| //===- DeadStoreElimination.cpp - Fast Dead Store Elimination -------------===// |
| // |
| // 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 |
| // |
| //===----------------------------------------------------------------------===// |
| // |
| // This file implements a trivial dead store elimination that only considers |
| // basic-block local redundant stores. |
| // |
| // FIXME: This should eventually be extended to be a post-dominator tree |
| // traversal. Doing so would be pretty trivial. |
| // |
| //===----------------------------------------------------------------------===// |
| |
| #include "llvm/Transforms/Scalar/DeadStoreElimination.h" |
| #include "llvm/ADT/APInt.h" |
| #include "llvm/ADT/DenseMap.h" |
| #include "llvm/ADT/MapVector.h" |
| #include "llvm/ADT/PostOrderIterator.h" |
| #include "llvm/ADT/SetVector.h" |
| #include "llvm/ADT/SmallPtrSet.h" |
| #include "llvm/ADT/SmallVector.h" |
| #include "llvm/ADT/Statistic.h" |
| #include "llvm/ADT/StringRef.h" |
| #include "llvm/Analysis/AliasAnalysis.h" |
| #include "llvm/Analysis/CaptureTracking.h" |
| #include "llvm/Analysis/GlobalsModRef.h" |
| #include "llvm/Analysis/MemoryBuiltins.h" |
| #include "llvm/Analysis/MemoryDependenceAnalysis.h" |
| #include "llvm/Analysis/MemoryLocation.h" |
| #include "llvm/Analysis/MemorySSA.h" |
| #include "llvm/Analysis/MemorySSAUpdater.h" |
| #include "llvm/Analysis/PostDominators.h" |
| #include "llvm/Analysis/TargetLibraryInfo.h" |
| #include "llvm/Analysis/ValueTracking.h" |
| #include "llvm/IR/Argument.h" |
| #include "llvm/IR/BasicBlock.h" |
| #include "llvm/IR/Constant.h" |
| #include "llvm/IR/Constants.h" |
| #include "llvm/IR/DataLayout.h" |
| #include "llvm/IR/Dominators.h" |
| #include "llvm/IR/Function.h" |
| #include "llvm/IR/InstIterator.h" |
| #include "llvm/IR/InstrTypes.h" |
| #include "llvm/IR/Instruction.h" |
| #include "llvm/IR/Instructions.h" |
| #include "llvm/IR/IntrinsicInst.h" |
| #include "llvm/IR/Intrinsics.h" |
| #include "llvm/IR/LLVMContext.h" |
| #include "llvm/IR/Module.h" |
| #include "llvm/IR/PassManager.h" |
| #include "llvm/IR/PatternMatch.h" |
| #include "llvm/IR/Value.h" |
| #include "llvm/InitializePasses.h" |
| #include "llvm/Pass.h" |
| #include "llvm/Support/Casting.h" |
| #include "llvm/Support/CommandLine.h" |
| #include "llvm/Support/Debug.h" |
| #include "llvm/Support/DebugCounter.h" |
| #include "llvm/Support/ErrorHandling.h" |
| #include "llvm/Support/MathExtras.h" |
| #include "llvm/Support/raw_ostream.h" |
| #include "llvm/Transforms/Scalar.h" |
| #include "llvm/Transforms/Utils/AssumeBundleBuilder.h" |
| #include "llvm/Transforms/Utils/Local.h" |
| #include <algorithm> |
| #include <cassert> |
| #include <cstddef> |
| #include <cstdint> |
| #include <iterator> |
| #include <map> |
| #include <utility> |
| |
| using namespace llvm; |
| using namespace PatternMatch; |
| |
| #define DEBUG_TYPE "dse" |
| |
| STATISTIC(NumRemainingStores, "Number of stores remaining after DSE"); |
| STATISTIC(NumRedundantStores, "Number of redundant stores deleted"); |
| STATISTIC(NumFastStores, "Number of stores deleted"); |
| STATISTIC(NumFastOther, "Number of other instrs removed"); |
| STATISTIC(NumCompletePartials, "Number of stores dead by later partials"); |
| STATISTIC(NumModifiedStores, "Number of stores modified"); |
| STATISTIC(NumCFGChecks, "Number of stores modified"); |
| STATISTIC(NumCFGTries, "Number of stores modified"); |
| STATISTIC(NumCFGSuccess, "Number of stores modified"); |
| STATISTIC(NumGetDomMemoryDefPassed, |
| "Number of times a valid candidate is returned from getDomMemoryDef"); |
| STATISTIC(NumDomMemDefChecks, |
| "Number iterations check for reads in getDomMemoryDef"); |
| |
| DEBUG_COUNTER(MemorySSACounter, "dse-memoryssa", |
| "Controls which MemoryDefs are eliminated."); |
| |
| static cl::opt<bool> |
| EnablePartialOverwriteTracking("enable-dse-partial-overwrite-tracking", |
| cl::init(true), cl::Hidden, |
| cl::desc("Enable partial-overwrite tracking in DSE")); |
| |
| static cl::opt<bool> |
| EnablePartialStoreMerging("enable-dse-partial-store-merging", |
| cl::init(true), cl::Hidden, |
| cl::desc("Enable partial store merging in DSE")); |
| |
| static cl::opt<bool> |
| EnableMemorySSA("enable-dse-memoryssa", cl::init(true), cl::Hidden, |
| cl::desc("Use the new MemorySSA-backed DSE.")); |
| |
| static cl::opt<unsigned> |
| MemorySSAScanLimit("dse-memoryssa-scanlimit", cl::init(150), cl::Hidden, |
| cl::desc("The number of memory instructions to scan for " |
| "dead store elimination (default = 100)")); |
| static cl::opt<unsigned> MemorySSAUpwardsStepLimit( |
| "dse-memoryssa-walklimit", cl::init(90), cl::Hidden, |
| cl::desc("The maximum number of steps while walking upwards to find " |
| "MemoryDefs that may be killed (default = 90)")); |
| |
| static cl::opt<unsigned> MemorySSAPartialStoreLimit( |
| "dse-memoryssa-partial-store-limit", cl::init(5), cl::Hidden, |
| cl::desc("The maximum number candidates that only partially overwrite the " |
| "killing MemoryDef to consider" |
| " (default = 5)")); |
| |
| static cl::opt<unsigned> MemorySSADefsPerBlockLimit( |
| "dse-memoryssa-defs-per-block-limit", cl::init(5000), cl::Hidden, |
| cl::desc("The number of MemoryDefs we consider as candidates to eliminated " |
| "other stores per basic block (default = 5000)")); |
| |
| static cl::opt<unsigned> MemorySSASameBBStepCost( |
| "dse-memoryssa-samebb-cost", cl::init(1), cl::Hidden, |
| cl::desc( |
| "The cost of a step in the same basic block as the killing MemoryDef" |
| "(default = 1)")); |
| |
| static cl::opt<unsigned> |
| MemorySSAOtherBBStepCost("dse-memoryssa-otherbb-cost", cl::init(5), |
| cl::Hidden, |
| cl::desc("The cost of a step in a different basic " |
| "block than the killing MemoryDef" |
| "(default = 5)")); |
| |
| static cl::opt<unsigned> MemorySSAPathCheckLimit( |
| "dse-memoryssa-path-check-limit", cl::init(50), cl::Hidden, |
| cl::desc("The maximum number of blocks to check when trying to prove that " |
| "all paths to an exit go through a killing block (default = 50)")); |
| |
| //===----------------------------------------------------------------------===// |
| // Helper functions |
| //===----------------------------------------------------------------------===// |
| using OverlapIntervalsTy = std::map<int64_t, int64_t>; |
| using InstOverlapIntervalsTy = DenseMap<Instruction *, OverlapIntervalsTy>; |
| |
| /// Delete this instruction. Before we do, go through and zero out all the |
| /// operands of this instruction. If any of them become dead, delete them and |
| /// the computation tree that feeds them. |
| /// If ValueSet is non-null, remove any deleted instructions from it as well. |
| static void |
| deleteDeadInstruction(Instruction *I, BasicBlock::iterator *BBI, |
| MemoryDependenceResults &MD, const TargetLibraryInfo &TLI, |
| InstOverlapIntervalsTy &IOL, |
| MapVector<Instruction *, bool> &ThrowableInst, |
| SmallSetVector<const Value *, 16> *ValueSet = nullptr) { |
| SmallVector<Instruction*, 32> NowDeadInsts; |
| |
| NowDeadInsts.push_back(I); |
| --NumFastOther; |
| |
| // Keeping the iterator straight is a pain, so we let this routine tell the |
| // caller what the next instruction is after we're done mucking about. |
| BasicBlock::iterator NewIter = *BBI; |
| |
| // Before we touch this instruction, remove it from memdep! |
| do { |
| Instruction *DeadInst = NowDeadInsts.pop_back_val(); |
| // Mark the DeadInst as dead in the list of throwable instructions. |
| auto It = ThrowableInst.find(DeadInst); |
| if (It != ThrowableInst.end()) |
| ThrowableInst[It->first] = false; |
| ++NumFastOther; |
| |
| // Try to preserve debug information attached to the dead instruction. |
| salvageDebugInfo(*DeadInst); |
| salvageKnowledge(DeadInst); |
| |
| // This instruction is dead, zap it, in stages. Start by removing it from |
| // MemDep, which needs to know the operands and needs it to be in the |
| // function. |
| MD.removeInstruction(DeadInst); |
| |
| for (unsigned op = 0, e = DeadInst->getNumOperands(); op != e; ++op) { |
| Value *Op = DeadInst->getOperand(op); |
| DeadInst->setOperand(op, nullptr); |
| |
| // If this operand just became dead, add it to the NowDeadInsts list. |
| if (!Op->use_empty()) continue; |
| |
| if (Instruction *OpI = dyn_cast<Instruction>(Op)) |
| if (isInstructionTriviallyDead(OpI, &TLI)) |
| NowDeadInsts.push_back(OpI); |
| } |
| |
| if (ValueSet) ValueSet->remove(DeadInst); |
| IOL.erase(DeadInst); |
| |
| if (NewIter == DeadInst->getIterator()) |
| NewIter = DeadInst->eraseFromParent(); |
| else |
| DeadInst->eraseFromParent(); |
| } while (!NowDeadInsts.empty()); |
| *BBI = NewIter; |
| // Pop dead entries from back of ThrowableInst till we find an alive entry. |
| while (!ThrowableInst.empty() && !ThrowableInst.back().second) |
| ThrowableInst.pop_back(); |
| } |
| |
| /// Does this instruction write some memory? This only returns true for things |
| /// that we can analyze with other helpers below. |
| static bool hasAnalyzableMemoryWrite(Instruction *I, |
| const TargetLibraryInfo &TLI) { |
| if (isa<StoreInst>(I)) |
| return true; |
| if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(I)) { |
| switch (II->getIntrinsicID()) { |
| default: |
| return false; |
| case Intrinsic::memset: |
| case Intrinsic::memmove: |
| case Intrinsic::memcpy: |
| case Intrinsic::memcpy_inline: |
| case Intrinsic::memcpy_element_unordered_atomic: |
| case Intrinsic::memmove_element_unordered_atomic: |
| case Intrinsic::memset_element_unordered_atomic: |
| case Intrinsic::init_trampoline: |
| case Intrinsic::lifetime_end: |
| case Intrinsic::masked_store: |
| return true; |
| } |
| } |
| if (auto *CB = dyn_cast<CallBase>(I)) { |
| LibFunc LF; |
| if (TLI.getLibFunc(*CB, LF) && TLI.has(LF)) { |
| switch (LF) { |
| case LibFunc_strcpy: |
| case LibFunc_strncpy: |
| case LibFunc_strcat: |
| case LibFunc_strncat: |
| return true; |
| default: |
| return false; |
| } |
| } |
| } |
| return false; |
| } |
| |
| /// Return a Location stored to by the specified instruction. If isRemovable |
| /// returns true, this function and getLocForRead completely describe the memory |
| /// operations for this instruction. |
| static MemoryLocation getLocForWrite(Instruction *Inst, |
| const TargetLibraryInfo &TLI) { |
| if (StoreInst *SI = dyn_cast<StoreInst>(Inst)) |
| return MemoryLocation::get(SI); |
| |
| // memcpy/memmove/memset. |
| if (auto *MI = dyn_cast<AnyMemIntrinsic>(Inst)) |
| return MemoryLocation::getForDest(MI); |
| |
| if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(Inst)) { |
| switch (II->getIntrinsicID()) { |
| default: |
| return MemoryLocation(); // Unhandled intrinsic. |
| case Intrinsic::init_trampoline: |
| return MemoryLocation::getAfter(II->getArgOperand(0)); |
| case Intrinsic::masked_store: |
| return MemoryLocation::getForArgument(II, 1, TLI); |
| case Intrinsic::lifetime_end: { |
| uint64_t Len = cast<ConstantInt>(II->getArgOperand(0))->getZExtValue(); |
| return MemoryLocation(II->getArgOperand(1), Len); |
| } |
| } |
| } |
| if (auto *CB = dyn_cast<CallBase>(Inst)) |
| // All the supported TLI functions so far happen to have dest as their |
| // first argument. |
| return MemoryLocation::getAfter(CB->getArgOperand(0)); |
| return MemoryLocation(); |
| } |
| |
| /// Return the location read by the specified "hasAnalyzableMemoryWrite" |
| /// instruction if any. |
| static MemoryLocation getLocForRead(Instruction *Inst, |
| const TargetLibraryInfo &TLI) { |
| assert(hasAnalyzableMemoryWrite(Inst, TLI) && "Unknown instruction case"); |
| |
| // The only instructions that both read and write are the mem transfer |
| // instructions (memcpy/memmove). |
| if (auto *MTI = dyn_cast<AnyMemTransferInst>(Inst)) |
| return MemoryLocation::getForSource(MTI); |
