| //===- MergedLoadStoreMotion.cpp - merge and hoist/sink load/stores -------===// |
| // |
| // The LLVM Compiler Infrastructure |
| // |
| // This file is distributed under the University of Illinois Open Source |
| // License. See LICENSE.TXT for details. |
| // |
| //===----------------------------------------------------------------------===// |
| // |
| //! \file |
| //! \brief This pass performs merges of loads and stores on both sides of a |
| // diamond (hammock). It hoists the loads and sinks the stores. |
| // |
| // The algorithm iteratively hoists two loads to the same address out of a |
| // diamond (hammock) and merges them into a single load in the header. Similar |
| // it sinks and merges two stores to the tail block (footer). The algorithm |
| // iterates over the instructions of one side of the diamond and attempts to |
| // find a matching load/store on the other side. It hoists / sinks when it |
| // thinks it safe to do so. This optimization helps with eg. hiding load |
| // latencies, triggering if-conversion, and reducing static code size. |
| // |
| //===----------------------------------------------------------------------===// |
| // |
| // |
| // Example: |
| // Diamond shaped code before merge: |
| // |
| // header: |
| // br %cond, label %if.then, label %if.else |
| // + + |
| // + + |
| // + + |
| // if.then: if.else: |
| // %lt = load %addr_l %le = load %addr_l |
| // <use %lt> <use %le> |
| // <...> <...> |
| // store %st, %addr_s store %se, %addr_s |
| // br label %if.end br label %if.end |
| // + + |
| // + + |
| // + + |
| // if.end ("footer"): |
| // <...> |
| // |
| // Diamond shaped code after merge: |
| // |
| // header: |
| // %l = load %addr_l |
| // br %cond, label %if.then, label %if.else |
| // + + |
| // + + |
| // + + |
| // if.then: if.else: |
| // <use %l> <use %l> |
| // <...> <...> |
| // br label %if.end br label %if.end |
| // + + |
| // + + |
| // + + |
| // if.end ("footer"): |
| // %s.sink = phi [%st, if.then], [%se, if.else] |
| // <...> |
| // store %s.sink, %addr_s |
| // <...> |
| // |
| // |
| //===----------------------- TODO -----------------------------------------===// |
| // |
| // 1) Generalize to regions other than diamonds |
| // 2) Be more aggressive merging memory operations |
| // Note that both changes require register pressure control |
| // |
| //===----------------------------------------------------------------------===// |
| |
| #include "llvm/Transforms/Scalar.h" |
| #include "llvm/ADT/SetVector.h" |
| #include "llvm/ADT/SmallPtrSet.h" |
| #include "llvm/ADT/Statistic.h" |
| #include "llvm/Analysis/AliasAnalysis.h" |
| #include "llvm/Analysis/CFG.h" |
| #include "llvm/Analysis/GlobalsModRef.h" |
| #include "llvm/Analysis/Loads.h" |
| #include "llvm/Analysis/MemoryBuiltins.h" |
| #include "llvm/Analysis/MemoryDependenceAnalysis.h" |
| #include "llvm/Analysis/TargetLibraryInfo.h" |
| #include "llvm/IR/Metadata.h" |
| #include "llvm/IR/PatternMatch.h" |
| #include "llvm/Support/Allocator.h" |
| #include "llvm/Support/CommandLine.h" |
| #include "llvm/Support/Debug.h" |
| #include "llvm/Support/raw_ostream.h" |
| #include "llvm/Transforms/Utils/BasicBlockUtils.h" |
| #include "llvm/Transforms/Utils/SSAUpdater.h" |
| #include <vector> |
| |
| using namespace llvm; |
| |
| #define DEBUG_TYPE "mldst-motion" |
| |
| //===----------------------------------------------------------------------===// |
