| //===- MergedLoadStoreMotion.cpp - merge and hoist/sink load/stores -------===// | |
| // | |
| // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions. | |
| // See https://llvm.org/LICENSE.txt for license information. | |
| // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception | |
| // | |
| //===----------------------------------------------------------------------===// | |
| // | |
| //! \file | |
| //! 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. New tail/footer block may be | |
| // insterted if the tail/footer block has more predecessors (not only the two | |
| // predecessors that are forming the diamond). 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. | |
| // | |
| // NOTE: This code no longer performs load hoisting, it is subsumed by GVNHoist. | |
| // | |
| //===----------------------------------------------------------------------===// | |
| // | |
| // | |
| // 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/MergedLoadStoreMotion.h" | |
| #include "llvm/Analysis/AliasAnalysis.h" | |
| #include "llvm/Analysis/GlobalsModRef.h" | |
| #include "llvm/IR/Instructions.h" | |
| #include "llvm/InitializePasses.h" | |
| #include "llvm/Support/Debug.h" | |
| #include "llvm/Support/raw_ostream.h" | |
| #include "llvm/Transforms/Scalar.h" | |
| #include "llvm/Transforms/Utils/BasicBlockUtils.h" | |
| using namespace llvm; | |
| #define DEBUG_TYPE "mldst-motion" | |
| namespace { | |
| //===----------------------------------------------------------------------===// | |
| // MergedLoadStoreMotion Pass | |
| //===----------------------------------------------------------------------===// | |
| class MergedLoadStoreMotion { | |
| AliasAnalysis *AA = nullptr; | |
| // 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 = 250; | |
| const bool SplitFooterBB; | |
| public: | |
| MergedLoadStoreMotion(bool SplitFooterBB) : SplitFooterBB(SplitFooterBB) {} | |
| bool run(Function &F, AliasAnalysis &AA); | |
| private: | |
| BasicBlock *getDiamondTail(BasicBlock *BB); | |
| bool isDiamondHead(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 canSinkStoresAndGEPs(StoreInst *S0, StoreInst *S1) const; | |
| void sinkStoresAndGEPs(BasicBlock *BB, StoreInst *SinkCand, | |
| StoreInst *ElseInst); | |
| bool mergeStores(BasicBlock *BB); | |
| }; | |
| } // end anonymous namespace | |
| /// | |
| /// Return tail block of a diamond. | |
| /// | |
| BasicBlock *MergedLoadStoreMotion::getDiamondTail(BasicBlock *BB) { | |
| assert(isDiamondHead(BB) && "Basic block is not head of a diamond"); | |
| return BB->getTerminator()->getSuccessor(0)->getSingleSuccessor(); | |
| } | |
| /// | |
| /// True when BB is the head of a diamond (hammock) | |
| /// | |
| bool MergedLoadStoreMotion::isDiamondHead(BasicBlock *BB) { | |
| if (!BB) | |
| return false; | |
| auto *BI = dyn_cast<BranchInst>(BB->getTerminator()); | |
| if (!BI || !BI->isConditional()) | |
| return false; | |
| BasicBlock *Succ0 = BI->getSuccessor(0); | |
| BasicBlock *Succ1 = BI->getSuccessor(1); | |
| if (!Succ0->getSinglePredecessor()) | |
| return false; | |
| if (!Succ1->getSinglePredecessor()) | |
| return false; | |
| BasicBlock *Succ0Succ = Succ0->getSingleSuccessor(); | |
| BasicBlock *Succ1Succ = Succ1->getSingleSuccessor(); | |
| // Ignore triangles. | |
| if (!Succ0Succ || !Succ1Succ || Succ0Succ != Succ1Succ) | |
| return false; | |
| return true; | |
| } | |
| /// | |
| /// 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) { | |
| for (const Instruction &Inst : | |
| make_range(Start.getIterator(), End.getIterator())) | |
| if (Inst.mayThrow()) | |
| return true; | |
| return AA->canInstructionRangeModRef(Start, End, Loc, ModRefInfo::ModRef); | |
| } | |
| /// | |
| /// 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) { | |
| LLVM_DEBUG(dbgs() << "can Sink? : "; Store0->dump(); dbgs() << "\n"); | |
| BasicBlock *BB0 = Store0->getParent(); | |
| for (Instruction &Inst : reverse(*BB1)) { | |
| auto *Store1 = dyn_cast<StoreInst>(&Inst); | |
| if (!Store1) | |
| continue; | |
| MemoryLocation Loc0 = MemoryLocation::get(Store0); | |
