| //===- 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/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/ValueTracking.h" |
| #include "llvm/IR/Metadata.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 together with corresponding GEPs |
| /// |
| bool MergedLoadStoreMotion::canSinkStoresAndGEPs(StoreInst *S0, |
| StoreInst *S1) const { |
| auto *A0 = dyn_cast<Instruction>(S0->getPointerOperand()); |
| auto *A1 = dyn_cast<Instruction>(S1->getPointerOperand()); |
| return A0 && A1 && A0->isIdenticalTo(A1) && A0->hasOneUse() && |
| (A0->getParent() == S0->getParent()) && A1->hasOneUse() && |
| (A1->getParent() == S1->getParent()) && isa<GetElementPtrInst>(A0); |
| } |
| |
| /// |
| /// Merge two stores to same address and sink into \p BB |
| /// |
| /// Also sinks GEP instruction computing the store address |
| /// |
| void MergedLoadStoreMotion::sinkStoresAndGEPs(BasicBlock *BB, StoreInst *S0, |
| StoreInst *S1) { |
| // Only one definition? |
| auto *A0 = dyn_cast<Instruction>(S0->getPointerOperand()); |
| auto *A1 = dyn_cast<Instruction>(S1->getPointerOperand()); |
| 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(); |
| |
| // Create the new store to be inserted at the join point. |
| StoreInst *SNew = cast<StoreInst>(S0->clone()); |
| Instruction *ANew = A0->clone(); |
| SNew->insertBefore(&*InsertPt); |
| ANew->insertBefore(SNew); |
| |
| assert(S0->getParent() == A0->getParent()); |
| assert(S1->getParent() == A1->getParent()); |
| |
| // New PHI operand? Use it. |
| if (PHINode *NewPN = getPHIOperand(BB, S0, S1)) |
| SNew->setOperand(0, NewPN); |
| S0->eraseFromParent(); |
| S1->eraseFromParent(); |
| A0->replaceAllUsesWith(ANew); |
| A0->eraseFromParent(); |
| A1->replaceAllUsesWith(ANew); |
| A1->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 (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 |= 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>(); |
| PA.preserve<GlobalsAA>(); |
| return PA; |
| } |