| // MergedLoadStoreMotion Pass |
| //===----------------------------------------------------------------------===// |
| |
| namespace { |
| class MergedLoadStoreMotion : public FunctionPass { |
| AliasAnalysis *AA; |
| MemoryDependenceAnalysis *MD; |
| |
| public: |
| static char ID; // Pass identification, replacement for typeid |
| MergedLoadStoreMotion() |
| : FunctionPass(ID), MD(nullptr), MagicCompileTimeControl(250) { |
| initializeMergedLoadStoreMotionPass(*PassRegistry::getPassRegistry()); |
| } |
| |
| bool runOnFunction(Function &F) override; |
| |
| private: |
| // This transformation requires dominator postdominator info |
| void getAnalysisUsage(AnalysisUsage &AU) const override { |
| AU.setPreservesCFG(); |
| AU.addRequired<TargetLibraryInfoWrapperPass>(); |
| AU.addRequired<AAResultsWrapperPass>(); |
| AU.addPreserved<GlobalsAAWrapperPass>(); |
| AU.addPreserved<MemoryDependenceAnalysis>(); |
| } |
| |
| // Helper routines |
| |
| /// |
| /// \brief Remove instruction from parent and update memory dependence |
| /// analysis. |
| /// |
| void removeInstruction(Instruction *Inst); |
| BasicBlock *getDiamondTail(BasicBlock *BB); |
| bool isDiamondHead(BasicBlock *BB); |
| // Routines for hoisting loads |
| bool isLoadHoistBarrierInRange(const Instruction& Start, |
| const Instruction& End, |
| LoadInst* LI); |
| LoadInst *canHoistFromBlock(BasicBlock *BB, LoadInst *LI); |
| void hoistInstruction(BasicBlock *BB, Instruction *HoistCand, |
| Instruction *ElseInst); |
| bool isSafeToHoist(Instruction *I) const; |
| bool hoistLoad(BasicBlock *BB, LoadInst *HoistCand, LoadInst *ElseInst); |
| bool mergeLoads(BasicBlock *BB); |
| // Routines for sinking stores |
| StoreInst *canSinkFromBlock(BasicBlock *BB, StoreInst *SI); |
| PHINode *getPHIOperand(BasicBlock *BB, StoreInst *S0, StoreInst *S1); |
| bool isStoreSinkBarrierInRange(const Instruction &Start, |
| const Instruction &End, MemoryLocation Loc); |
| bool sinkStore(BasicBlock *BB, StoreInst *SinkCand, StoreInst *ElseInst); |
| bool mergeStores(BasicBlock *BB); |
| // The mergeLoad/Store algorithms could have Size0 * Size1 complexity, |
| // where Size0 and Size1 are the #instructions on the two sides of |
| // the diamond. The constant chosen here is arbitrary. Compiler Time |
| // Control is enforced by the check Size0 * Size1 < MagicCompileTimeControl. |
| const int MagicCompileTimeControl; |
| }; |
| |
| char MergedLoadStoreMotion::ID = 0; |
| } // anonymous namespace |
| |
| /// |
| /// \brief createMergedLoadStoreMotionPass - The public interface to this file. |
| /// |
| FunctionPass *llvm::createMergedLoadStoreMotionPass() { |
| return new MergedLoadStoreMotion(); |
| } |
| |
| INITIALIZE_PASS_BEGIN(MergedLoadStoreMotion, "mldst-motion", |
| "MergedLoadStoreMotion", false, false) |
| INITIALIZE_PASS_DEPENDENCY(MemoryDependenceAnalysis) |
| INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfoWrapperPass) |
| INITIALIZE_PASS_DEPENDENCY(AAResultsWrapperPass) |
| INITIALIZE_PASS_DEPENDENCY(GlobalsAAWrapperPass) |
| INITIALIZE_PASS_END(MergedLoadStoreMotion, "mldst-motion", |
| "MergedLoadStoreMotion", false, false) |
| |
| /// |
| /// \brief Remove instruction from parent and update memory dependence analysis. |
| /// |
| void MergedLoadStoreMotion::removeInstruction(Instruction *Inst) { |
| // Notify the memory dependence analysis. |