| MemoryLocation Loc1 = MemoryLocation::get(Store1); | |
| if (AA->isMustAlias(Loc0, Loc1) && Store0->isSameOperationAs(Store1) && | |
| !isStoreSinkBarrierInRange(*Store1->getNextNode(), BB1->back(), Loc1) && | |
| !isStoreSinkBarrierInRange(*Store0->getNextNode(), BB0->back(), Loc0)) { | |
| return Store1; | |
| } | |
| } | |
| return nullptr; | |
| } | |
| /// | |
| /// 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. | |
| Value *Opd1 = S0->getValueOperand(); | |
| Value *Opd2 = S1->getValueOperand(); | |
| if (Opd1 == Opd2) | |
| return nullptr; | |
| auto *NewPN = PHINode::Create(Opd1->getType(), 2, Opd2->getName() + ".sink", | |
| &BB->front()); | |
| NewPN->applyMergedLocation(S0->getDebugLoc(), S1->getDebugLoc()); | |
| NewPN->addIncoming(Opd1, S0->getParent()); | |
| NewPN->addIncoming(Opd2, S1->getParent()); | |
| return NewPN; | |
| } | |
| /// | |
| /// Check if 2 stores can be sunk, optionally together with corresponding GEPs. | |
| /// | |
| bool MergedLoadStoreMotion::canSinkStoresAndGEPs(StoreInst *S0, | |
| StoreInst *S1) const { | |
| if (S0->getPointerOperand() == S1->getPointerOperand()) | |
| return true; | |
| auto *GEP0 = dyn_cast<GetElementPtrInst>(S0->getPointerOperand()); | |
| auto *GEP1 = dyn_cast<GetElementPtrInst>(S1->getPointerOperand()); | |
| return GEP0 && GEP1 && GEP0->isIdenticalTo(GEP1) && GEP0->hasOneUse() && | |
| (GEP0->getParent() == S0->getParent()) && GEP1->hasOneUse() && | |
| (GEP1->getParent() == S1->getParent()); | |
| } | |
| /// | |
| /// Merge two stores to same address and sink into \p BB | |
| /// | |
| /// Optionally also sinks GEP instruction computing the store address | |
| /// | |
| void MergedLoadStoreMotion::sinkStoresAndGEPs(BasicBlock *BB, StoreInst *S0, | |
| StoreInst *S1) { | |
| Value *Ptr0 = S0->getPointerOperand(); | |
| Value *Ptr1 = S1->getPointerOperand(); | |
| // Only one definition? | |
| LLVM_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->andIRFlags(S1); | |
| S0->dropUnknownNonDebugMetadata(); | |
| S0->applyMergedLocation(S0->getDebugLoc(), S1->getDebugLoc()); | |
| S0->mergeDIAssignID(S1); | |
| // Create the new store to be inserted at the join point. | |
| StoreInst *SNew = cast<StoreInst>(S0->clone()); | |
| SNew->insertBefore(&*InsertPt); | |
| // New PHI operand? Use it. | |
| if (PHINode *NewPN = getPHIOperand(BB, S0, S1)) | |
| SNew->setOperand(0, NewPN); | |
| S0->eraseFromParent(); | |
| S1->eraseFromParent(); | |
| if (Ptr0 != Ptr1) { | |
| auto *GEP0 = cast<GetElementPtrInst>(Ptr0); | |
| auto *GEP1 = cast<GetElementPtrInst>(Ptr1); | |
| Instruction *GEPNew = GEP0->clone(); | |
| GEPNew->insertBefore(SNew); | |
| GEPNew->applyMergedLocation(GEP0->getDebugLoc(), GEP1->getDebugLoc()); | |
| SNew->setOperand(1, GEPNew); | |
| GEP0->replaceAllUsesWith(GEPNew); | |
| GEP0->eraseFromParent(); | |
| GEP1->replaceAllUsesWith(GEPNew); | |
| GEP1->eraseFromParent(); | |
| } | |
| } | |
| /// | |
| /// True when two stores are equivalent and can sink into the footer | |
| /// | |
| /// Starting from a diamond head block, iterate over the instructions in one | |
| /// successor block and try to match a store in the second successor. | |
| /// | |
| bool MergedLoadStoreMotion::mergeStores(BasicBlock *HeadBB) { | |
| bool MergedStores = false; | |
| BasicBlock *TailBB = getDiamondTail(HeadBB); | |
| BasicBlock *SinkBB = TailBB; | |
| assert(SinkBB && "Footer of a diamond cannot be empty"); | |
| succ_iterator SI = succ_begin(HeadBB); | |
| assert(SI != succ_end(HeadBB) && "Diamond head cannot have zero successors"); | |
| BasicBlock *Pred0 = *SI; | |
| ++SI; | |
| assert(SI != succ_end(HeadBB) && "Diamond head cannot have single successor"); | |
| BasicBlock *Pred1 = *SI; | |
| // tail block of a diamond/hammock? | |
| if (Pred0 == Pred1) | |
| return false; // No. | |
| // bail out early if we can not merge into the footer BB | |
| if (!SplitFooterBB && TailBB->hasNPredecessorsOrMore(3)) | |
| return false; | |
| // #Instructions in Pred1 for Compile Time Control | |
| auto InstsNoDbg = Pred1->instructionsWithoutDebug(); | |
| int Size1 = std::distance(InstsNoDbg.begin(), InstsNoDbg.end()); | |