| return MemoryLocation(); |
| } |
| |
| /// If the value of this instruction and the memory it writes to is unused, may |
| /// we delete this instruction? |
| static bool isRemovable(Instruction *I) { |
| // Don't remove volatile/atomic stores. |
| if (StoreInst *SI = dyn_cast<StoreInst>(I)) |
| return SI->isUnordered(); |
| |
| if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(I)) { |
| switch (II->getIntrinsicID()) { |
| default: llvm_unreachable("doesn't pass 'hasAnalyzableMemoryWrite' predicate"); |
| case Intrinsic::lifetime_end: |
| // Never remove dead lifetime_end's, e.g. because it is followed by a |
| // free. |
| return false; |
| case Intrinsic::init_trampoline: |
| // Always safe to remove init_trampoline. |
| return true; |
| case Intrinsic::memset: |
| case Intrinsic::memmove: |
| case Intrinsic::memcpy: |
| case Intrinsic::memcpy_inline: |
| // Don't remove volatile memory intrinsics. |
| return !cast<MemIntrinsic>(II)->isVolatile(); |
| case Intrinsic::memcpy_element_unordered_atomic: |
| case Intrinsic::memmove_element_unordered_atomic: |
| case Intrinsic::memset_element_unordered_atomic: |
| case Intrinsic::masked_store: |
| return true; |
| } |
| } |
| |
| // note: only get here for calls with analyzable writes - i.e. libcalls |
| if (auto *CB = dyn_cast<CallBase>(I)) |
| return CB->use_empty(); |
| |
| return false; |
| } |
| |
| /// Returns true if the end of this instruction can be safely shortened in |
| /// length. |
| static bool isShortenableAtTheEnd(Instruction *I) { |
| // Don't shorten stores for now |
| if (isa<StoreInst>(I)) |
| return false; |
| |
| if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(I)) { |
| switch (II->getIntrinsicID()) { |
| default: return false; |
| case Intrinsic::memset: |
| case Intrinsic::memcpy: |
| case Intrinsic::memcpy_element_unordered_atomic: |
| case Intrinsic::memset_element_unordered_atomic: |
| // Do shorten memory intrinsics. |
| // FIXME: Add memmove if it's also safe to transform. |
| return true; |
| } |
| } |
| |
| // Don't shorten libcalls calls for now. |
| |
| return false; |
| } |
| |
| /// Returns true if the beginning of this instruction can be safely shortened |
| /// in length. |
| static bool isShortenableAtTheBeginning(Instruction *I) { |
| // FIXME: Handle only memset for now. Supporting memcpy/memmove should be |
| // easily done by offsetting the source address. |
| return isa<AnyMemSetInst>(I); |
| } |
| |
| /// Return the pointer that is being written to. |
| static Value *getStoredPointerOperand(Instruction *I, |
| const TargetLibraryInfo &TLI) { |
| //TODO: factor this to reuse getLocForWrite |
| MemoryLocation Loc = getLocForWrite(I, TLI); |
| assert(Loc.Ptr && |
| "unable to find pointer written for analyzable instruction?"); |
| // TODO: most APIs don't expect const Value * |
| return const_cast<Value*>(Loc.Ptr); |
| } |
| |
| static uint64_t getPointerSize(const Value *V, const DataLayout &DL, |
| const TargetLibraryInfo &TLI, |
| const Function *F) { |
| uint64_t Size; |
| ObjectSizeOpts Opts; |
| Opts.NullIsUnknownSize = NullPointerIsDefined(F); |
| |
| if (getObjectSize(V, Size, DL, &TLI, Opts)) |
| return Size; |
| return MemoryLocation::UnknownSize; |
| } |
| |
| namespace { |
| |
| enum OverwriteResult { |
| OW_Begin, |
| OW_Complete, |
| OW_End, |
| OW_PartialEarlierWithFullLater, |
| OW_MaybePartial, |
| OW_Unknown |
| }; |
| |
| } // end anonymous namespace |
| |
| /// Check if two instruction are masked stores that completely |
| /// overwrite one another. More specifically, \p Later has to |
| /// overwrite \p Earlier. |
| template <typename AATy> |
| static OverwriteResult isMaskedStoreOverwrite(const Instruction *Later, |
| const Instruction *Earlier, |
| AATy &AA) { |
| const auto *IIL = dyn_cast<IntrinsicInst>(Later); |
| const auto *IIE = dyn_cast<IntrinsicInst>(Earlier); |
| if (IIL == nullptr || IIE == nullptr) |
| return OW_Unknown; |
| if (IIL->getIntrinsicID() != Intrinsic::masked_store || |
| IIE->getIntrinsicID() != Intrinsic::masked_store) |
| return OW_Unknown; |
| // Pointers. |
| Value *LP = IIL->getArgOperand(1)->stripPointerCasts(); |
| Value *EP = IIE->getArgOperand(1)->stripPointerCasts(); |
| if (LP != EP && !AA.isMustAlias(LP, EP)) |
| return OW_Unknown; |
| // Masks. |
| // TODO: check that Later's mask is a superset of the Earlier's mask. |
| if (IIL->getArgOperand(3) != IIE->getArgOperand(3)) |
| return OW_Unknown; |
| return OW_Complete; |
| } |
| |
| /// Return 'OW_Complete' if a store to the 'Later' location (by \p LaterI |
| /// instruction) completely overwrites a store to the 'Earlier' location. |
| /// (by \p EarlierI instruction). |
| /// Return OW_MaybePartial if \p Later does not completely overwrite |
| /// \p Earlier, but they both write to the same underlying object. In that |
| /// case, use isPartialOverwrite to check if \p Later partially overwrites |
| /// \p Earlier. Returns 'OW_Unknown' if nothing can be determined. |
| template <typename AATy> |
| static OverwriteResult |
| isOverwrite(const Instruction *LaterI, const Instruction *EarlierI, |
| const MemoryLocation &Later, const MemoryLocation &Earlier, |
| const DataLayout &DL, const TargetLibraryInfo &TLI, |
| int64_t &EarlierOff, int64_t &LaterOff, AATy &AA, |
| const Function *F) { |
| // FIXME: Vet that this works for size upper-bounds. Seems unlikely that we'll |
| // get imprecise values here, though (except for unknown sizes). |
| if (!Later.Size.isPrecise() || !Earlier.Size.isPrecise()) { |
| // Masked stores have imprecise locations, but we can reason about them |
| // to some extent. |
| return isMaskedStoreOverwrite(LaterI, EarlierI, AA); |
| } |
| |
| const uint64_t LaterSize = Later.Size.getValue(); |
| const uint64_t EarlierSize = Earlier.Size.getValue(); |
| |
| const Value *P1 = Earlier.Ptr->stripPointerCasts(); |
| const Value *P2 = Later.Ptr->stripPointerCasts(); |
| |
| // If the start pointers are the same, we just have to compare sizes to see if |
| // the later store was larger than the earlier store. |
| if (P1 == P2 || AA.isMustAlias(P1, P2)) { |
| // Make sure that the Later size is >= the Earlier size. |
| if (LaterSize >= EarlierSize) |
| return OW_Complete; |
| } |
| |
| // Check to see if the later store is to the entire object (either a global, |
| // an alloca, or a byval/inalloca argument). If so, then it clearly |
| // overwrites any other store to the same object. |
| const Value *UO1 = getUnderlyingObject(P1), *UO2 = getUnderlyingObject(P2); |
| |
| // If we can't resolve the same pointers to the same object, then we can't |
| // analyze them at all. |
| if (UO1 != UO2) |
| return OW_Unknown; |
| |
| // If the "Later" store is to a recognizable object, get its size. |
| uint64_t ObjectSize = getPointerSize(UO2, DL, TLI, F); |
| if (ObjectSize != MemoryLocation::UnknownSize) |
| if (ObjectSize == LaterSize && ObjectSize >= EarlierSize) |
| return OW_Complete; |
| |
| // Okay, we have stores to two completely different pointers. Try to |
| // decompose the pointer into a "base + constant_offset" form. If the base |
| // pointers are equal, then we can reason about the two stores. |
| EarlierOff = 0; |
| LaterOff = 0; |
| const Value *BP1 = GetPointerBaseWithConstantOffset(P1, EarlierOff, DL); |
| const Value *BP2 = GetPointerBaseWithConstantOffset(P2, LaterOff, DL); |
| |
| // If the base pointers still differ, we have two completely different stores. |
| if (BP1 != BP2) |
| return OW_Unknown; |
| |
| // The later access completely overlaps the earlier store if and only if |
| // both start and end of the earlier one is "inside" the later one: |
| // |<->|--earlier--|<->| |
| // |-------later-------| |
| // Accesses may overlap if and only if start of one of them is "inside" |
| // another one: |
| // |<->|--earlier--|<----->| |
| // |-------later-------| |
| // OR |
| // |----- earlier -----| |
| // |<->|---later---|<----->| |
| // |
| // We have to be careful here as *Off is signed while *.Size is unsigned. |
| |
| // Check if the earlier access starts "not before" the later one. |
| if (EarlierOff >= LaterOff) { |
| // If the earlier access ends "not after" the later access then the earlier |
| // one is completely overwritten by the later one. |
| if (uint64_t(EarlierOff - LaterOff) + EarlierSize <= LaterSize) |
| return OW_Complete; |
| // If start of the earlier access is "before" end of the later access then |
| // accesses overlap. |
| else if ((uint64_t)(EarlierOff - LaterOff) < LaterSize) |
| return OW_MaybePartial; |
| } |
| // If start of the later access is "before" end of the earlier access then |
| // accesses overlap. |
| else if ((uint64_t)(LaterOff - EarlierOff) < EarlierSize) { |
| return OW_MaybePartial; |
| } |
| |
| // Can reach here only if accesses are known not to overlap. There is no |
| // dedicated code to indicate no overlap so signal "unknown". |
| return OW_Unknown; |
| } |
| |
| /// Return 'OW_Complete' if a store to the 'Later' location completely |
| /// overwrites a store to the 'Earlier' location, 'OW_End' if the end of the |
| /// 'Earlier' location is completely overwritten by 'Later', 'OW_Begin' if the |
| /// beginning of the 'Earlier' location is overwritten by 'Later'. |
| /// 'OW_PartialEarlierWithFullLater' means that an earlier (big) store was |
| /// overwritten by a latter (smaller) store which doesn't write outside the big |
| /// store's memory locations. Returns 'OW_Unknown' if nothing can be determined. |
| /// NOTE: This function must only be called if both \p Later and \p Earlier |
| /// write to the same underlying object with valid \p EarlierOff and \p |
| /// LaterOff. |
| static OverwriteResult isPartialOverwrite(const MemoryLocation &Later, |
| const MemoryLocation &Earlier, |
| int64_t EarlierOff, int64_t LaterOff, |
| Instruction *DepWrite, |
| InstOverlapIntervalsTy &IOL) { |
| const uint64_t LaterSize = Later.Size.getValue(); |
| const uint64_t EarlierSize = Earlier.Size.getValue(); |
| // We may now overlap, although the overlap is not complete. There might also |
| // be other incomplete overlaps, and together, they might cover the complete |
| // earlier write. |
| // Note: The correctness of this logic depends on the fact that this function |
| // is not even called providing DepWrite when there are any intervening reads. |
| if (EnablePartialOverwriteTracking && |
| LaterOff < int64_t(EarlierOff + EarlierSize) && |
| int64_t(LaterOff + LaterSize) >= EarlierOff) { |
| |
| // Insert our part of the overlap into the map. |
| auto &IM = IOL[DepWrite]; |
| LLVM_DEBUG(dbgs() << "DSE: Partial overwrite: Earlier [" << EarlierOff |
| << ", " << int64_t(EarlierOff + EarlierSize) |
| << ") Later [" << LaterOff << ", " |
| << int64_t(LaterOff + LaterSize) << ")\n"); |
| |
| // Make sure that we only insert non-overlapping intervals and combine |
| // adjacent intervals. The intervals are stored in the map with the ending |
| // offset as the key (in the half-open sense) and the starting offset as |
| // the value. |
| int64_t LaterIntStart = LaterOff, LaterIntEnd = LaterOff + LaterSize; |
| |
| // Find any intervals ending at, or after, LaterIntStart which start |
| // before LaterIntEnd. |
| auto ILI = IM.lower_bound(LaterIntStart); |
| if (ILI != IM.end() && ILI->second <= LaterIntEnd) { |
| // This existing interval is overlapped with the current store somewhere |
| // in [LaterIntStart, LaterIntEnd]. Merge them by erasing the existing |
| // intervals and adjusting our start and end. |
| LaterIntStart = std::min(LaterIntStart, ILI->second); |
| LaterIntEnd = std::max(LaterIntEnd, ILI->first); |
| ILI = IM.erase(ILI); |
| |
| // Continue erasing and adjusting our end in case other previous |
| // intervals are also overlapped with the current store. |
| // |