| if (MD) { |
| MD->removeInstruction(Inst); |
| if (LoadInst *LI = dyn_cast<LoadInst>(Inst)) |
| MD->invalidateCachedPointerInfo(LI->getPointerOperand()); |
| if (Inst->getType()->getScalarType()->isPointerTy()) { |
| MD->invalidateCachedPointerInfo(Inst); |
| } |
| } |
| Inst->eraseFromParent(); |
| } |
| |
| /// |
| /// \brief Return tail block of a diamond. |
| /// |
| BasicBlock *MergedLoadStoreMotion::getDiamondTail(BasicBlock *BB) { |
| assert(isDiamondHead(BB) && "Basic block is not head of a diamond"); |
| BranchInst *BI = (BranchInst *)(BB->getTerminator()); |
| BasicBlock *Succ0 = BI->getSuccessor(0); |
| BasicBlock *Tail = Succ0->getTerminator()->getSuccessor(0); |
| return Tail; |
| } |
| |
| /// |
| /// \brief True when BB is the head of a diamond (hammock) |
| /// |
| bool MergedLoadStoreMotion::isDiamondHead(BasicBlock *BB) { |
| if (!BB) |
| return false; |
| if (!isa<BranchInst>(BB->getTerminator())) |
| return false; |
| if (BB->getTerminator()->getNumSuccessors() != 2) |
| return false; |
| |
| BranchInst *BI = (BranchInst *)(BB->getTerminator()); |
| BasicBlock *Succ0 = BI->getSuccessor(0); |
| BasicBlock *Succ1 = BI->getSuccessor(1); |
| |
| if (!Succ0->getSinglePredecessor() || |
| Succ0->getTerminator()->getNumSuccessors() != 1) |
| return false; |
| if (!Succ1->getSinglePredecessor() || |
| Succ1->getTerminator()->getNumSuccessors() != 1) |
| return false; |
| |
| BasicBlock *Tail = Succ0->getTerminator()->getSuccessor(0); |
| // Ignore triangles. |
| if (Succ1->getTerminator()->getSuccessor(0) != Tail) |
| return false; |
| return true; |
| } |
| |
| /// |
| /// \brief True when instruction is a hoist barrier for a load |
| /// |
| /// Whenever an instruction could possibly modify the value |
| /// being loaded or protect against the load from happening |
| /// it is considered a hoist barrier. |
| /// |
| bool MergedLoadStoreMotion::isLoadHoistBarrierInRange(const Instruction& Start, |
| const Instruction& End, |
| LoadInst* LI) { |
| MemoryLocation Loc = MemoryLocation::get(LI); |
| return AA->canInstructionRangeModRef(Start, End, Loc, MRI_Mod); |
| } |
| |
| /// |
| /// \brief Decide if a load can be hoisted |
| /// |
| /// When there is a load in \p BB to the same address as \p LI |
| /// and it can be hoisted from \p BB, return that load. |
| /// Otherwise return Null. |
| /// |
| LoadInst *MergedLoadStoreMotion::canHoistFromBlock(BasicBlock *BB1, |
| LoadInst *Load0) { |
| |
| for (BasicBlock::iterator BBI = BB1->begin(), BBE = BB1->end(); BBI != BBE; |
| ++BBI) { |
| Instruction *Inst = &*BBI; |
| |
| // Only merge and hoist loads when their result in used only in BB |
| if (!isa<LoadInst>(Inst) || Inst->isUsedOutsideOfBlock(BB1)) |
| continue; |
| |
| LoadInst *Load1 = dyn_cast<LoadInst>(Inst); |
| BasicBlock *BB0 = Load0->getParent(); |
| |
| MemoryLocation Loc0 = MemoryLocation::get(Load0); |
| MemoryLocation Loc1 = MemoryLocation::get(Load1); |
| if (AA->isMustAlias(Loc0, Loc1) && Load0->isSameOperationAs(Load1) && |
| !isLoadHoistBarrierInRange(BB1->front(), *Load1, Load1) && |
| !isLoadHoistBarrierInRange(BB0->front(), *Load0, Load0)) { |
| return Load1; |
| } |
| } |
| return nullptr; |
| } |
| |
| /// |
| /// \brief Merge two equivalent instructions \p HoistCand and \p ElseInst into |
| /// \p BB |
| /// |
| /// BB is the head of a diamond |
| /// |
