLoad(BasicBlock *BB, LoadInst *HoistCand, LoadInst *ElseInst);
    130   bool mergeLoads(BasicBlock *BB);
    131   // Routines for sinking stores
    132   StoreInst *canSinkFromBlock(BasicBlock *BB, StoreInst *SI);
    133   PHINode *getPHIOperand(BasicBlock *BB, StoreInst *S0, StoreInst *S1);
    134   bool isStoreSinkBarrierInRange(const Instruction &Start,
    135                                  const Instruction &End, MemoryLocation Loc);
    136   bool sinkStore(BasicBlock *BB, StoreInst *SinkCand, StoreInst *ElseInst);
    137   bool mergeStores(BasicBlock *BB);
    138 };
    139 } // end anonymous namespace
    140 
    141 ///
    142 /// \brief Remove instruction from parent and update memory dependence analysis.
    143 ///
    144 void MergedLoadStoreMotion::removeInstruction(Instruction *Inst) {
    145   // Notify the memory dependence analysis.
    146   if (MD) {
    147     MD->removeInstruction(Inst);
    148     if (auto *LI = dyn_cast<LoadInst>(Inst))
    149       MD->invalidateCachedPointerInfo(LI->getPointerOperand());
    150     if (Inst->getType()->isPtrOrPtrVectorTy()) {
    151       MD->invalidateCachedPointerInfo(Inst);
    152     }
    153   }
    154   Inst->eraseFromParent();
    155 }
    156 
    157 ///
    158 /// \brief Return tail block of a diamond.
    159 ///
    160 BasicBlock *MergedLoadStoreMotion::getDiamondTail(BasicBlock *BB) {
    161   assert(isDiamondHead(BB) && "Basic block is not head of a diamond");
    162   return BB->getTerminator()->getSuccessor(0)->getSingleSuccessor();
    163 }
    164 
    165 ///
    166 /// \brief True when BB is the head of a diamond (hammock)
    167 ///
    168 bool MergedLoadStoreMotion::isDiamondHead(BasicBlock *BB) {
    169   if (!BB)
    170     return false;
    171   auto *BI = dyn_cast<BranchInst>(BB->getTerminator());
    172   if (!BI || !BI->isConditional())
    173     return false;
    174 
    175   BasicBlock *Succ0 = BI->getSuccessor(0);
    176   BasicBlock *Succ1 = BI->getSuccessor(1);
    177 
    178   if (!Succ0->getSinglePredecessor())
    179     return false;
    180   if (!Succ1->getSinglePredecessor())
    181     return false;
    182 
    183   BasicBlock *Succ0Succ = Succ0->getSingleSuccessor();
    184   BasicBlock *Succ1Succ = Succ1->getSingleSuccessor();
    185   // Ignore triangles.
    186   if (!Succ0Succ || !Succ1Succ || Succ0Succ != Succ1Succ)
    187     return false;
    188   return true;
    189 }
    190 
    191 ///
    192 /// \brief True when instruction is a hoist barrier for a load
    193 ///
    194 /// Whenever an instruction could possibly modify the value
    195 /// being loaded or protect against the load from happening
    196 /// it is considered a hoist barrier.
    197 ///
    198 bool MergedLoadStoreMotion::isLoadHoistBarrierInRange(
    199     const Instruction &Start, const Instruction &End, LoadInst *LI,
    200     bool SafeToLoadUnconditionally) {
    201   if (!SafeToLoadUnconditionally)
    202     for (const Instruction &Inst :
    203          make_range(Start.getIterator(), End.getIterator()))
    204       if (!isGuaranteedToTransferExecutionToSuccessor(&Inst))
    205         return true;
    206   MemoryLocation Loc = MemoryLocation::get(LI);
    207   return AA->canInstructionRangeModRef(Start, End, Loc, MRI_Mod);
    208 }
    209 
    210 ///
    211 /// \brief Decide if a load can be hoisted
    212 ///
    213 /// When there is a load in \p BB to the same address as \p LI
    214 /// and it can be hoisted from \p BB, return that load.
    215 /// Otherwise return Null.