| // |--- ealier 1 ---| |--- ealier 2 ---| |
| // |------- later---------| |
| // |
| while (ILI != IM.end() && ILI->second <= LaterIntEnd) { |
| assert(ILI->second > LaterIntStart && "Unexpected interval"); |
| LaterIntEnd = std::max(LaterIntEnd, ILI->first); |
| ILI = IM.erase(ILI); |
| } |
| } |
| |
| IM[LaterIntEnd] = LaterIntStart; |
| |
| ILI = IM.begin(); |
| if (ILI->second <= EarlierOff && |
| ILI->first >= int64_t(EarlierOff + EarlierSize)) { |
| LLVM_DEBUG(dbgs() << "DSE: Full overwrite from partials: Earlier [" |
| << EarlierOff << ", " |
| << int64_t(EarlierOff + EarlierSize) |
| << ") Composite Later [" << ILI->second << ", " |
| << ILI->first << ")\n"); |
| ++NumCompletePartials; |
| return OW_Complete; |
| } |
| } |
| |
| // Check for an earlier store which writes to all the memory locations that |
| // the later store writes to. |
| if (EnablePartialStoreMerging && LaterOff >= EarlierOff && |
| int64_t(EarlierOff + EarlierSize) > LaterOff && |
| uint64_t(LaterOff - EarlierOff) + LaterSize <= EarlierSize) { |
| LLVM_DEBUG(dbgs() << "DSE: Partial overwrite an earlier load [" |
| << EarlierOff << ", " |
| << int64_t(EarlierOff + EarlierSize) |
| << ") by a later store [" << LaterOff << ", " |
| << int64_t(LaterOff + LaterSize) << ")\n"); |
| // TODO: Maybe come up with a better name? |
| return OW_PartialEarlierWithFullLater; |
| } |
| |
| // Another interesting case is if the later store overwrites the end of the |
| // earlier store. |
| // |
| // |--earlier--| |
| // |-- later --| |
| // |
| // In this case we may want to trim the size of earlier to avoid generating |
| // writes to addresses which will definitely be overwritten later |
| if (!EnablePartialOverwriteTracking && |
| (LaterOff > EarlierOff && LaterOff < int64_t(EarlierOff + EarlierSize) && |
| int64_t(LaterOff + LaterSize) >= int64_t(EarlierOff + EarlierSize))) |
| return OW_End; |
| |
| // Finally, we also need to check if the later store overwrites the beginning |
| // of the earlier store. |
| // |
| // |--earlier--| |
| // |-- later --| |
| // |
| // In this case we may want to move the destination address and trim the size |
| // of earlier to avoid generating writes to addresses which will definitely |
| // be overwritten later. |
| if (!EnablePartialOverwriteTracking && |
| (LaterOff <= EarlierOff && int64_t(LaterOff + LaterSize) > EarlierOff)) { |
| assert(int64_t(LaterOff + LaterSize) < int64_t(EarlierOff + EarlierSize) && |
| "Expect to be handled as OW_Complete"); |
| return OW_Begin; |
| } |
| // Otherwise, they don't completely overlap. |
| return OW_Unknown; |
| } |
| |
| /// If 'Inst' might be a self read (i.e. a noop copy of a |
| /// memory region into an identical pointer) then it doesn't actually make its |
| /// input dead in the traditional sense. Consider this case: |
| /// |
| /// memmove(A <- B) |
| /// memmove(A <- A) |
| /// |
| /// In this case, the second store to A does not make the first store to A dead. |
| /// The usual situation isn't an explicit A<-A store like this (which can be |
| /// trivially removed) but a case where two pointers may alias. |
| /// |
| /// This function detects when it is unsafe to remove a dependent instruction |
| /// because the DSE inducing instruction may be a self-read. |
| static bool isPossibleSelfRead(Instruction *Inst, |
| const MemoryLocation &InstStoreLoc, |
| Instruction *DepWrite, |
| const TargetLibraryInfo &TLI, |
| AliasAnalysis &AA) { |
| // Self reads can only happen for instructions that read memory. Get the |
| // location read. |
| MemoryLocation InstReadLoc = getLocForRead(Inst, TLI); |
| if (!InstReadLoc.Ptr) |
| return false; // Not a reading instruction. |
| |
| // If the read and written loc obviously don't alias, it isn't a read. |
| if (AA.isNoAlias(InstReadLoc, InstStoreLoc)) |
| return false; |
| |
| if (isa<AnyMemCpyInst>(Inst)) { |
| // LLVM's memcpy overlap semantics are not fully fleshed out (see PR11763) |
| // but in practice memcpy(A <- B) either means that A and B are disjoint or |
| // are equal (i.e. there are not partial overlaps). Given that, if we have: |
| // |
| // memcpy/memmove(A <- B) // DepWrite |
| // memcpy(A <- B) // Inst |
| // |
| // with Inst reading/writing a >= size than DepWrite, we can reason as |
| // follows: |
| // |
| // - If A == B then both the copies are no-ops, so the DepWrite can be |
| // removed. |
| // - If A != B then A and B are disjoint locations in Inst. Since |
| // Inst.size >= DepWrite.size A and B are disjoint in DepWrite too. |
| // Therefore DepWrite can be removed. |
| MemoryLocation DepReadLoc = getLocForRead(DepWrite, TLI); |
| |
| if (DepReadLoc.Ptr && AA.isMustAlias(InstReadLoc.Ptr, DepReadLoc.Ptr)) |
| return false; |
| } |
| |
| // If DepWrite doesn't read memory or if we can't prove it is a must alias, |
| // then it can't be considered dead. |
| return true; |
| } |
| |
| /// Returns true if the memory which is accessed by the second instruction is not |
| /// modified between the first and the second instruction. |
| /// Precondition: Second instruction must be dominated by the first |
| /// instruction. |
| template <typename AATy> |
| static bool |
| memoryIsNotModifiedBetween(Instruction *FirstI, Instruction *SecondI, AATy &AA, |
| const DataLayout &DL, DominatorTree *DT) { |
| // Do a backwards scan through the CFG from SecondI to FirstI. Look for |
| // instructions which can modify the memory location accessed by SecondI. |
| // |
| // While doing the walk keep track of the address to check. It might be |
| // different in different basic blocks due to PHI translation. |
| using BlockAddressPair = std::pair<BasicBlock *, PHITransAddr>; |
| SmallVector<BlockAddressPair, 16> WorkList; |
| // Keep track of the address we visited each block with. Bail out if we |
| // visit a block with different addresses. |
| DenseMap<BasicBlock *, Value *> Visited; |
| |
| BasicBlock::iterator FirstBBI(FirstI); |
| ++FirstBBI; |
| BasicBlock::iterator SecondBBI(SecondI); |
| BasicBlock *FirstBB = FirstI->getParent(); |
| BasicBlock *SecondBB = SecondI->getParent(); |
| MemoryLocation MemLoc = MemoryLocation::get(SecondI); |
| auto *MemLocPtr = const_cast<Value *>(MemLoc.Ptr); |
| |
| // Start checking the SecondBB. |
| WorkList.push_back( |
| std::make_pair(SecondBB, PHITransAddr(MemLocPtr, DL, nullptr))); |
| bool isFirstBlock = true; |
| |
| // Check all blocks going backward until we reach the FirstBB. |
| while (!WorkList.empty()) { |
| BlockAddressPair Current = WorkList.pop_back_val(); |
| BasicBlock *B = Current.first; |
| PHITransAddr &Addr = Current.second; |
| Value *Ptr = Addr.getAddr(); |
| |
| // Ignore instructions before FirstI if this is the FirstBB. |
| BasicBlock::iterator BI = (B == FirstBB ? FirstBBI : B->begin()); |
| |
| BasicBlock::iterator EI; |
| if (isFirstBlock) { |
| // Ignore instructions after SecondI if this is the first visit of SecondBB. |
| assert(B == SecondBB && "first block is not the store block"); |
| EI = SecondBBI; |
| isFirstBlock = false; |
| } else { |
| // It's not SecondBB or (in case of a loop) the second visit of SecondBB. |
| // In this case we also have to look at instructions after SecondI. |
| EI = B->end(); |
| } |
| for (; BI != EI; ++BI) { |
| Instruction *I = &*BI; |
| if (I->mayWriteToMemory() && I != SecondI) |
| if (isModSet(AA.getModRefInfo(I, MemLoc.getWithNewPtr(Ptr)))) |
| return false; |
| } |
| if (B != FirstBB) { |
| assert(B != &FirstBB->getParent()->getEntryBlock() && |
| "Should not hit the entry block because SI must be dominated by LI"); |
| for (auto PredI = pred_begin(B), PE = pred_end(B); PredI != PE; ++PredI) { |
| PHITransAddr PredAddr = Addr; |
| if (PredAddr.NeedsPHITranslationFromBlock(B)) { |
| if (!PredAddr.IsPotentiallyPHITranslatable()) |
| return false; |
| if (PredAddr.PHITranslateValue(B, *PredI, DT, false)) |
| return false; |
| } |
| Value *TranslatedPtr = PredAddr.getAddr(); |
| auto Inserted = Visited.insert(std::make_pair(*PredI, TranslatedPtr)); |
| if (!Inserted.second) { |
| // We already visited this block before. If it was with a different |
| // address - bail out! |
| if (TranslatedPtr != Inserted.first->second) |
| return false; |
| // ... otherwise just skip it. |
| continue; |
| } |
| WorkList.push_back(std::make_pair(*PredI, PredAddr)); |
| } |
| } |
| } |
| return true; |
| } |
| |
| /// Find all blocks that will unconditionally lead to the block BB and append |
| /// them to F. |
| static void findUnconditionalPreds(SmallVectorImpl<BasicBlock *> &Blocks, |
| BasicBlock *BB, DominatorTree *DT) { |
| for (pred_iterator I = pred_begin(BB), E = pred_end(BB); I != E; ++I) { |
| BasicBlock *Pred = *I; |
| if (Pred == BB) continue; |
| Instruction *PredTI = Pred->getTerminator(); |
| if (PredTI->getNumSuccessors() != 1) |
| continue; |
| |
| if (DT->isReachableFromEntry(Pred)) |
| Blocks.push_back(Pred); |
| } |
| } |
| |
| /// Handle frees of entire structures whose dependency is a store |
| /// to a field of that structure. |
| static bool handleFree(CallInst *F, AliasAnalysis *AA, |
| MemoryDependenceResults *MD, DominatorTree *DT, |
| const TargetLibraryInfo *TLI, |
| InstOverlapIntervalsTy &IOL, |
| MapVector<Instruction *, bool> &ThrowableInst) { |
| bool MadeChange = false; |
| |
| MemoryLocation Loc = MemoryLocation::getAfter(F->getOperand(0)); |
| SmallVector<BasicBlock *, 16> Blocks; |
| Blocks.push_back(F->getParent()); |
| |
| while (!Blocks.empty()) { |
| BasicBlock *BB = Blocks.pop_back_val(); |
| Instruction *InstPt = BB->getTerminator(); |
| if (BB == F->getParent()) InstPt = F; |
| |
| MemDepResult Dep = |
| MD->getPointerDependencyFrom(Loc, false, InstPt->getIterator(), BB); |
| while (Dep.isDef() || Dep.isClobber()) { |
| Instruction *Dependency = Dep.getInst(); |
| if (!hasAnalyzableMemoryWrite(Dependency, *TLI) || |
| !isRemovable(Dependency)) |
| break; |
| |
| Value *DepPointer = |
| getUnderlyingObject(getStoredPointerOperand(Dependency, *TLI)); |
| |
| // Check for aliasing. |
| if (!AA->isMustAlias(F->getArgOperand(0), DepPointer)) |
| break; |
| |
| LLVM_DEBUG( |
| dbgs() << "DSE: Dead Store to soon to be freed memory:\n DEAD: " |
| << *Dependency << '\n'); |
| |
| // DCE instructions only used to calculate that store. |
| BasicBlock::iterator BBI(Dependency); |
| deleteDeadInstruction(Dependency, &BBI, *MD, *TLI, IOL, |
| ThrowableInst); |
| ++NumFastStores; |
| MadeChange = true; |
| |
| // Inst's old Dependency is now deleted. Compute the next dependency, |
| // which may also be dead, as in |
| // s[0] = 0; |
| // s[1] = 0; // This has just been deleted. |
| // free(s); |
| Dep = MD->getPointerDependencyFrom(Loc, false, BBI, BB); |
| } |
| |
| if (Dep.isNonLocal()) |
| findUnconditionalPreds(Blocks, BB, DT); |
| } |
| |
| return MadeChange; |
| } |
| |
| /// Check to see if the specified location may alias any of the stack objects in |
| /// the DeadStackObjects set. If so, they become live because the location is |
| /// being loaded. |
| static void removeAccessedObjects(const MemoryLocation &LoadedLoc, |
| SmallSetVector<const Value *, 16> &DeadStackObjects, |
| const DataLayout &DL, AliasAnalysis *AA, |
| const TargetLibraryInfo *TLI, |
| const Function *F) { |
| const Value *UnderlyingPointer = getUnderlyingObject(LoadedLoc.Ptr); |
| |
| // A constant can't be in the dead pointer set. |
| if (isa<Constant>(UnderlyingPointer)) |
| return; |
| |
| // If the kill pointer can be easily reduced to an alloca, don't bother doing |
| // extraneous AA queries. |
| if (isa<AllocaInst>(UnderlyingPointer) || isa<Argument>(UnderlyingPointer)) { |
| DeadStackObjects.remove(UnderlyingPointer); |
| return; |
| } |
| |
| // Remove objects that could alias LoadedLoc. |
| DeadStackObjects.remove_if([&](const Value *I) { |