| void MergedLoadStoreMotion::hoistInstruction(BasicBlock *BB, |
| Instruction *HoistCand, |
| Instruction *ElseInst) { |
| DEBUG(dbgs() << " Hoist Instruction into BB \n"; BB->dump(); |
| dbgs() << "Instruction Left\n"; HoistCand->dump(); dbgs() << "\n"; |
| dbgs() << "Instruction Right\n"; ElseInst->dump(); dbgs() << "\n"); |
| // Hoist the instruction. |
| assert(HoistCand->getParent() != BB); |
| |
| // Intersect optional metadata. |
| HoistCand->intersectOptionalDataWith(ElseInst); |
| HoistCand->dropUnknownNonDebugMetadata(); |
| |
| // Prepend point for instruction insert |
| Instruction *HoistPt = BB->getTerminator(); |
| |
| // Merged instruction |
| Instruction *HoistedInst = HoistCand->clone(); |
| |
| // Hoist instruction. |
| HoistedInst->insertBefore(HoistPt); |
| |
| HoistCand->replaceAllUsesWith(HoistedInst); |
| removeInstruction(HoistCand); |
| // Replace the else block instruction. |
| ElseInst->replaceAllUsesWith(HoistedInst); |
| removeInstruction(ElseInst); |
| } |
| |
| /// |
| /// \brief Return true if no operand of \p I is defined in I's parent block |
| /// |
| bool MergedLoadStoreMotion::isSafeToHoist(Instruction *I) const { |
| BasicBlock *Parent = I->getParent(); |
| for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i) { |
| Instruction *Instr = dyn_cast<Instruction>(I->getOperand(i)); |
| if (Instr && Instr->getParent() == Parent) |
| return false; |
| } |
| return true; |
| } |
| |
| /// |
| /// \brief Merge two equivalent loads and GEPs and hoist into diamond head |
| /// |
| bool MergedLoadStoreMotion::hoistLoad(BasicBlock *BB, LoadInst *L0, |
| LoadInst *L1) { |
| // Only one definition? |
| Instruction *A0 = dyn_cast<Instruction>(L0->getPointerOperand()); |
| Instruction *A1 = dyn_cast<Instruction>(L1->getPointerOperand()); |
| if (A0 && A1 && A0->isIdenticalTo(A1) && isSafeToHoist(A0) && |
| A0->hasOneUse() && (A0->getParent() == L0->getParent()) && |
| A1->hasOneUse() && (A1->getParent() == L1->getParent()) && |
| isa<GetElementPtrInst>(A0)) { |
| DEBUG(dbgs() << "Hoist Instruction into BB \n"; BB->dump(); |
| dbgs() << "Instruction Left\n"; L0->dump(); dbgs() << "\n"; |
| dbgs() << "Instruction Right\n"; L1->dump(); dbgs() << "\n"); |
| hoistInstruction(BB, A0, A1); |
| hoistInstruction(BB, L0, L1); |
| return true; |
| } else |
| return false; |
| } |
| |
| /// |
| /// \brief Try to hoist two loads to same address into diamond header |
| /// |
| /// Starting from a diamond head block, iterate over the instructions in one |
| /// successor block and try to match a load in the second successor. |
| /// |
| bool MergedLoadStoreMotion::mergeLoads(BasicBlock *BB) { |
| bool MergedLoads = false; |
| assert(isDiamondHead(BB)); |
| BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator()); |
| BasicBlock *Succ0 = BI->getSuccessor(0); |
| BasicBlock *Succ1 = BI->getSuccessor(1); |
| // #Instructions in Succ1 for Compile Time Control |
| int Size1 = Succ1->size(); |
| int NLoads = 0; |
| for (BasicBlock::iterator BBI = Succ0->begin(), BBE = Succ0->end(); |
| BBI != BBE;) { |
| Instruction *I = &*BBI; |
| ++BBI; |
| |
| // Only move non-simple (atomic, volatile) loads. |
| LoadInst *L0 = dyn_cast<LoadInst>(I); |
| if (!L0 || !L0->isSimple() || L0->isUsedOutsideOfBlock(Succ0)) |
| continue; |
| |
| ++NLoads; |
| if (NLoads * Size1 >= MagicCompileTimeControl) |
| break; |
| if (LoadInst *L1 = canHoistFromBlock(Succ1, L0)) { |