| int NStores = 0; | |
| for (BasicBlock::reverse_iterator RBI = Pred0->rbegin(), RBE = Pred0->rend(); | |
| RBI != RBE;) { | |
| Instruction *I = &*RBI; | |
| ++RBI; | |
| // Don't sink non-simple (atomic, volatile) stores. | |
| auto *S0 = dyn_cast<StoreInst>(I); | |
| if (!S0 || !S0->isSimple()) | |
| continue; | |
| ++NStores; | |
| if (NStores * Size1 >= MagicCompileTimeControl) | |
| break; | |
| if (StoreInst *S1 = canSinkFromBlock(Pred1, S0)) { | |
| if (!canSinkStoresAndGEPs(S0, S1)) | |
| // 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. | |
| break; | |
| if (SinkBB == TailBB && TailBB->hasNPredecessorsOrMore(3)) { | |
| // We have more than 2 predecessors. Insert a new block | |
| // postdominating 2 predecessors we're going to sink from. | |
| SinkBB = SplitBlockPredecessors(TailBB, {Pred0, Pred1}, ".sink.split"); | |
| if (!SinkBB) | |
| break; | |
| } | |
| MergedStores = true; | |
| sinkStoresAndGEPs(SinkBB, S0, S1); | |
| RBI = Pred0->rbegin(); | |
| RBE = Pred0->rend(); | |
| LLVM_DEBUG(dbgs() << "Search again\n"; Instruction *I = &*RBI; I->dump()); | |
| } | |
| } | |
| return MergedStores; | |
| } | |
| bool MergedLoadStoreMotion::run(Function &F, AliasAnalysis &AA) { | |
| this->AA = &AA; | |
| bool Changed = false; | |
| LLVM_DEBUG(dbgs() << "Instruction Merger\n"); | |
| // Merge unconditional branches, allowing PRE to catch more | |
| // optimization opportunities. | |
| // This loop doesn't care about newly inserted/split blocks | |
| // since they never will be diamond heads. | |
| for (BasicBlock &BB : make_early_inc_range(F)) | |
| // Hoist equivalent loads and sink stores | |
| // outside diamonds when possible | |
| if (isDiamondHead(&BB)) | |
| Changed |= mergeStores(&BB); | |
| return Changed; | |
| } | |
| namespace { | |
| class MergedLoadStoreMotionLegacyPass : public FunctionPass { | |
| const bool SplitFooterBB; | |
| public: | |
| static char ID; // Pass identification, replacement for typeid | |
| MergedLoadStoreMotionLegacyPass(bool SplitFooterBB = false) | |
| : FunctionPass(ID), SplitFooterBB(SplitFooterBB) { | |
| initializeMergedLoadStoreMotionLegacyPassPass( | |
| *PassRegistry::getPassRegistry()); | |
| } | |
| /// | |
| /// Run the transformation for each function | |
| /// | |
| bool runOnFunction(Function &F) override { | |
| if (skipFunction(F)) | |
| return false; | |
| MergedLoadStoreMotion Impl(SplitFooterBB); | |
| return Impl.run(F, getAnalysis<AAResultsWrapperPass>().getAAResults()); | |
| } | |
| private: | |
| void getAnalysisUsage(AnalysisUsage &AU) const override { | |
| if (!SplitFooterBB) | |
| AU.setPreservesCFG(); | |
| AU.addRequired<AAResultsWrapperPass>(); | |
| AU.addPreserved<GlobalsAAWrapperPass>(); | |
| } | |
| }; | |
| char MergedLoadStoreMotionLegacyPass::ID = 0; | |
| } // anonymous namespace | |
| /// | |
| /// createMergedLoadStoreMotionPass - The public interface to this file. | |
| /// | |
| FunctionPass *llvm::createMergedLoadStoreMotionPass(bool SplitFooterBB) { | |
| return new MergedLoadStoreMotionLegacyPass(SplitFooterBB); | |
| } | |
| INITIALIZE_PASS_BEGIN(MergedLoadStoreMotionLegacyPass, "mldst-motion", | |
| "MergedLoadStoreMotion", false, false) | |
| INITIALIZE_PASS_DEPENDENCY(AAResultsWrapperPass) | |
| INITIALIZE_PASS_END(MergedLoadStoreMotionLegacyPass, "mldst-motion", | |
| "MergedLoadStoreMotion", false, false) | |
| PreservedAnalyses | |
| MergedLoadStoreMotionPass::run(Function &F, FunctionAnalysisManager &AM) { | |
| MergedLoadStoreMotion Impl(Options.SplitFooterBB); | |
| auto &AA = AM.getResult<AAManager>(F); | |
| if (!Impl.run(F, AA)) | |
| return PreservedAnalyses::all(); | |
| PreservedAnalyses PA; | |
| if (!Options.SplitFooterBB) | |
| PA.preserveSet<CFGAnalyses>(); | |
| return PA; | |
| } | |
| void MergedLoadStoreMotionPass::printPipeline( | |
| raw_ostream &OS, function_ref<StringRef(StringRef)> MapClassName2PassName) { | |
| static_cast<PassInfoMixin<MergedLoadStoreMotionPass> *>(this)->printPipeline( | |
| OS, MapClassName2PassName); | |
| OS << "<"; | |
| OS << (Options.SplitFooterBB ? "" : "no-") << "split-footer-bb"; | |
| OS << ">"; | |
| } |