| // See if the loaded location could alias the stack location. |
| MemoryLocation StackLoc(I, getPointerSize(I, DL, *TLI, F)); |
| return !AA->isNoAlias(StackLoc, LoadedLoc); |
| }); |
| } |
| |
| /// Remove dead stores to stack-allocated locations in the function end block. |
| /// Ex: |
| /// %A = alloca i32 |
| /// ... |
| /// store i32 1, i32* %A |
| /// ret void |
| static bool handleEndBlock(BasicBlock &BB, AliasAnalysis *AA, |
| MemoryDependenceResults *MD, |
| const TargetLibraryInfo *TLI, |
| InstOverlapIntervalsTy &IOL, |
| MapVector<Instruction *, bool> &ThrowableInst) { |
| bool MadeChange = false; |
| |
| // Keep track of all of the stack objects that are dead at the end of the |
| // function. |
| SmallSetVector<const Value*, 16> DeadStackObjects; |
| |
| // Find all of the alloca'd pointers in the entry block. |
| BasicBlock &Entry = BB.getParent()->front(); |
| for (Instruction &I : Entry) { |
| if (isa<AllocaInst>(&I)) |
| DeadStackObjects.insert(&I); |
| |
| // Okay, so these are dead heap objects, but if the pointer never escapes |
| // then it's leaked by this function anyways. |
| else if (isAllocLikeFn(&I, TLI) && !PointerMayBeCaptured(&I, true, true)) |
| DeadStackObjects.insert(&I); |
| } |
| |
| // Treat byval or inalloca arguments the same, stores to them are dead at the |
| // end of the function. |
| for (Argument &AI : BB.getParent()->args()) |
| if (AI.hasPassPointeeByValueCopyAttr()) |
| DeadStackObjects.insert(&AI); |
| |
| const DataLayout &DL = BB.getModule()->getDataLayout(); |
| |
| // Scan the basic block backwards |
| for (BasicBlock::iterator BBI = BB.end(); BBI != BB.begin(); ){ |
| --BBI; |
| |
| // If we find a store, check to see if it points into a dead stack value. |
| if (hasAnalyzableMemoryWrite(&*BBI, *TLI) && isRemovable(&*BBI)) { |
| // See through pointer-to-pointer bitcasts |
| SmallVector<const Value *, 4> Pointers; |
| getUnderlyingObjects(getStoredPointerOperand(&*BBI, *TLI), Pointers); |
| |
| // Stores to stack values are valid candidates for removal. |
| bool AllDead = true; |
| for (const Value *Pointer : Pointers) |
| if (!DeadStackObjects.count(Pointer)) { |
| AllDead = false; |
| break; |
| } |
| |
| if (AllDead) { |
| Instruction *Dead = &*BBI; |
| |
| LLVM_DEBUG(dbgs() << "DSE: Dead Store at End of Block:\n DEAD: " |
| << *Dead << "\n Objects: "; |
| for (SmallVectorImpl<const Value *>::iterator I = |
| Pointers.begin(), |
| E = Pointers.end(); |
| I != E; ++I) { |
| dbgs() << **I; |
| if (std::next(I) != E) |
| dbgs() << ", "; |
| } dbgs() |
| << '\n'); |
| |
| // DCE instructions only used to calculate that store. |
| deleteDeadInstruction(Dead, &BBI, *MD, *TLI, IOL, ThrowableInst, |
| &DeadStackObjects); |
| ++NumFastStores; |
| MadeChange = true; |
| continue; |
| } |
| } |
| |
| // Remove any dead non-memory-mutating instructions. |
| if (isInstructionTriviallyDead(&*BBI, TLI)) { |
| LLVM_DEBUG(dbgs() << "DSE: Removing trivially dead instruction:\n DEAD: " |
| << *&*BBI << '\n'); |
| deleteDeadInstruction(&*BBI, &BBI, *MD, *TLI, IOL, ThrowableInst, |
| &DeadStackObjects); |
| ++NumFastOther; |
| MadeChange = true; |
| continue; |
| } |
| |
| if (isa<AllocaInst>(BBI)) { |
| // Remove allocas from the list of dead stack objects; there can't be |
| // any references before the definition. |
| DeadStackObjects.remove(&*BBI); |
| continue; |
| } |
| |
| if (auto *Call = dyn_cast<CallBase>(&*BBI)) { |
| // Remove allocation function calls from the list of dead stack objects; |
| // there can't be any references before the definition. |
| if (isAllocLikeFn(&*BBI, TLI)) |
| DeadStackObjects.remove(&*BBI); |
| |
| // If this call does not access memory, it can't be loading any of our |
| // pointers. |
| if (AA->doesNotAccessMemory(Call)) |
| continue; |
| |
| // If the call might load from any of our allocas, then any store above |
| // the call is live. |
| DeadStackObjects.remove_if([&](const Value *I) { |
| // See if the call site touches the value. |
| return isRefSet(AA->getModRefInfo( |
| Call, I, getPointerSize(I, DL, *TLI, BB.getParent()))); |
| }); |
| |
| // If all of the allocas were clobbered by the call then we're not going |
| // to find anything else to process. |
| if (DeadStackObjects.empty()) |
| break; |
| |
| continue; |
| } |
| |
| // We can remove the dead stores, irrespective of the fence and its ordering |
| // (release/acquire/seq_cst). Fences only constraints the ordering of |
| // already visible stores, it does not make a store visible to other |
| // threads. So, skipping over a fence does not change a store from being |
| // dead. |
| if (isa<FenceInst>(*BBI)) |
| continue; |
| |
| MemoryLocation LoadedLoc; |
| |
| // If we encounter a use of the pointer, it is no longer considered dead |
| if (LoadInst *L = dyn_cast<LoadInst>(BBI)) { |
| if (!L->isUnordered()) // Be conservative with atomic/volatile load |
| break; |
| LoadedLoc = MemoryLocation::get(L); |
| } else if (VAArgInst *V = dyn_cast<VAArgInst>(BBI)) { |
| LoadedLoc = MemoryLocation::get(V); |
| } else if (!BBI->mayReadFromMemory()) { |
| // Instruction doesn't read memory. Note that stores that weren't removed |
| // above will hit this case. |
| continue; |
| } else { |
| // Unknown inst; assume it clobbers everything. |
| break; |
| } |
| |
| // Remove any allocas from the DeadPointer set that are loaded, as this |
| // makes any stores above the access live. |
| removeAccessedObjects(LoadedLoc, DeadStackObjects, DL, AA, TLI, BB.getParent()); |
| |
| // If all of the allocas were clobbered by the access then we're not going |
| // to find anything else to process. |
| if (DeadStackObjects.empty()) |
| break; |
| } |
| |
| return MadeChange; |
| } |
| |
| static bool tryToShorten(Instruction *EarlierWrite, int64_t &EarlierOffset, |
| uint64_t &EarlierSize, int64_t LaterOffset, |
| uint64_t LaterSize, bool IsOverwriteEnd) { |
| // TODO: base this on the target vector size so that if the earlier |
| // store was too small to get vector writes anyway then its likely |
| // a good idea to shorten it |
| // Power of 2 vector writes are probably always a bad idea to optimize |
| // as any store/memset/memcpy is likely using vector instructions so |
| // shortening it to not vector size is likely to be slower |
| auto *EarlierIntrinsic = cast<AnyMemIntrinsic>(EarlierWrite); |
| unsigned EarlierWriteAlign = EarlierIntrinsic->getDestAlignment(); |
| if (!IsOverwriteEnd) |
| LaterOffset = int64_t(LaterOffset + LaterSize); |
| |
| if (!(isPowerOf2_64(LaterOffset) && EarlierWriteAlign <= LaterOffset) && |
| !((EarlierWriteAlign != 0) && LaterOffset % EarlierWriteAlign == 0)) |
| return false; |
| |
| int64_t NewLength = IsOverwriteEnd |
| ? LaterOffset - EarlierOffset |
| : EarlierSize - (LaterOffset - EarlierOffset); |
| |
| if (auto *AMI = dyn_cast<AtomicMemIntrinsic>(EarlierWrite)) { |
| // When shortening an atomic memory intrinsic, the newly shortened |
| // length must remain an integer multiple of the element size. |
| const uint32_t ElementSize = AMI->getElementSizeInBytes(); |
| if (0 != NewLength % ElementSize) |
| return false; |
| } |
| |
| LLVM_DEBUG(dbgs() << "DSE: Remove Dead Store:\n OW " |
| << (IsOverwriteEnd ? "END" : "BEGIN") << ": " |
| << *EarlierWrite << "\n KILLER (offset " << LaterOffset |
| << ", " << EarlierSize << ")\n"); |
| |
| Value *EarlierWriteLength = EarlierIntrinsic->getLength(); |
| Value *TrimmedLength = |
| ConstantInt::get(EarlierWriteLength->getType(), NewLength); |
| EarlierIntrinsic->setLength(TrimmedLength); |
| |
| EarlierSize = NewLength; |
| if (!IsOverwriteEnd) { |
| int64_t OffsetMoved = (LaterOffset - EarlierOffset); |
| Value *Indices[1] = { |
| ConstantInt::get(EarlierWriteLength->getType(), OffsetMoved)}; |
| GetElementPtrInst *NewDestGEP = GetElementPtrInst::CreateInBounds( |
| EarlierIntrinsic->getRawDest()->getType()->getPointerElementType(), |
| EarlierIntrinsic->getRawDest(), Indices, "", EarlierWrite); |
| NewDestGEP->setDebugLoc(EarlierIntrinsic->getDebugLoc()); |
| EarlierIntrinsic->setDest(NewDestGEP); |
| EarlierOffset = EarlierOffset + OffsetMoved; |
| } |
| return true; |
| } |
| |
| static bool tryToShortenEnd(Instruction *EarlierWrite, |
| OverlapIntervalsTy &IntervalMap, |
| int64_t &EarlierStart, uint64_t &EarlierSize) { |
| if (IntervalMap.empty() || !isShortenableAtTheEnd(EarlierWrite)) |
| return false; |
| |
| OverlapIntervalsTy::iterator OII = --IntervalMap.end(); |
| int64_t LaterStart = OII->second; |
| uint64_t LaterSize = OII->first - LaterStart; |
| |
| assert(OII->first - LaterStart >= 0 && "Size expected to be positive"); |
| |
| if (LaterStart > EarlierStart && |
| // Note: "LaterStart - EarlierStart" is known to be positive due to |
| // preceding check. |
| (uint64_t)(LaterStart - EarlierStart) < EarlierSize && |
| // Note: "EarlierSize - (uint64_t)(LaterStart - EarlierStart)" is known to |
| // be non negative due to preceding checks. |
| LaterSize >= EarlierSize - (uint64_t)(LaterStart - EarlierStart)) { |
| if (tryToShorten(EarlierWrite, EarlierStart, EarlierSize, LaterStart, |
| LaterSize, true)) { |
| IntervalMap.erase(OII); |
| return true; |
| } |
| } |
| return false; |
| } |
| |
| static bool tryToShortenBegin(Instruction *EarlierWrite, |
| OverlapIntervalsTy &IntervalMap, |
| int64_t &EarlierStart, uint64_t &EarlierSize) { |
| if (IntervalMap.empty() || !isShortenableAtTheBeginning(EarlierWrite)) |
| return false; |
| |
| OverlapIntervalsTy::iterator OII = IntervalMap.begin(); |
| int64_t LaterStart = OII->second; |
| uint64_t LaterSize = OII->first - LaterStart; |
| |
| assert(OII->first - LaterStart >= 0 && "Size expected to be positive"); |
| |
| if (LaterStart <= EarlierStart && |
| // Note: "EarlierStart - LaterStart" is known to be non negative due to |
| // preceding check. |
| LaterSize > (uint64_t)(EarlierStart - LaterStart)) { |
| // Note: "LaterSize - (uint64_t)(EarlierStart - LaterStart)" is known to be |
| // positive due to preceding checks. |
| assert(LaterSize - (uint64_t)(EarlierStart - LaterStart) < EarlierSize && |
| "Should have been handled as OW_Complete"); |
| if (tryToShorten(EarlierWrite, EarlierStart, EarlierSize, LaterStart, |
| LaterSize, false)) { |
| IntervalMap.erase(OII); |
| return true; |
| } |
| } |
| return false; |
| } |
| |
| static bool removePartiallyOverlappedStores(const DataLayout &DL, |
| InstOverlapIntervalsTy &IOL, |
| const TargetLibraryInfo &TLI) { |
| bool Changed = false; |
| for (auto OI : IOL) { |
| Instruction *EarlierWrite = OI.first; |
| MemoryLocation Loc = getLocForWrite(EarlierWrite, TLI); |
| assert(isRemovable(EarlierWrite) && "Expect only removable instruction"); |
| |
| const Value *Ptr = Loc.Ptr->stripPointerCasts(); |
| int64_t EarlierStart = 0; |
| uint64_t EarlierSize = Loc.Size.getValue(); |
| GetPointerBaseWithConstantOffset(Ptr, EarlierStart, DL); |
| OverlapIntervalsTy &IntervalMap = OI.second; |
| Changed |= |
| tryToShortenEnd(EarlierWrite, IntervalMap, EarlierStart, EarlierSize); |
| if (IntervalMap.empty()) |
| continue; |
| Changed |= |
| tryToShortenBegin(EarlierWrite, IntervalMap, EarlierStart, EarlierSize); |
| } |
| return Changed; |
| } |
| |
| static bool eliminateNoopStore(Instruction *Inst, BasicBlock::iterator &BBI, |
| AliasAnalysis *AA, MemoryDependenceResults *MD, |
| const DataLayout &DL, |
| const TargetLibraryInfo *TLI, |
| InstOverlapIntervalsTy &IOL, |
| MapVector<Instruction *, bool> &ThrowableInst, |
| DominatorTree *DT) { |
| // Must be a store instruction. |
| StoreInst *SI = dyn_cast<StoreInst>(Inst); |
| if (!SI) |
| return false; |
| |
| // If we're storing the same value back to a pointer that we just loaded from, |
| // then the store can be removed. |
| if (LoadInst *DepLoad = dyn_cast<LoadInst>(SI->getValueOperand())) { |
| if (SI->getPointerOperand() == DepLoad->getPointerOperand() && |