| bool Res = hoistLoad(BB, L0, L1); |
| MergedLoads |= Res; |
| // Don't attempt to hoist above loads that had not been hoisted. |
| if (!Res) |
| break; |
| } |
| } |
| return MergedLoads; |
| } |
| |
| /// |
| /// \brief True when instruction is a sink barrier for a store |
| /// located in Loc |
| /// |
| /// Whenever an instruction could possibly read or modify the |
| /// value being stored or protect against the store from |
| /// happening it is considered a sink barrier. |
| /// |
| bool MergedLoadStoreMotion::isStoreSinkBarrierInRange(const Instruction &Start, |
| const Instruction &End, |
| MemoryLocation Loc) { |
| return AA->canInstructionRangeModRef(Start, End, Loc, MRI_ModRef); |
| } |
| |
| /// |
| /// \brief Check if \p BB contains a store to the same address as \p SI |
| /// |
| /// \return The store in \p when it is safe to sink. Otherwise return Null. |
| /// |
| StoreInst *MergedLoadStoreMotion::canSinkFromBlock(BasicBlock *BB1, |
| StoreInst *Store0) { |
| DEBUG(dbgs() << "can Sink? : "; Store0->dump(); dbgs() << "\n"); |
| BasicBlock *BB0 = Store0->getParent(); |
| for (BasicBlock::reverse_iterator RBI = BB1->rbegin(), RBE = BB1->rend(); |
| RBI != RBE; ++RBI) { |
| Instruction *Inst = &*RBI; |
| |
| if (!isa<StoreInst>(Inst)) |
| continue; |
| |
| StoreInst *Store1 = cast<StoreInst>(Inst); |
| |
| MemoryLocation Loc0 = MemoryLocation::get(Store0); |
| MemoryLocation Loc1 = MemoryLocation::get(Store1); |
| if (AA->isMustAlias(Loc0, Loc1) && Store0->isSameOperationAs(Store1) && |
| !isStoreSinkBarrierInRange(*(std::next(BasicBlock::iterator(Store1))), |
| BB1->back(), Loc1) && |
| !isStoreSinkBarrierInRange(*(std::next(BasicBlock::iterator(Store0))), |
| BB0->back(), Loc0)) { |
| return Store1; |
| } |
| } |
| return nullptr; |
| } |
| |
| /// |
| /// \brief Create a PHI node in BB for the operands of S0 and S1 |
| /// |
| PHINode *MergedLoadStoreMotion::getPHIOperand(BasicBlock *BB, StoreInst *S0, |
| StoreInst *S1) { |
| // Create a phi if the values mismatch. |
| PHINode *NewPN = nullptr; |
| Value *Opd1 = S0->getValueOperand(); |
| Value *Opd2 = S1->getValueOperand(); |
| if (Opd1 != Opd2) { |
| NewPN = PHINode::Create(Opd1->getType(), 2, Opd2->getName() + ".sink", |
| &BB->front()); |
| NewPN->addIncoming(Opd1, S0->getParent()); |
| NewPN->addIncoming(Opd2, S1->getParent()); |
| if (MD && NewPN->getType()->getScalarType()->isPointerTy()) |
| MD->invalidateCachedPointerInfo(NewPN); |
| } |
| return NewPN; |
| } |
| |
| /// |
| /// \brief Merge two stores to same address and sink into \p BB |
| /// |
| /// Also sinks GEP instruction computing the store address |
| /// |
| bool MergedLoadStoreMotion::sinkStore(BasicBlock *BB, StoreInst *S0, |
| StoreInst *S1) { |
| // Only one definition? |
| Instruction *A0 = dyn_cast<Instruction>(S0->getPointerOperand()); |
| Instruction *A1 = dyn_cast<Instruction>(S1->getPointerOperand()); |
| if (A0 && A1 && A0->isIdenticalTo(A1) && A0->hasOneUse() && |
| (A0->getParent() == S0->getParent()) && A1->hasOneUse() && |
| (A1->getParent() == S1->getParent()) && isa<GetElementPtrInst>(A0)) { |
| DEBUG(dbgs() << "Sink Instruction into BB \n"; BB->dump(); |
| dbgs() << "Instruction Left\n"; S0->dump(); dbgs() << "\n"; |
| dbgs() << "Instruction Right\n"; S1->dump(); dbgs() << "\n"); |
| // Hoist the instruction. |
| BasicBlock::iterator InsertPt = BB->getFirstInsertionPt(); |