    216 ///
    217 LoadInst *MergedLoadStoreMotion::canHoistFromBlock(BasicBlock *BB1,
    218                                                    LoadInst *Load0) {
    219   BasicBlock *BB0 = Load0->getParent();
    220   BasicBlock *Head = BB0->getSinglePredecessor();
    221   bool SafeToLoadUnconditionally = isSafeToLoadUnconditionally(
    222       Load0->getPointerOperand(), Load0->getAlignment(),
    223       Load0->getModule()->getDataLayout(),
    224       /*ScanFrom=*/Head->getTerminator());
    225   for (BasicBlock::iterator BBI = BB1->begin(), BBE = BB1->end(); BBI != BBE;
    226        ++BBI) {
    227     Instruction *Inst = &*BBI;
    228 
    229     // Only merge and hoist loads when their result in used only in BB
    230     auto *Load1 = dyn_cast<LoadInst>(Inst);
    231     if (!Load1 || Inst->isUsedOutsideOfBlock(BB1))
    232       continue;
    233 
    234     MemoryLocation Loc0 = MemoryLocation::get(Load0);
    235     MemoryLocation Loc1 = MemoryLocation::get(Load1);
    236     if (Load0->isSameOperationAs(Load1) && AA->isMustAlias(Loc0, Loc1) &&
    237         !isLoadHoistBarrierInRange(BB1->front(), *Load1, Load1,
    238                                    SafeToLoadUnconditionally) &&
    239         !isLoadHoistBarrierInRange(BB0->front(), *Load0, Load0,
    240                                    SafeToLoadUnconditionally)) {
    241       return Load1;
    242     }
    243   }
    244   return nullptr;
    245 }
    246 
    247 ///
    248 /// \brief Merge two equivalent instructions \p HoistCand and \p ElseInst into
    249 /// \p BB
    250 ///
    251 /// BB is the head of a diamond
    252 ///
    253 void MergedLoadStoreMotion::hoistInstruction(BasicBlock *BB,
    254                                              Instruction *HoistCand,
    255                                              Instruction *ElseInst) {
    256   DEBUG(dbgs() << " Hoist Instruction into BB \n"; BB->dump();
    257         dbgs() << "Instruction Left\n"; HoistCand->dump(); dbgs() << "\n";
    258         dbgs() << "Instruction Right\n"; ElseInst->dump(); dbgs() << "\n");
    259   // Hoist the instruction.
    260   assert(HoistCand->getParent() != BB);
    261 
    262   // Intersect optional metadata.
    263   HoistCand->intersectOptionalDataWith(ElseInst);
    264   HoistCand->dropUnknownNonDebugMetadata();
    265 
    266   // Prepend point for instruction insert
    267   Instruction *HoistPt = BB->getTerminator();
    268 
    269   // Merged instruction
    270   Instruction *HoistedInst = HoistCand->clone();
    271 
    272   // Hoist instruction.
    273   HoistedInst->insertBefore(HoistPt);
    274 
    275   HoistCand->replaceAllUsesWith(HoistedInst);
    276   removeInstruction(HoistCand);
    277   // Replace the else block instruction.
    278   ElseInst->replaceAllUsesWith(HoistedInst);
    279   removeInstruction(ElseInst);
    280 }
    281 
    282 ///
    283 /// \brief Return true if no operand of \p I is defined in I's parent block
    284 ///
    285 bool MergedLoadStoreMotion::isSafeToHoist(Instruction *I) const {
    286   BasicBlock *Parent = I->getParent();
    287   for (Use &U : I->operands())
    288     if (auto *Instr = dyn_cast<Instruction>(&U))
    289       if (Instr->getParent() == Parent)
    290         return false;
    291   return true;
    292 }
    293 
    294 ///
    295 /// \brief Merge two equivalent loads and GEPs and hoist into diamond head
    296 ///
    297 bool MergedLoadStoreMotion::hoistLoad(BasicBlock *BB, LoadInst *L0,
    298                                       LoadInst *L1) {
    299   // Only one definition?
    300   auto *A0 = dyn_cast<Instruction>(L0->getPointerOperand());
    301   auto *A1 = dyn_cast<Instruction>(L1->getPointerOperand());
    302   if (A0 && A1 && A0->isIdenticalTo(A1) && isSafeToHoist(A0) &&
    303       A0->hasOneUse() && (A0->getParent() == L0->getParent()) &&
    304       A1->hasOneUse() && (A1->getParent() == L1->getParent()) &&
    305       isa<GetElementPtrInst>(A0)) {
    306     DEBUG(dbgs() << "Hoist Instruction into BB \n"; BB->dump();
    307           dbgs() << "Instruction Left\n"; L0->dump(); dbgs() << "\n";
    308           dbgs() << "Instruction Right\n"; L1->dump(); dbgs() << "\n");
    309     hoistInstruction(BB, A0, A1);
    310     hoistInstruction(BB, L0, L1);
    311     return true;
    312   }
    313   return false;
    314 }
    315 
    316 ///
    317 /// \brief Try to hoist two loads to same address into diamond header
    318 ///
    319 /// Starting from a diamond head block, iterate over the instructions in one
    320 /// successor block and try to match a load in the second successor.