| isRemovable(SI) && |
| memoryIsNotModifiedBetween(DepLoad, SI, *AA, DL, DT)) { |
| |
| LLVM_DEBUG( |
| dbgs() << "DSE: Remove Store Of Load from same pointer:\n LOAD: " |
| << *DepLoad << "\n STORE: " << *SI << '\n'); |
| |
| deleteDeadInstruction(SI, &BBI, *MD, *TLI, IOL, ThrowableInst); |
| ++NumRedundantStores; |
| return true; |
| } |
| } |
| |
| // Remove null stores into the calloc'ed objects |
| Constant *StoredConstant = dyn_cast<Constant>(SI->getValueOperand()); |
| if (StoredConstant && StoredConstant->isNullValue() && isRemovable(SI)) { |
| Instruction *UnderlyingPointer = |
| dyn_cast<Instruction>(getUnderlyingObject(SI->getPointerOperand())); |
| |
| if (UnderlyingPointer && isCallocLikeFn(UnderlyingPointer, TLI) && |
| memoryIsNotModifiedBetween(UnderlyingPointer, SI, *AA, DL, DT)) { |
| LLVM_DEBUG( |
| dbgs() << "DSE: Remove null store to the calloc'ed object:\n DEAD: " |
| << *Inst << "\n OBJECT: " << *UnderlyingPointer << '\n'); |
| |
| deleteDeadInstruction(SI, &BBI, *MD, *TLI, IOL, ThrowableInst); |
| ++NumRedundantStores; |
| return true; |
| } |
| } |
| return false; |
| } |
| |
| template <typename AATy> |
| static Constant *tryToMergePartialOverlappingStores( |
| StoreInst *Earlier, StoreInst *Later, int64_t InstWriteOffset, |
| int64_t DepWriteOffset, const DataLayout &DL, AATy &AA, DominatorTree *DT) { |
| |
| if (Earlier && isa<ConstantInt>(Earlier->getValueOperand()) && |
| DL.typeSizeEqualsStoreSize(Earlier->getValueOperand()->getType()) && |
| Later && isa<ConstantInt>(Later->getValueOperand()) && |
| DL.typeSizeEqualsStoreSize(Later->getValueOperand()->getType()) && |
| memoryIsNotModifiedBetween(Earlier, Later, AA, DL, DT)) { |
| // If the store we find is: |
| // a) partially overwritten by the store to 'Loc' |
| // b) the later store is fully contained in the earlier one and |
| // c) they both have a constant value |
| // d) none of the two stores need padding |
| // Merge the two stores, replacing the earlier store's value with a |
| // merge of both values. |
| // TODO: Deal with other constant types (vectors, etc), and probably |
| // some mem intrinsics (if needed) |
| |
| APInt EarlierValue = |
| cast<ConstantInt>(Earlier->getValueOperand())->getValue(); |
| APInt LaterValue = cast<ConstantInt>(Later->getValueOperand())->getValue(); |
| unsigned LaterBits = LaterValue.getBitWidth(); |
| assert(EarlierValue.getBitWidth() > LaterValue.getBitWidth()); |
| LaterValue = LaterValue.zext(EarlierValue.getBitWidth()); |
| |
| // Offset of the smaller store inside the larger store |
| unsigned BitOffsetDiff = (InstWriteOffset - DepWriteOffset) * 8; |
| unsigned LShiftAmount = DL.isBigEndian() ? EarlierValue.getBitWidth() - |
| BitOffsetDiff - LaterBits |
| : BitOffsetDiff; |
| APInt Mask = APInt::getBitsSet(EarlierValue.getBitWidth(), LShiftAmount, |
| LShiftAmount + LaterBits); |
| // Clear the bits we'll be replacing, then OR with the smaller |
| // store, shifted appropriately. |
| APInt Merged = (EarlierValue & ~Mask) | (LaterValue << LShiftAmount); |
| LLVM_DEBUG(dbgs() << "DSE: Merge Stores:\n Earlier: " << *Earlier |
| << "\n Later: " << *Later |
| << "\n Merged Value: " << Merged << '\n'); |
| return ConstantInt::get(Earlier->getValueOperand()->getType(), Merged); |
| } |
| return nullptr; |
| } |
| |
| static bool eliminateDeadStores(BasicBlock &BB, AliasAnalysis *AA, |
| MemoryDependenceResults *MD, DominatorTree *DT, |
| const TargetLibraryInfo *TLI) { |
| const DataLayout &DL = BB.getModule()->getDataLayout(); |
| bool MadeChange = false; |
| |
| MapVector<Instruction *, bool> ThrowableInst; |
| |
| // A map of interval maps representing partially-overwritten value parts. |
| InstOverlapIntervalsTy IOL; |
| |
| // Do a top-down walk on the BB. |
| for (BasicBlock::iterator BBI = BB.begin(), BBE = BB.end(); BBI != BBE; ) { |
| // Handle 'free' calls specially. |
| if (CallInst *F = isFreeCall(&*BBI, TLI)) { |
| MadeChange |= handleFree(F, AA, MD, DT, TLI, IOL, ThrowableInst); |
| // Increment BBI after handleFree has potentially deleted instructions. |
| // This ensures we maintain a valid iterator. |
| ++BBI; |
| continue; |
| } |
| |
| Instruction *Inst = &*BBI++; |
| |
| if (Inst->mayThrow()) { |
| ThrowableInst[Inst] = true; |
| continue; |
| } |
| |
| // Check to see if Inst writes to memory. If not, continue. |
| if (!hasAnalyzableMemoryWrite(Inst, *TLI)) |
| continue; |
| |
| // eliminateNoopStore will update in iterator, if necessary. |
| if (eliminateNoopStore(Inst, BBI, AA, MD, DL, TLI, IOL, |
| ThrowableInst, DT)) { |
| MadeChange = true; |
| continue; |
| } |
| |
| // If we find something that writes memory, get its memory dependence. |
| MemDepResult InstDep = MD->getDependency(Inst); |
| |
| // Ignore any store where we can't find a local dependence. |
| // FIXME: cross-block DSE would be fun. :) |
| if (!InstDep.isDef() && !InstDep.isClobber()) |
| continue; |
| |
| // Figure out what location is being stored to. |
| MemoryLocation Loc = getLocForWrite(Inst, *TLI); |
| |
| // If we didn't get a useful location, fail. |
| if (!Loc.Ptr) |
| continue; |
| |
| // Loop until we find a store we can eliminate or a load that |
| // invalidates the analysis. Without an upper bound on the number of |
| // instructions examined, this analysis can become very time-consuming. |
| // However, the potential gain diminishes as we process more instructions |
| // without eliminating any of them. Therefore, we limit the number of |
| // instructions we look at. |
| auto Limit = MD->getDefaultBlockScanLimit(); |
| while (InstDep.isDef() || InstDep.isClobber()) { |
| // Get the memory clobbered by the instruction we depend on. MemDep will |
| // skip any instructions that 'Loc' clearly doesn't interact with. If we |
| // end up depending on a may- or must-aliased load, then we can't optimize |
| // away the store and we bail out. However, if we depend on something |
| // that overwrites the memory location we *can* potentially optimize it. |
| // |
| // Find out what memory location the dependent instruction stores. |
| Instruction *DepWrite = InstDep.getInst(); |
| if (!hasAnalyzableMemoryWrite(DepWrite, *TLI)) |
| break; |
| MemoryLocation DepLoc = getLocForWrite(DepWrite, *TLI); |
| // If we didn't get a useful location, or if it isn't a size, bail out. |
| if (!DepLoc.Ptr) |
| break; |
| |
| // Find the last throwable instruction not removed by call to |
| // deleteDeadInstruction. |
| Instruction *LastThrowing = nullptr; |
| if (!ThrowableInst.empty()) |
| LastThrowing = ThrowableInst.back().first; |
| |
| // Make sure we don't look past a call which might throw. This is an |
| // issue because MemoryDependenceAnalysis works in the wrong direction: |
| // it finds instructions which dominate the current instruction, rather than |
| // instructions which are post-dominated by the current instruction. |
| // |
| // If the underlying object is a non-escaping memory allocation, any store |
| // to it is dead along the unwind edge. Otherwise, we need to preserve |
| // the store. |
| if (LastThrowing && DepWrite->comesBefore(LastThrowing)) { |
| const Value *Underlying = getUnderlyingObject(DepLoc.Ptr); |
| bool IsStoreDeadOnUnwind = isa<AllocaInst>(Underlying); |
| if (!IsStoreDeadOnUnwind) { |
| // We're looking for a call to an allocation function |
| // where the allocation doesn't escape before the last |
| // throwing instruction; PointerMayBeCaptured |
| // reasonably fast approximation. |
| IsStoreDeadOnUnwind = isAllocLikeFn(Underlying, TLI) && |
| !PointerMayBeCaptured(Underlying, false, true); |
| } |
| if (!IsStoreDeadOnUnwind) |
| break; |
| } |
| |
| // If we find a write that is a) removable (i.e., non-volatile), b) is |
| // completely obliterated by the store to 'Loc', and c) which we know that |
| // 'Inst' doesn't load from, then we can remove it. |
| // Also try to merge two stores if a later one only touches memory written |
| // to by the earlier one. |
| if (isRemovable(DepWrite) && |
| !isPossibleSelfRead(Inst, Loc, DepWrite, *TLI, *AA)) { |
| int64_t InstWriteOffset, DepWriteOffset; |
| OverwriteResult OR = isOverwrite(Inst, DepWrite, Loc, DepLoc, DL, *TLI, |
| DepWriteOffset, InstWriteOffset, *AA, |
| BB.getParent()); |
| if (OR == OW_MaybePartial) |
| OR = isPartialOverwrite(Loc, DepLoc, DepWriteOffset, InstWriteOffset, |
| DepWrite, IOL); |
| |
| if (OR == OW_Complete) { |
| LLVM_DEBUG(dbgs() << "DSE: Remove Dead Store:\n DEAD: " << *DepWrite |
| << "\n KILLER: " << *Inst << '\n'); |
| |
| // Delete the store and now-dead instructions that feed it. |
| deleteDeadInstruction(DepWrite, &BBI, *MD, *TLI, IOL, |
| ThrowableInst); |
| ++NumFastStores; |
| MadeChange = true; |
| |
| // We erased DepWrite; start over. |
| InstDep = MD->getDependency(Inst); |
| continue; |
| } else if ((OR == OW_End && isShortenableAtTheEnd(DepWrite)) || |
| ((OR == OW_Begin && |
| isShortenableAtTheBeginning(DepWrite)))) { |
| assert(!EnablePartialOverwriteTracking && "Do not expect to perform " |
| "when partial-overwrite " |
| "tracking is enabled"); |
| // The overwrite result is known, so these must be known, too. |
| uint64_t EarlierSize = DepLoc.Size.getValue(); |
| uint64_t LaterSize = Loc.Size.getValue(); |
| bool IsOverwriteEnd = (OR == OW_End); |
| MadeChange |= tryToShorten(DepWrite, DepWriteOffset, EarlierSize, |
| InstWriteOffset, LaterSize, IsOverwriteEnd); |
| } else if (EnablePartialStoreMerging && |
| OR == OW_PartialEarlierWithFullLater) { |
| auto *Earlier = dyn_cast<StoreInst>(DepWrite); |
| auto *Later = dyn_cast<StoreInst>(Inst); |
| if (Constant *C = tryToMergePartialOverlappingStores( |
| Earlier, Later, InstWriteOffset, DepWriteOffset, DL, *AA, |
| DT)) { |
| auto *SI = new StoreInst( |
| C, Earlier->getPointerOperand(), false, Earlier->getAlign(), |
| Earlier->getOrdering(), Earlier->getSyncScopeID(), DepWrite); |
| |
| unsigned MDToKeep[] = {LLVMContext::MD_dbg, LLVMContext::MD_tbaa, |
| LLVMContext::MD_alias_scope, |
| LLVMContext::MD_noalias, |
| LLVMContext::MD_nontemporal}; |
| SI->copyMetadata(*DepWrite, MDToKeep); |
| ++NumModifiedStores; |
| |
| // Delete the old stores and now-dead instructions that feed them. |
| deleteDeadInstruction(Inst, &BBI, *MD, *TLI, IOL, |
| ThrowableInst); |
| deleteDeadInstruction(DepWrite, &BBI, *MD, *TLI, IOL, |
| ThrowableInst); |
| MadeChange = true; |
| |
| // We erased DepWrite and Inst (Loc); start over. |
| break; |
| } |
| } |
| } |
| |
| // If this is a may-aliased store that is clobbering the store value, we |
| // can keep searching past it for another must-aliased pointer that stores |
| // to the same location. For example, in: |
| // store -> P |
| // store -> Q |
| // store -> P |
| // we can remove the first store to P even though we don't know if P and Q |
| // alias. |
| if (DepWrite == &BB.front()) break; |
| |
| // Can't look past this instruction if it might read 'Loc'. |
| if (isRefSet(AA->getModRefInfo(DepWrite, Loc))) |
| break; |
| |
| InstDep = MD->getPointerDependencyFrom(Loc, /*isLoad=*/ false, |
| DepWrite->getIterator(), &BB, |
| /*QueryInst=*/ nullptr, &Limit); |
| } |
| } |
| |
| if (EnablePartialOverwriteTracking) |
| MadeChange |= removePartiallyOverlappedStores(DL, IOL, *TLI); |
| |
| // If this block ends in a return, unwind, or unreachable, all allocas are |
| // dead at its end, which means stores to them are also dead. |
| if (BB.getTerminator()->getNumSuccessors() == 0) |
| MadeChange |= handleEndBlock(BB, AA, MD, TLI, IOL, ThrowableInst); |
| |
| return MadeChange; |
| } |
| |
| static bool eliminateDeadStores(Function &F, AliasAnalysis *AA, |
| MemoryDependenceResults *MD, DominatorTree *DT, |
| const TargetLibraryInfo *TLI) { |
| bool MadeChange = false; |
| for (BasicBlock &BB : F) |