| // Intersect optional metadata. |
| S0->intersectOptionalDataWith(S1); |
| S0->dropUnknownNonDebugMetadata(); |
| |
| // Create the new store to be inserted at the join point. |
| StoreInst *SNew = (StoreInst *)(S0->clone()); |
| Instruction *ANew = A0->clone(); |
| SNew->insertBefore(&*InsertPt); |
| ANew->insertBefore(SNew); |
| |
| assert(S0->getParent() == A0->getParent()); |
| assert(S1->getParent() == A1->getParent()); |
| |
| PHINode *NewPN = getPHIOperand(BB, S0, S1); |
| // New PHI operand? Use it. |
| if (NewPN) |
| SNew->setOperand(0, NewPN); |
| removeInstruction(S0); |
| removeInstruction(S1); |
| A0->replaceAllUsesWith(ANew); |
| removeInstruction(A0); |
| A1->replaceAllUsesWith(ANew); |
| removeInstruction(A1); |
| return true; |
| } |
| return false; |
| } |
| |
| /// |
| /// \brief True when two stores are equivalent and can sink into the footer |
| /// |
| /// Starting from a diamond tail block, iterate over the instructions in one |
| /// predecessor block and try to match a store in the second predecessor. |
| /// |
| bool MergedLoadStoreMotion::mergeStores(BasicBlock *T) { |
| |
| bool MergedStores = false; |
| assert(T && "Footer of a diamond cannot be empty"); |
| |
| pred_iterator PI = pred_begin(T), E = pred_end(T); |
| assert(PI != E); |
| BasicBlock *Pred0 = *PI; |
| ++PI; |
| BasicBlock *Pred1 = *PI; |
| ++PI; |
| // tail block of a diamond/hammock? |
| if (Pred0 == Pred1) |
| return false; // No. |
| if (PI != E) |
| return false; // No. More than 2 predecessors. |
| |
| // #Instructions in Succ1 for Compile Time Control |
| int Size1 = Pred1->size(); |
| int NStores = 0; |
| |
| for (BasicBlock::reverse_iterator RBI = Pred0->rbegin(), RBE = Pred0->rend(); |
| RBI != RBE;) { |
| |
| Instruction *I = &*RBI; |
| ++RBI; |
| |
| // Sink move non-simple (atomic, volatile) stores |
| if (!isa<StoreInst>(I)) |
| continue; |
| StoreInst *S0 = (StoreInst *)I; |
| if (!S0->isSimple()) |
| continue; |
| |
| ++NStores; |
| if (NStores * Size1 >= MagicCompileTimeControl) |
| break; |
| if (StoreInst *S1 = canSinkFromBlock(Pred1, S0)) { |
| bool Res = sinkStore(T, S0, S1); |
| MergedStores |= Res; |
| // Don't attempt to sink below stores that had to stick around |
| // But after removal of a store and some of its feeding |
| // instruction search again from the beginning since the iterator |
| // is likely stale at this point. |
| if (!Res) |
| break; |
| else { |
| RBI = Pred0->rbegin(); |
| RBE = Pred0->rend(); |
| DEBUG(dbgs() << "Search again\n"; Instruction *I = &*RBI; I->dump()); |
| } |
| } |
| } |
| return MergedStores; |
| } |
| |
| /// |
| /// \brief Run the transformation for each function |
| /// |
| bool MergedLoadStoreMotion::runOnFunction(Function &F) { |
| MD = getAnalysisIfAvailable<MemoryDependenceAnalysis>(); |
| AA = &getAnalysis<AAResultsWrapperPass>().getAAResults(); |
| |
| bool Changed = false; |
| DEBUG(dbgs() << "Instruction Merger\n"); |
| |
| // Merge unconditional branches, allowing PRE to catch more |
| // optimization opportunities. |
| for (Function::iterator FI = F.begin(), FE = F.end(); FI != FE;) { |
| BasicBlock *BB = &*FI++; |
| |
| // Hoist equivalent loads and sink stores |
| // outside diamonds when possible |
| if (isDiamondHead(BB)) { |
| Changed |= mergeLoads(BB); |
| Changed |= mergeStores(getDiamondTail(BB)); |
| } |
| } |
| return Changed; |
| } |