    321 ///
    322 bool MergedLoadStoreMotion::mergeLoads(BasicBlock *BB) {
    323   bool MergedLoads = false;
    324   assert(isDiamondHead(BB));
    325   BranchInst *BI = cast<BranchInst>(BB->getTerminator());
    326   BasicBlock *Succ0 = BI->getSuccessor(0);
    327   BasicBlock *Succ1 = BI->getSuccessor(1);
    328   // #Instructions in Succ1 for Compile Time Control
    329   int Size1 = Succ1->size();
    330   int NLoads = 0;
    331   for (BasicBlock::iterator BBI = Succ0->begin(), BBE = Succ0->end();
    332        BBI != BBE;) {
    333     Instruction *I = &*BBI;
    334     ++BBI;
    335 
    336     // Don't move non-simple (atomic, volatile) loads.
    337     auto *L0 = dyn_cast<LoadInst>(I);
    338     if (!L0 || !L0->isSimple() || L0->isUsedOutsideOfBlock(Succ0))
    339       continue;
    340 
    341     ++NLoads;
    342     if (NLoads * Size1 >= MagicCompileTimeControl)
    343       break;
    344     if (LoadInst *L1 = canHoistFromBlock(Succ1, L0)) {
    345       bool Res = hoistLoad(BB, L0, L1);
    346       MergedLoads |= Res;
    347       // Don't attempt to hoist above loads that had not been hoisted.
    348       if (!Res)
    349         break;
    350     }
    351   }
    352   return MergedLoads;
    353 }
    354 
    355 ///
    356 /// \brief True when instruction is a sink barrier for a store
    357 /// located in Loc
    358 ///
    359 /// Whenever an instruction could possibly read or modify the
    360 /// value being stored or protect against the store from
    361 /// happening it is considered a sink barrier.
    362 ///
    363 bool MergedLoadStoreMotion::isStoreSinkBarrierInRange(const Instruction &Start,
    364                                                       const Instruction &End,
    365                                                       MemoryLocation Loc) {
    366   for (const Instruction &Inst :
    367        make_range(Start.getIterator(), End.getIterator()))
    368     if (Inst.mayThrow())
    369       return true;
    370   return AA->canInstructionRangeModRef(Start, End, Loc, MRI_ModRef);
    371 }
    372 
    373 ///
    374 /// \brief Check if \p BB contains a store to the same address as \p SI
    375 ///
    376 /// \return The store in \p  when it is safe to sink. Otherwise return Null.
    377 ///
    378 StoreInst *MergedLoadStoreMotion::canSinkFromBlock(BasicBlock *BB1,
    379                                                    StoreInst *Store0) {
    380   DEBUG(dbgs() << "can Sink? : "; Store0->dump(); dbgs() << "\n");
    381   BasicBlock *BB0 = Store0->getParent();
    382   for (Instruction &Inst : reverse(*BB1)) {
    383     auto *Store1 = dyn_cast<StoreInst>(&Inst);
    384     if (!Store1)
    385       continue;
    386 
    387     MemoryLocation Loc0 = MemoryLocation::get(Store0);
    388     MemoryLocation Loc1 = MemoryLocation::get(Store1);
    389     if (AA->isMustAlias(Loc0, Loc1) && Store0->isSameOperationAs(Store1) &&
    390         !isStoreSinkBarrierInRange(*Store1->getNextNode(), BB1->back(), Loc1) &&
    391         !isStoreSinkBarrierInRange(*Store0->getNextNode(), BB0->back(), Loc0)) {
    392       return Store1;
    393     }
    394   }
    395   return nullptr;
    396 }
    397 
    398 ///
    399 /// \brief Create a PHI node in BB for the operands of S0 and S1
    400 ///
    401 PHINode *MergedLoadStoreMotion::getPHIOperand(BasicBlock *BB, StoreInst *S0,
    402                                               StoreInst *S1) {
    403   // Create a phi if the values mismatch.