| // Only check non-dead blocks. Dead blocks may have strange pointer |
| // cycles that will confuse alias analysis. |
| if (DT->isReachableFromEntry(&BB)) |
| MadeChange |= eliminateDeadStores(BB, AA, MD, DT, TLI); |
| |
| return MadeChange; |
| } |
| |
| namespace { |
| //============================================================================= |
| // MemorySSA backed dead store elimination. |
| // |
| // The code below implements dead store elimination using MemorySSA. It uses |
| // the following general approach: given a MemoryDef, walk upwards to find |
| // clobbering MemoryDefs that may be killed by the starting def. Then check |
| // that there are no uses that may read the location of the original MemoryDef |
| // in between both MemoryDefs. A bit more concretely: |
| // |
| // For all MemoryDefs StartDef: |
| // 1. Get the next dominating clobbering MemoryDef (EarlierAccess) by walking |
| // upwards. |
| // 2. Check that there are no reads between EarlierAccess and the StartDef by |
| // checking all uses starting at EarlierAccess and walking until we see |
| // StartDef. |
| // 3. For each found CurrentDef, check that: |
| // 1. There are no barrier instructions between CurrentDef and StartDef (like |
| // throws or stores with ordering constraints). |
| // 2. StartDef is executed whenever CurrentDef is executed. |
| // 3. StartDef completely overwrites CurrentDef. |
| // 4. Erase CurrentDef from the function and MemorySSA. |
| |
| // Returns true if \p I is an intrisnic that does not read or write memory. |
| bool isNoopIntrinsic(Instruction *I) { |
| if (const IntrinsicInst *II = dyn_cast<IntrinsicInst>(I)) { |
| switch (II->getIntrinsicID()) { |
| case Intrinsic::lifetime_start: |
| case Intrinsic::lifetime_end: |
| case Intrinsic::invariant_end: |
| case Intrinsic::launder_invariant_group: |
| case Intrinsic::assume: |
| return true; |
| case Intrinsic::dbg_addr: |
| case Intrinsic::dbg_declare: |
| case Intrinsic::dbg_label: |
| case Intrinsic::dbg_value: |
| llvm_unreachable("Intrinsic should not be modeled in MemorySSA"); |
| default: |
| return false; |
| } |
| } |
| return false; |
| } |
| |
| // Check if we can ignore \p D for DSE. |
| bool canSkipDef(MemoryDef *D, bool DefVisibleToCaller) { |
| Instruction *DI = D->getMemoryInst(); |
| // Calls that only access inaccessible memory cannot read or write any memory |
| // locations we consider for elimination. |
| if (auto *CB = dyn_cast<CallBase>(DI)) |
| if (CB->onlyAccessesInaccessibleMemory()) |
| return true; |
| |
| // We can eliminate stores to locations not visible to the caller across |
| // throwing instructions. |
| if (DI->mayThrow() && !DefVisibleToCaller) |
| return true; |
| |
| // We can remove the dead stores, irrespective of the fence and its ordering |
| // (release/acquire/seq_cst). Fences only constraints the ordering of |
| // already visible stores, it does not make a store visible to other |
| // threads. So, skipping over a fence does not change a store from being |
| // dead. |
| if (isa<FenceInst>(DI)) |
| return true; |
| |
| // Skip intrinsics that do not really read or modify memory. |
| if (isNoopIntrinsic(D->getMemoryInst())) |
| return true; |
| |
| return false; |
| } |
| |
| struct DSEState { |
| Function &F; |
| AliasAnalysis &AA; |
| |
| /// The single BatchAA instance that is used to cache AA queries. It will |
| /// not be invalidated over the whole run. This is safe, because: |
| /// 1. Only memory writes are removed, so the alias cache for memory |
| /// locations remains valid. |
| /// 2. No new instructions are added (only instructions removed), so cached |
| /// information for a deleted value cannot be accessed by a re-used new |
| /// value pointer. |
| BatchAAResults BatchAA; |
| |
| MemorySSA &MSSA; |
| DominatorTree &DT; |
| PostDominatorTree &PDT; |
| const TargetLibraryInfo &TLI; |
| const DataLayout &DL; |
| |
| // All MemoryDefs that potentially could kill other MemDefs. |
| SmallVector<MemoryDef *, 64> MemDefs; |
| // Any that should be skipped as they are already deleted |
| SmallPtrSet<MemoryAccess *, 4> SkipStores; |
| // Keep track of all of the objects that are invisible to the caller before |
| // the function returns. |
| // SmallPtrSet<const Value *, 16> InvisibleToCallerBeforeRet; |
| DenseMap<const Value *, bool> InvisibleToCallerBeforeRet; |
| // Keep track of all of the objects that are invisible to the caller after |
| // the function returns. |
| DenseMap<const Value *, bool> InvisibleToCallerAfterRet; |
| // Keep track of blocks with throwing instructions not modeled in MemorySSA. |
| SmallPtrSet<BasicBlock *, 16> ThrowingBlocks; |
| // Post-order numbers for each basic block. Used to figure out if memory |
| // accesses are executed before another access. |
| DenseMap<BasicBlock *, unsigned> PostOrderNumbers; |
| |
| /// Keep track of instructions (partly) overlapping with killing MemoryDefs per |
| /// basic block. |
| DenseMap<BasicBlock *, InstOverlapIntervalsTy> IOLs; |
| |
| DSEState(Function &F, AliasAnalysis &AA, MemorySSA &MSSA, DominatorTree &DT, |
| PostDominatorTree &PDT, const TargetLibraryInfo &TLI) |
| : F(F), AA(AA), BatchAA(AA), MSSA(MSSA), DT(DT), PDT(PDT), TLI(TLI), |
| DL(F.getParent()->getDataLayout()) {} |
| |
| static DSEState get(Function &F, AliasAnalysis &AA, MemorySSA &MSSA, |
| DominatorTree &DT, PostDominatorTree &PDT, |
| const TargetLibraryInfo &TLI) { |
| DSEState State(F, AA, MSSA, DT, PDT, TLI); |
| // Collect blocks with throwing instructions not modeled in MemorySSA and |
| // alloc-like objects. |
| unsigned PO = 0; |
| for (BasicBlock *BB : post_order(&F)) { |
| State.PostOrderNumbers[BB] = PO++; |
| for (Instruction &I : *BB) { |
| MemoryAccess *MA = MSSA.getMemoryAccess(&I); |
| if (I.mayThrow() && !MA) |
| State.ThrowingBlocks.insert(I.getParent()); |
| |
| auto *MD = dyn_cast_or_null<MemoryDef>(MA); |
| if (MD && State.MemDefs.size() < MemorySSADefsPerBlockLimit && |
| (State.getLocForWriteEx(&I) || State.isMemTerminatorInst(&I))) |
| State.MemDefs.push_back(MD); |
| } |
| } |
| |
| // Treat byval or inalloca arguments the same as Allocas, stores to them are |
| // dead at the end of the function. |
| for (Argument &AI : F.args()) |
| if (AI.hasPassPointeeByValueCopyAttr()) { |
| // For byval, the caller doesn't know the address of the allocation. |
| if (AI.hasByValAttr()) |
| State.InvisibleToCallerBeforeRet.insert({&AI, true}); |
| State.InvisibleToCallerAfterRet.insert({&AI, true}); |
| } |
| |
| return State; |
| } |
| |
| bool isInvisibleToCallerAfterRet(const Value *V) { |
| if (isa<AllocaInst>(V)) |
| return true; |
| auto I = InvisibleToCallerAfterRet.insert({V, false}); |
| if (I.second) { |
| if (!isInvisibleToCallerBeforeRet(V)) { |
| I.first->second = false; |
| } else { |
| auto *Inst = dyn_cast<Instruction>(V); |
| if (Inst && isAllocLikeFn(Inst, &TLI)) |
| I.first->second = !PointerMayBeCaptured(V, true, false); |
| } |
| } |
| return I.first->second; |
| } |
| |
| bool isInvisibleToCallerBeforeRet(const Value *V) { |
| if (isa<AllocaInst>(V)) |
| return true; |
| auto I = InvisibleToCallerBeforeRet.insert({V, false}); |
| if (I.second) { |
| auto *Inst = dyn_cast<Instruction>(V); |
| if (Inst && isAllocLikeFn(Inst, &TLI)) |
| // NOTE: This could be made more precise by PointerMayBeCapturedBefore |
| // with the killing MemoryDef. But we refrain from doing so for now to |
| // limit compile-time and this does not cause any changes to the number |
| // of stores removed on a large test set in practice. |
| I.first->second = !PointerMayBeCaptured(V, false, true); |
| } |
| return I.first->second; |
| } |
| |
| Optional<MemoryLocation> getLocForWriteEx(Instruction *I) const { |
| if (!I->mayWriteToMemory()) |
| return None; |
| |
| if (auto *MTI = dyn_cast<AnyMemIntrinsic>(I)) |
| return {MemoryLocation::getForDest(MTI)}; |
| |
| if (auto *CB = dyn_cast<CallBase>(I)) { |
| // If the functions may write to memory we do not know about, bail out. |
| if (!CB->onlyAccessesArgMemory() && |
| !CB->onlyAccessesInaccessibleMemOrArgMem()) |
| return None; |
| |
| LibFunc LF; |
| if (TLI.getLibFunc(*CB, LF) && TLI.has(LF)) { |
| switch (LF) { |
| case LibFunc_strcpy: |
| case LibFunc_strncpy: |
| case LibFunc_strcat: |
| case LibFunc_strncat: |
| return {MemoryLocation::getAfter(CB->getArgOperand(0))}; |
| default: |
| break; |
| } |
| } |
| switch (CB->getIntrinsicID()) { |
| case Intrinsic::init_trampoline: |
| return {MemoryLocation::getAfter(CB->getArgOperand(0))}; |
| case Intrinsic::masked_store: |
| return {MemoryLocation::getForArgument(CB, 1, TLI)}; |
| default: |
| break; |
| } |
| return None; |
| } |
| |
| return MemoryLocation::getOrNone(I); |
| } |
| |
| /// Returns true if \p UseInst completely overwrites \p DefLoc |
| /// (stored by \p DefInst). |
| bool isCompleteOverwrite(const MemoryLocation &DefLoc, Instruction *DefInst, |
| Instruction *UseInst) { |
| // UseInst has a MemoryDef associated in MemorySSA. It's possible for a |
| // MemoryDef to not write to memory, e.g. a volatile load is modeled as a |
| // MemoryDef. |
| if (!UseInst->mayWriteToMemory()) |
| return false; |
| |
| if (auto *CB = dyn_cast<CallBase>(UseInst)) |
| if (CB->onlyAccessesInaccessibleMemory()) |
| return false; |
| |
| int64_t InstWriteOffset, DepWriteOffset; |
| if (auto CC = getLocForWriteEx(UseInst)) |
| return isOverwrite(UseInst, DefInst, *CC, DefLoc, DL, TLI, DepWriteOffset, |
| InstWriteOffset, BatchAA, &F) == OW_Complete; |
| return false; |
| } |
| |
| /// Returns true if \p Def is not read before returning from the function. |
| bool isWriteAtEndOfFunction(MemoryDef *Def) { |
| LLVM_DEBUG(dbgs() << " Check if def " << *Def << " (" |
| << *Def->getMemoryInst() |
| << ") is at the end the function \n"); |
| |
| auto MaybeLoc = getLocForWriteEx(Def->getMemoryInst()); |
| if (!MaybeLoc) { |
| LLVM_DEBUG(dbgs() << " ... could not get location for write.\n"); |
| return false; |
| } |
| |
| SmallVector<MemoryAccess *, 4> WorkList; |
| SmallPtrSet<MemoryAccess *, 8> Visited; |
| auto PushMemUses = [&WorkList, &Visited](MemoryAccess *Acc) { |
| if (!Visited.insert(Acc).second) |
| return; |
| for (Use &U : Acc->uses()) |
| WorkList.push_back(cast<MemoryAccess>(U.getUser())); |
| }; |
| PushMemUses(Def); |
| for (unsigned I = 0; I < WorkList.size(); I++) { |
| if (WorkList.size() >= MemorySSAScanLimit) { |
| LLVM_DEBUG(dbgs() << " ... hit exploration limit.\n"); |
| return false; |
| } |
| |
| MemoryAccess *UseAccess = WorkList[I]; |
| // Simply adding the users of MemoryPhi to the worklist is not enough, |
| // because we might miss read clobbers in different iterations of a loop, |
| // for example. |
| // TODO: Add support for phi translation to handle the loop case. |
| if (isa<MemoryPhi>(UseAccess)) |
| return false; |
| |
| // TODO: Checking for aliasing is expensive. Consider reducing the amount |
| // of times this is called and/or caching it. |
| Instruction *UseInst = cast<MemoryUseOrDef>(UseAccess)->getMemoryInst(); |
| if (isReadClobber(*MaybeLoc, UseInst)) { |
| LLVM_DEBUG(dbgs() << " ... hit read clobber " << *UseInst << ".\n"); |
| return false; |
| } |
| |
| if (MemoryDef *UseDef = dyn_cast<MemoryDef>(UseAccess)) |
| PushMemUses(UseDef); |
| } |
| return true; |
| } |
| |
| /// If \p I is a memory terminator like llvm.lifetime.end or free, return a |
| /// pair with the MemoryLocation terminated by \p I and a boolean flag |
| /// indicating whether \p I is a free-like call. |
| Optional<std::pair<MemoryLocation, bool>> |
| getLocForTerminator(Instruction *I) const { |
| uint64_t Len; |
| Value *Ptr; |
| if (match(I, m_Intrinsic<Intrinsic::lifetime_end>(m_ConstantInt(Len), |
| m_Value(Ptr)))) |
| return {std::make_pair(MemoryLocation(Ptr, Len), false)}; |
| |
| if (auto *CB = dyn_cast<CallBase>(I)) { |