    404   Value *Opd1 = S0->getValueOperand();
    405   Value *Opd2 = S1->getValueOperand();
    406   if (Opd1 == Opd2)
    407     return nullptr;
    408 
    409   auto *NewPN = PHINode::Create(Opd1->getType(), 2, Opd2->getName() + ".sink",
    410                                 &BB->front());
    411   NewPN->addIncoming(Opd1, S0->getParent());
    412   NewPN->addIncoming(Opd2, S1->getParent());
    413   if (MD && NewPN->getType()->getScalarType()->isPointerTy())
    414     MD->invalidateCachedPointerInfo(NewPN);
    415   return NewPN;
    416 }
    417 
    418 ///
    419 /// \brief Merge two stores to same address and sink into \p BB
    420 ///
    421 /// Also sinks GEP instruction computing the store address
    422 ///
    423 bool MergedLoadStoreMotion::sinkStore(BasicBlock *BB, StoreInst *S0,
    424                                       StoreInst *S1) {
    425   // Only one definition?
    426   auto *A0 = dyn_cast<Instruction>(S0->getPointerOperand());
    427   auto *A1 = dyn_cast<Instruction>(S1->getPointerOperand());
    428   if (A0 && A1 && A0->isIdenticalTo(A1) && A0->hasOneUse() &&
    429       (A0->getParent() == S0->getParent()) && A1->hasOneUse() &&
    430       (A1->getParent() == S1->getParent()) && isa<GetElementPtrInst>(A0)) {
    431     DEBUG(dbgs() << "Sink Instruction into BB \n"; BB->dump();
    432           dbgs() << "Instruction Left\n"; S0->dump(); dbgs() << "\n";
    433           dbgs() << "Instruction Right\n"; S1->dump(); dbgs() << "\n");
    434     // Hoist the instruction.
    435     BasicBlock::iterator InsertPt = BB->getFirstInsertionPt();
    436     // Intersect optional metadata.
    437     S0->intersectOptionalDataWith(S1);
    438     S0->dropUnknownNonDebugMetadata();
    439 
    440     // Create the new store to be inserted at the join point.
    441     StoreInst *SNew = cast<StoreInst>(S0->clone());
    442     Instruction *ANew = A0->clone();
    443     SNew->insertBefore(&*InsertPt);
    444     ANew->insertBefore(SNew);
    445 
    446     assert(S0->getParent() == A0->getParent());
    447     assert(S1->getParent() == A1->getParent());
    448 
    449     // New PHI operand? Use it.
    450     if (PHINode *NewPN = getPHIOperand(BB, S0, S1))
    451       SNew->setOperand(0, NewPN);
    452     removeInstruction(S0);
    453     removeInstruction(S1);
    454     A0->replaceAllUsesWith(ANew);
    455     removeInstruction(A0);
    456     A1->replaceAllUsesWith(ANew);
    457     removeInstruction(A1);
    458     return true;
    459   }
    460   return false;
    461 }
    462 
    463 ///
    464 /// \brief True when two stores are equivalent and can sink into the footer
    465 ///
    466 /// Starting from a diamond tail block, iterate over the instructions in one
    467 /// predecessor block and try to match a store in the second predecessor.
    468 ///
    469 bool MergedLoadStoreMotion::mergeStores(BasicBlock *T) {
    470 
    471   bool MergedStores = false;
    472   assert(T && "Footer of a diamond cannot be empty");
    473 
    474   pred_iterator PI = pred_begin(T), E = pred_end(T);
    475   assert(PI != E);
    476   BasicBlock *Pred0 = *PI;
    477   ++PI;
    478   BasicBlock *Pred1 = *PI;
    479   ++PI;
    480   // tail block  of a diamond/hammock?
    481   if (Pred0 == Pred1)
    482     return false; // No.
    483   if (PI != E)
    484     return false; // No. More than 2 predecessors.
    485 
    486   // #Instructions in Succ1 for Compile Time Control
    487   int Size1 = Pred1->size();
    488   int NStores = 0;
    489 
    490   for (BasicBlock::reverse_iterator RBI = Pred0->rbegin(), RBE = Pred0->rend();
    491        RBI != RBE;) {
    492 
    493     Instruction *I = &*RBI;
    494     ++RBI;
    495 
    496     // Don't sink non-simple (atomic, volatile) stores.
    497     auto *S0 = dyn_cast<StoreInst>(I);
    498     if (!S0 || !S0->isSimple())
    499       continue;
    500 
    501     ++NStores;
    502     if (NStores * Size1 >= MagicCompileTimeControl)
    503       break;
    504     if (StoreInst *S1 = canSinkFromBlock(Pred1, S0)) {
    505       bool Res = sinkStore(T, S0, S1);
    506       MergedStores |= Res;
    507       // Don't attempt to sink below stores that had to stick around
    508       // But after removal of a store and some of its feeding
    509       // instruction search again from the beginning since the iterator
    510       // is likely stale at this point.
    511       if (!Res)
    512         break;
    513       RBI = Pred0->rbegin();
    514       RBE = Pred0->rend();
    515       DEBUG(dbgs() << "Search again\n"; Instruction *I = &*RBI; I->dump());
    516     }
    517   }
    518   return MergedStores;
    519 }
    520 
    521 bool MergedLoadStoreMotion::run(Function &F, MemoryDependenceResults *MD,
    522                                 AliasAnalysis &AA) {
    523   this->MD = MD;
    524   this->AA = &AA;
    525 
    526   bool Changed = false;
    527   DEBUG(dbgs() << "Instruction Merger\n");
    528 
    529   // Merge unconditional branches, allowing PRE to catch more
    530   // optimization opportunities.
    531   for (Function::iterator FI = F.begin(), FE = F.end(); FI != FE;) {
    532     BasicBlock *BB = &*FI++;
    533 
    534     // Hoist equivalent loads and sink stores
    535     // outside diamonds when possible
    536     if (isDiamondHead(BB)) {
    537       Changed |= mergeLoads(BB);
    538       Changed |= mergeStores(getDiamondTail(BB));
    539     }
    540   }
    541   return Changed;
    542 }
    543 
    544 namespace {
    545 class MergedLoadStoreMotionLegacyPass : public FunctionPass {
    546 public:
    547   static char ID; // Pass identification, replacement for typeid
    548   MergedLoadStoreMotionLegacyPass() : FunctionPass(ID) {
    549     initializeMergedLoadStoreMotionLegacyPassPass(
    550         *PassRegistry::getPassRegistry());
    551   }
    552 
    553   ///
    554   /// \brief Run the transformation for each function
    555   ///
    556   bool runOnFunction(Function &F) override {
    557     if (skipFunction(F))
    558       return false;
    559     MergedLoadStoreMotion Impl;
    560     auto *MDWP = getAnalysisIfAvailable<MemoryDependenceWrapperPass>();
    561     return Impl.run(F, MDWP ? &MDWP->getMemDep() : nullptr,
    562                     getAnalysis<AAResultsWrapperPass>().getAAResults());
    563   }
    564 
    565 private:
    566   // This transformation requires dominator postdominator info
    567   void getAnalysisUsage(AnalysisUsage &AU) const override {
    568     AU.setPreservesCFG();
    569     AU.addRequired<AAResultsWrapperPass>();
    570     AU.addPreserved<GlobalsAAWrapperPass>();
    571     AU.addPreserved<MemoryDependenceWrapperPass>();
    572   }
    573 };
    574 
    575 char MergedLoadStoreMotionLegacyPass::ID = 0;
    576 } // anonymous namespace
    577 
    578 ///
    579 /// \brief createMergedLoadStoreMotionPass - The public interface to this file.
    580 ///
    581 FunctionPass *llvm::createMergedLoadStoreMotionPass() {
    582   return new MergedLoadStoreMotionLegacyPass();
    583 }
    584 
    585 INITIALIZE_PASS_BEGIN(MergedLoadStoreMotionLegacyPass, "mldst-motion",
    586                       "MergedLoadStoreMotion", false, false)
    587 INITIALIZE_PASS_DEPENDENCY(MemoryDependenceWrapperPass)
    588 INITIALIZE_PASS_DEPENDENCY(AAResultsWrapperPass)
    589 INITIALIZE_PASS_END(MergedLoadStoreMotionLegacyPass, "mldst-motion",
    590                     "MergedLoadStoreMotion", false, false)
    591 
    592 PreservedAnalyses
    593 MergedLoadStoreMotionPass::run(Function &F, AnalysisManager<Function> &AM) {
    594   MergedLoadStoreMotion Impl;
    595   auto *MD = AM.getCachedResult<MemoryDependenceAnalysis>(F);
    596   auto &AA = AM.getResult<AAManager>(F);
    597   if (!Impl.run(F, MD, AA))
    598     return PreservedAnalyses::all();
    599 
    600   // FIXME: This should also 'preserve the CFG'.
    601   PreservedAnalyses PA;
    602   PA.preserve<GlobalsAA>();
    603   PA.preserve<MemoryDependenceAnalysis>();
    604   return PA;
    605 }
    606