| if (isFreeCall(I, &TLI)) |
| return {std::make_pair(MemoryLocation::getAfter(CB->getArgOperand(0)), |
| true)}; |
| } |
| |
| return None; |
| } |
| |
| /// Returns true if \p I is a memory terminator instruction like |
| /// llvm.lifetime.end or free. |
| bool isMemTerminatorInst(Instruction *I) const { |
| IntrinsicInst *II = dyn_cast<IntrinsicInst>(I); |
| return (II && II->getIntrinsicID() == Intrinsic::lifetime_end) || |
| isFreeCall(I, &TLI); |
| } |
| |
| /// Returns true if \p MaybeTerm is a memory terminator for \p Loc from |
| /// instruction \p AccessI. |
| bool isMemTerminator(const MemoryLocation &Loc, Instruction *AccessI, |
| Instruction *MaybeTerm) { |
| Optional<std::pair<MemoryLocation, bool>> MaybeTermLoc = |
| getLocForTerminator(MaybeTerm); |
| |
| if (!MaybeTermLoc) |
| return false; |
| |
| // If the terminator is a free-like call, all accesses to the underlying |
| // object can be considered terminated. |
| if (getUnderlyingObject(Loc.Ptr) != |
| getUnderlyingObject(MaybeTermLoc->first.Ptr)) |
| return false; |
| |
| auto TermLoc = MaybeTermLoc->first; |
| if (MaybeTermLoc->second) { |
| const Value *LocUO = getUnderlyingObject(Loc.Ptr); |
| return BatchAA.isMustAlias(TermLoc.Ptr, LocUO); |
| } |
| int64_t InstWriteOffset, DepWriteOffset; |
| return isOverwrite(MaybeTerm, AccessI, TermLoc, Loc, DL, TLI, |
| DepWriteOffset, InstWriteOffset, BatchAA, |
| &F) == OW_Complete; |
| } |
| |
| // Returns true if \p Use may read from \p DefLoc. |
| bool isReadClobber(const MemoryLocation &DefLoc, Instruction *UseInst) { |
| if (isNoopIntrinsic(UseInst)) |
| return false; |
| |
| // Monotonic or weaker atomic stores can be re-ordered and do not need to be |
| // treated as read clobber. |
| if (auto SI = dyn_cast<StoreInst>(UseInst)) |
| return isStrongerThan(SI->getOrdering(), AtomicOrdering::Monotonic); |
| |
| if (!UseInst->mayReadFromMemory()) |
| return false; |
| |
| if (auto *CB = dyn_cast<CallBase>(UseInst)) |
| if (CB->onlyAccessesInaccessibleMemory()) |
| return false; |
| |
| // NOTE: For calls, the number of stores removed could be slightly improved |
| // by using AA.callCapturesBefore(UseInst, DefLoc, &DT), but that showed to |
| // be expensive compared to the benefits in practice. For now, avoid more |
| // expensive analysis to limit compile-time. |
| return isRefSet(BatchAA.getModRefInfo(UseInst, DefLoc)); |
| } |
| |
| /// Returns true if \p Ptr is guaranteed to be loop invariant for any possible |
| /// loop. In particular, this guarantees that it only references a single |
| /// MemoryLocation during execution of the containing function. |
| bool IsGuaranteedLoopInvariant(Value *Ptr) { |
| auto IsGuaranteedLoopInvariantBase = [this](Value *Ptr) { |
| Ptr = Ptr->stripPointerCasts(); |
| if (auto *I = dyn_cast<Instruction>(Ptr)) { |
| if (isa<AllocaInst>(Ptr)) |
| return true; |
| |
| if (isAllocLikeFn(I, &TLI)) |
| return true; |
| |
| return false; |
| } |
| return true; |
| }; |
| |
| Ptr = Ptr->stripPointerCasts(); |
| if (auto *GEP = dyn_cast<GEPOperator>(Ptr)) { |
| return IsGuaranteedLoopInvariantBase(GEP->getPointerOperand()) && |
| GEP->hasAllConstantIndices(); |
| } |
| return IsGuaranteedLoopInvariantBase(Ptr); |
| } |
| |
| // Find a MemoryDef writing to \p DefLoc and dominating \p StartAccess, with |
| // no read access between them or on any other path to a function exit block |
| // if \p DefLoc is not accessible after the function returns. If there is no |
| // such MemoryDef, return None. The returned value may not (completely) |
| // overwrite \p DefLoc. Currently we bail out when we encounter an aliasing |
| // MemoryUse (read). |
| Optional<MemoryAccess *> |
| getDomMemoryDef(MemoryDef *KillingDef, MemoryAccess *StartAccess, |
| const MemoryLocation &DefLoc, const Value *DefUO, |
| unsigned &ScanLimit, unsigned &WalkerStepLimit, |
| bool IsMemTerm, unsigned &PartialLimit) { |
| if (ScanLimit == 0 || WalkerStepLimit == 0) { |
| LLVM_DEBUG(dbgs() << "\n ... hit scan limit\n"); |
| return None; |
| } |
| |
| MemoryAccess *Current = StartAccess; |
| Instruction *KillingI = KillingDef->getMemoryInst(); |
| bool StepAgain; |
| LLVM_DEBUG(dbgs() << " trying to get dominating access\n"); |
| |
| // Find the next clobbering Mod access for DefLoc, starting at StartAccess. |
| Optional<MemoryLocation> CurrentLoc; |
| do { |
| StepAgain = false; |
| LLVM_DEBUG({ |
| dbgs() << " visiting " << *Current; |
| if (!MSSA.isLiveOnEntryDef(Current) && isa<MemoryUseOrDef>(Current)) |
| dbgs() << " (" << *cast<MemoryUseOrDef>(Current)->getMemoryInst() |
| << ")"; |
| dbgs() << "\n"; |
| }); |
| |
| // Reached TOP. |
| if (MSSA.isLiveOnEntryDef(Current)) { |
| LLVM_DEBUG(dbgs() << " ... found LiveOnEntryDef\n"); |
| return None; |
| } |
| |
| // Cost of a step. Accesses in the same block are more likely to be valid |
| // candidates for elimination, hence consider them cheaper. |
| unsigned StepCost = KillingDef->getBlock() == Current->getBlock() |
| ? MemorySSASameBBStepCost |
| : MemorySSAOtherBBStepCost; |
| if (WalkerStepLimit <= StepCost) { |
| LLVM_DEBUG(dbgs() << " ... hit walker step limit\n"); |
| return None; |
| } |
| WalkerStepLimit -= StepCost; |
| |
| // Return for MemoryPhis. They cannot be eliminated directly and the |
| // caller is responsible for traversing them. |
| if (isa<MemoryPhi>(Current)) { |
| LLVM_DEBUG(dbgs() << " ... found MemoryPhi\n"); |
| return Current; |
| } |
| |
| // Below, check if CurrentDef is a valid candidate to be eliminated by |
| // KillingDef. If it is not, check the next candidate. |
| MemoryDef *CurrentDef = cast<MemoryDef>(Current); |
| Instruction *CurrentI = CurrentDef->getMemoryInst(); |
| |
| if (canSkipDef(CurrentDef, !isInvisibleToCallerBeforeRet(DefUO))) { |
| StepAgain = true; |
| Current = CurrentDef->getDefiningAccess(); |
| continue; |
| } |
| |
| // Before we try to remove anything, check for any extra throwing |
| // instructions that block us from DSEing |
| if (mayThrowBetween(KillingI, CurrentI, DefUO)) { |
| LLVM_DEBUG(dbgs() << " ... skip, may throw!\n"); |
| return None; |
| } |
| |
| // Check for anything that looks like it will be a barrier to further |
| // removal |
| if (isDSEBarrier(DefUO, CurrentI)) { |
| LLVM_DEBUG(dbgs() << " ... skip, barrier\n"); |
| return None; |
| } |
| |
| // If Current is known to be on path that reads DefLoc or is a read |
| // clobber, bail out, as the path is not profitable. We skip this check |
| // for intrinsic calls, because the code knows how to handle memcpy |
| // intrinsics. |
| if (!isa<IntrinsicInst>(CurrentI) && isReadClobber(DefLoc, CurrentI)) |
| return None; |
| |
| // Quick check if there are direct uses that are read-clobbers. |
| if (any_of(Current->uses(), [this, &DefLoc, StartAccess](Use &U) { |
| if (auto *UseOrDef = dyn_cast<MemoryUseOrDef>(U.getUser())) |
| return !MSSA.dominates(StartAccess, UseOrDef) && |
| isReadClobber(DefLoc, UseOrDef->getMemoryInst()); |
| return false; |
| })) { |
| LLVM_DEBUG(dbgs() << " ... found a read clobber\n"); |
| return None; |
| } |
| |
| // If Current cannot be analyzed or is not removable, check the next |
| // candidate. |
| if (!hasAnalyzableMemoryWrite(CurrentI, TLI) || !isRemovable(CurrentI)) { |
| StepAgain = true; |
| Current = CurrentDef->getDefiningAccess(); |
| continue; |
| } |
| |
| // If Current does not have an analyzable write location, skip it |
| CurrentLoc = getLocForWriteEx(CurrentI); |
| if (!CurrentLoc) { |
| StepAgain = true; |
| Current = CurrentDef->getDefiningAccess(); |
| continue; |
| } |
| |
| // AliasAnalysis does not account for loops. Limit elimination to |
| // candidates for which we can guarantee they always store to the same |
| // memory location and not multiple locations in a loop. |
| if (Current->getBlock() != KillingDef->getBlock() && |
| !IsGuaranteedLoopInvariant(const_cast<Value *>(CurrentLoc->Ptr))) { |
| StepAgain = true; |
| Current = CurrentDef->getDefiningAccess(); |
| WalkerStepLimit -= 1; |
| continue; |
| } |
| |
| if (IsMemTerm) { |
| // If the killing def is a memory terminator (e.g. lifetime.end), check |
| // the next candidate if the current Current does not write the same |
| // underlying object as the terminator. |
| if (!isMemTerminator(*CurrentLoc, CurrentI, KillingI)) { |
| StepAgain = true; |
| Current = CurrentDef->getDefiningAccess(); |
| } |
| continue; |
| } else { |
| int64_t InstWriteOffset, DepWriteOffset; |
| auto OR = isOverwrite(KillingI, CurrentI, DefLoc, *CurrentLoc, DL, TLI, |
| DepWriteOffset, InstWriteOffset, BatchAA, &F); |
| // If Current does not write to the same object as KillingDef, check |
| // the next candidate. |
| if (OR == OW_Unknown) { |
| StepAgain = true; |
| Current = CurrentDef->getDefiningAccess(); |
| } else if (OR == OW_MaybePartial) { |
| // If KillingDef only partially overwrites Current, check the next |
| // candidate if the partial step limit is exceeded. This aggressively |
| // limits the number of candidates for partial store elimination, |
| // which are less likely to be removable in the end. |
| if (PartialLimit <= 1) { |
| StepAgain = true; |
| Current = CurrentDef->getDefiningAccess(); |
| WalkerStepLimit -= 1; |
| continue; |
| } |
| PartialLimit -= 1; |
| } |
| } |
| } while (StepAgain); |
| |
| // Accesses to objects accessible after the function returns can only be |
| // eliminated if the access is killed along all paths to the exit. Collect |
| // the blocks with killing (=completely overwriting MemoryDefs) and check if |
| // they cover all paths from EarlierAccess to any function exit. |
| SmallPtrSet<Instruction *, 16> KillingDefs; |
| KillingDefs.insert(KillingDef->getMemoryInst()); |
| MemoryAccess *EarlierAccess = Current; |
| Instruction *EarlierMemInst = |
| cast<MemoryDef>(EarlierAccess)->getMemoryInst(); |
| LLVM_DEBUG(dbgs() << " Checking for reads of " << *EarlierAccess << " (" |
| << *EarlierMemInst << ")\n"); |
| |
| SmallSetVector<MemoryAccess *, 32> WorkList; |
| auto PushMemUses = [&WorkList](MemoryAccess *Acc) { |
| for (Use &U : Acc->uses()) |
| WorkList.insert(cast<MemoryAccess>(U.getUser())); |
| }; |
| PushMemUses(EarlierAccess); |
| |
| // Optimistically collect all accesses for reads. If we do not find any |
| // read clobbers, add them to the cache. |
| SmallPtrSet<MemoryAccess *, 16> KnownNoReads; |
| if (!EarlierMemInst->mayReadFromMemory()) |
| KnownNoReads.insert(EarlierAccess); |
| // Check if EarlierDef may be read. |
| for (unsigned I = 0; I < WorkList.size(); I++) { |
| MemoryAccess *UseAccess = WorkList[I]; |
| |
| LLVM_DEBUG(dbgs() << " " << *UseAccess); |
| // Bail out if the number of accesses to check exceeds the scan limit. |
| if (ScanLimit < (WorkList.size() - I)) { |
| LLVM_DEBUG(dbgs() << "\n ... hit scan limit\n"); |
| return None; |
| } |
| --ScanLimit; |
| NumDomMemDefChecks++; |
| KnownNoReads.insert(UseAccess); |
| |
| if (isa<MemoryPhi>(UseAccess)) { |
| if (any_of(KillingDefs, [this, UseAccess](Instruction *KI) { |
| return DT.properlyDominates(KI->getParent(), |
| UseAccess->getBlock()); |
| })) { |
| LLVM_DEBUG(dbgs() << " ... skipping, dominated by killing block\n"); |
| continue; |
| } |
| LLVM_DEBUG(dbgs() << "\n ... adding PHI uses\n"); |
| PushMemUses(UseAccess); |
| continue; |
| } |
| |
| Instruction *UseInst = cast<MemoryUseOrDef>(UseAccess)->getMemoryInst(); |
| LLVM_DEBUG(dbgs() << " (" << *UseInst << ")\n"); |
| |
| if (any_of(KillingDefs, [this, UseInst](Instruction *KI) { |
| return DT.dominates(KI, UseInst); |
| })) { |
| LLVM_DEBUG(dbgs() << " ... skipping, dominated by killing def\n"); |
| continue; |
| } |
| |
| // A memory terminator kills all preceeding MemoryDefs and all succeeding |
| // MemoryAccesses. We do not have to check it's users. |
| if (isMemTerminator(*CurrentLoc, EarlierMemInst, UseInst)) { |
| LLVM_DEBUG( |
| dbgs() |
| << " ... skipping, memterminator invalidates following accesses\n"); |
| continue; |
| } |
| |
| if (isNoopIntrinsic(cast<MemoryUseOrDef>(UseAccess)->getMemoryInst())) { |
| LLVM_DEBUG(dbgs() << " ... adding uses of intrinsic\n"); |
| PushMemUses(UseAccess); |
| continue; |
| } |
| |
| if (UseInst->mayThrow() && !isInvisibleToCallerBeforeRet(DefUO)) { |
| LLVM_DEBUG(dbgs() << " ... found throwing instruction\n"); |
| return None; |
| } |
| |
| // Uses which may read the original MemoryDef mean we cannot eliminate the |
| // original MD. Stop walk. |
| if (isReadClobber(*CurrentLoc, UseInst)) { |
| LLVM_DEBUG(dbgs() << " ... found read clobber\n"); |
| return None; |
| } |
| |
| // For the KillingDef and EarlierAccess we only have to check if it reads |
| // the memory location. |
| // TODO: It would probably be better to check for self-reads before |
| // calling the function. |
| if (KillingDef == UseAccess || EarlierAccess == UseAccess) { |
| LLVM_DEBUG(dbgs() << " ... skipping killing def/dom access\n"); |
| continue; |
| } |
| |
| // Check all uses for MemoryDefs, except for defs completely overwriting |
| // the original location. Otherwise we have to check uses of *all* |
| // MemoryDefs we discover, including non-aliasing ones. Otherwise we might |
| // miss cases like the following |
| // 1 = Def(LoE) ; <----- EarlierDef stores [0,1] |
| // 2 = Def(1) ; (2, 1) = NoAlias, stores [2,3] |
| // Use(2) ; MayAlias 2 *and* 1, loads [0, 3]. |
| // (The Use points to the *first* Def it may alias) |
| // 3 = Def(1) ; <---- Current (3, 2) = NoAlias, (3,1) = MayAlias, |
| // stores [0,1] |
| if (MemoryDef *UseDef = dyn_cast<MemoryDef>(UseAccess)) { |
| if (isCompleteOverwrite(*CurrentLoc, EarlierMemInst, UseInst)) { |
| if (!isInvisibleToCallerAfterRet(DefUO) && |
| UseAccess != EarlierAccess) { |
| BasicBlock *MaybeKillingBlock = UseInst->getParent(); |
| if (PostOrderNumbers.find(MaybeKillingBlock)->second < |
| PostOrderNumbers.find(EarlierAccess->getBlock())->second) { |
| |
| LLVM_DEBUG(dbgs() |
| << " ... found killing def " << *UseInst << "\n"); |
| KillingDefs.insert(UseInst); |
| } |
| } |
| } else |
| PushMemUses(UseDef); |
| } |
| } |
| |
| // For accesses to locations visible after the function returns, make sure |
| // that the location is killed (=overwritten) along all paths from |
| // EarlierAccess to the exit. |
| if (!isInvisibleToCallerAfterRet(DefUO)) { |
| SmallPtrSet<BasicBlock *, 16> KillingBlocks; |
| for (Instruction *KD : KillingDefs) |
| KillingBlocks.insert(KD->getParent()); |
| assert(!KillingBlocks.empty() && |
| "Expected at least a single killing block"); |
| |
| // Find the common post-dominator of all killing blocks. |
| BasicBlock *CommonPred = *KillingBlocks.begin(); |
| for (auto I = std::next(KillingBlocks.begin()), E = KillingBlocks.end(); |
| I != E; I++) { |
| if (!CommonPred) |
| break; |
| CommonPred = PDT.findNearestCommonDominator(CommonPred, *I); |
| } |
| |
| // If CommonPred is in the set of killing blocks, just check if it |
| // post-dominates EarlierAccess. |
| if (KillingBlocks.count(CommonPred)) { |
| if (PDT.dominates(CommonPred, EarlierAccess->getBlock())) |
| return {EarlierAccess}; |
| return None; |
| } |
| |
| // If the common post-dominator does not post-dominate EarlierAccess, |
| // there is a path from EarlierAccess to an exit not going through a |
| // killing block. |
| if (PDT.dominates(CommonPred, EarlierAccess->getBlock())) { |
| SetVector<BasicBlock *> WorkList; |
| |
| // If CommonPred is null, there are multiple exits from the function. |
| // They all have to be added to the worklist. |
| if (CommonPred) |
| WorkList.insert(CommonPred); |
| else |
| for (BasicBlock *R : PDT.roots()) |
| WorkList.insert(R); |
| |
| NumCFGTries++; |
| // Check if all paths starting from an exit node go through one of the |
| // killing blocks before reaching EarlierAccess. |
| for (unsigned I = 0; I < WorkList.size(); I++) { |
| NumCFGChecks++; |
| BasicBlock *Current = WorkList[I]; |
| if (KillingBlocks.count(Current)) |
| continue; |
| if (Current == EarlierAccess->getBlock()) |
| return None; |
| |
| // EarlierAccess is reachable from the entry, so we don't have to |
| // explore unreachable blocks further. |
| if (!DT.isReachableFromEntry(Current)) |
| continue; |
| |
| for (BasicBlock *Pred : predecessors(Current)) |
| WorkList.insert(Pred); |
| |
| if (WorkList.size() >= MemorySSAPathCheckLimit) |
| return None; |
| } |
| NumCFGSuccess++; |
| return {EarlierAccess}; |
| } |
| return None; |
| } |
| |
| // No aliasing MemoryUses of EarlierAccess found, EarlierAccess is |
| // potentially dead. |
| return {EarlierAccess}; |
| } |
| |
| // Delete dead memory defs |
| void deleteDeadInstruction(Instruction *SI) { |
| MemorySSAUpdater Updater(&MSSA); |
| SmallVector<Instruction *, 32> NowDeadInsts; |
| NowDeadInsts.push_back(SI); |
| --NumFastOther; |
| |
| while (!NowDeadInsts.empty()) { |
| Instruction *DeadInst = NowDeadInsts.pop_back_val(); |
| ++NumFastOther; |
| |
| // Try to preserve debug information attached to the dead instruction. |
| salvageDebugInfo(*DeadInst); |
| salvageKnowledge(DeadInst); |
| |
| // Remove the Instruction from MSSA. |
| if (MemoryAccess *MA = MSSA.getMemoryAccess(DeadInst)) { |
| if (MemoryDef *MD = dyn_cast<MemoryDef>(MA)) { |
| SkipStores.insert(MD); |
| } |
| Updater.removeMemoryAccess(MA); |
| } |
| |
| auto I = IOLs.find(DeadInst->getParent()); |
| if (I != IOLs.end()) |
| I->second.erase(DeadInst); |
| // Remove its operands |
| for (Use &O : DeadInst->operands()) |
| if (Instruction *OpI = dyn_cast<Instruction>(O)) { |
| O = nullptr; |
| if (isInstructionTriviallyDead(OpI, &TLI)) |
| NowDeadInsts.push_back(OpI); |
| } |
| |
| DeadInst->eraseFromParent(); |
| } |
| } |
| |
| // Check for any extra throws between SI and NI that block DSE. This only |
| // checks extra maythrows (those that aren't MemoryDef's). MemoryDef that may |
| // throw are handled during the walk from one def to the next. |
| bool mayThrowBetween(Instruction *SI, Instruction *NI, |
| const Value *SILocUnd) { |
| // First see if we can ignore it by using the fact that SI is an |
| // alloca/alloca like object that is not visible to the caller during |
| // execution of the function. |
| if (SILocUnd && isInvisibleToCallerBeforeRet(SILocUnd)) |
| return false; |
| |
| if (SI->getParent() == NI->getParent()) |
| return ThrowingBlocks.count(SI->getParent()); |
| return !ThrowingBlocks.empty(); |
| } |
| |
| // Check if \p NI acts as a DSE barrier for \p SI. The following instructions |
| // act as barriers: |
| // * A memory instruction that may throw and \p SI accesses a non-stack |
| // object. |
| // * Atomic stores stronger that monotonic. |
| bool isDSEBarrier(const Value *SILocUnd, Instruction *NI) { |
| // If NI may throw it acts as a barrier, unless we are to an alloca/alloca |
| // like object that does not escape. |
| if (NI->mayThrow() && !isInvisibleToCallerBeforeRet(SILocUnd)) |
| return true; |
| |
| // If NI is an atomic load/store stronger than monotonic, do not try to |
| // eliminate/reorder it. |
| if (NI->isAtomic()) { |
| if (auto *LI = dyn_cast<LoadInst>(NI)) |
| return isStrongerThanMonotonic(LI->getOrdering()); |
| if (auto *SI = dyn_cast<StoreInst>(NI)) |
| return isStrongerThanMonotonic(SI->getOrdering()); |
| if (auto *ARMW = dyn_cast<AtomicRMWInst>(NI)) |
| return isStrongerThanMonotonic(ARMW->getOrdering()); |
| if (auto *CmpXchg = dyn_cast<AtomicCmpXchgInst>(NI)) |
| return isStrongerThanMonotonic(CmpXchg->getSuccessOrdering()) || |
| isStrongerThanMonotonic(CmpXchg->getFailureOrdering()); |
| llvm_unreachable("other instructions should be skipped in MemorySSA"); |
| } |
| return false; |
| } |
| |
| /// Eliminate writes to objects that are not visible in the caller and are not |
| /// accessed before returning from the function. |
| bool eliminateDeadWritesAtEndOfFunction() { |
| bool MadeChange = false; |
| LLVM_DEBUG( |
| dbgs() |
| << "Trying to eliminate MemoryDefs at the end of the function\n"); |
| for (int I = MemDefs.size() - 1; I >= 0; I--) { |
| MemoryDef *Def = MemDefs[I]; |
| if (SkipStores.contains(Def) || !isRemovable(Def->getMemoryInst())) |
| continue; |
| |
| Instruction *DefI = Def->getMemoryInst(); |
| SmallVector<const Value *, 4> Pointers; |
| auto DefLoc = getLocForWriteEx(DefI); |
| if (!DefLoc) |
| continue; |
| |
| // NOTE: Currently eliminating writes at the end of a function is limited |
| // to MemoryDefs with a single underlying object, to save compile-time. In |
| // practice it appears the case with multiple underlying objects is very |
| // uncommon. If it turns out to be important, we can use |
| // getUnderlyingObjects here instead. |
| const Value *UO = getUnderlyingObject(DefLoc->Ptr); |
| if (!UO || !isInvisibleToCallerAfterRet(UO)) |
| continue; |
| |
| if (isWriteAtEndOfFunction(Def)) { |
| // See through pointer-to-pointer bitcasts |
| LLVM_DEBUG(dbgs() << " ... MemoryDef is not accessed until the end " |
| "of the function\n"); |
| deleteDeadInstruction(DefI); |
| ++NumFastStores; |
| MadeChange = true; |
| } |
| } |
| return MadeChange; |
| } |
| |
| /// \returns true if \p Def is a no-op store, either because it |
| /// directly stores back a loaded value or stores zero to a calloced object. |
| bool storeIsNoop(MemoryDef *Def, const MemoryLocation &DefLoc, |
| const Value *DefUO) { |
| StoreInst *Store = dyn_cast<StoreInst>(Def->getMemoryInst()); |
| if (!Store) |
| return false; |
| |
| if (auto *LoadI = dyn_cast<LoadInst>(Store->getOperand(0))) { |
| if (LoadI->getPointerOperand() == Store->getOperand(1)) { |
| // Get the defining access for the load. |
| auto *LoadAccess = MSSA.getMemoryAccess(LoadI)->getDefiningAccess(); |
| // Fast path: the defining accesses are the same. |
| if (LoadAccess == Def->getDefiningAccess()) |
| return true; |
| |
| // Look through phi accesses. Recursively scan all phi accesses by |
| // adding them to a worklist. Bail when we run into a memory def that |
| // does not match LoadAccess. |
| SetVector<MemoryAccess *> ToCheck; |
| MemoryAccess *Current = |
| MSSA.getWalker()->getClobberingMemoryAccess(Def); |
| // We don't want to bail when we run into the store memory def. But, |
| // the phi access may point to it. So, pretend like we've already |
| // checked it. |
| ToCheck.insert(Def); |
| ToCheck.insert(Current); |
| // Start at current (1) to simulate already having checked Def. |
| for (unsigned I = 1; I < ToCheck.size(); ++I) { |
| Current = ToCheck[I]; |
| if (auto PhiAccess = dyn_cast<MemoryPhi>(Current)) { |
| // Check all the operands. |
| for (auto &Use : PhiAccess->incoming_values()) |
| ToCheck.insert(cast<MemoryAccess>(&Use)); |
| continue; |
| } |
| |
| // If we found a memory def, bail. This happens when we have an |
| // unrelated write in between an otherwise noop store. |
| assert(isa<MemoryDef>(Current) && |
| "Only MemoryDefs should reach here."); |
| // TODO: Skip no alias MemoryDefs that have no aliasing reads. |
| // We are searching for the definition of the store's destination. |
| // So, if that is the same definition as the load, then this is a |
| // noop. Otherwise, fail. |
| if (LoadAccess != Current) |
| return false; |
| } |
| return true; |
| } |
| } |
| |
| Constant *StoredConstant = dyn_cast<Constant>(Store->getOperand(0)); |
| if (StoredConstant && StoredConstant->isNullValue()) { |
| auto *DefUOInst = dyn_cast<Instruction>(DefUO); |
| if (DefUOInst && isCallocLikeFn(DefUOInst, &TLI)) { |
| auto *UnderlyingDef = cast<MemoryDef>(MSSA.getMemoryAccess(DefUOInst)); |
| // If UnderlyingDef is the clobbering access of Def, no instructions |
| // between them can modify the memory location. |
| auto *ClobberDef = |
| MSSA.getSkipSelfWalker()->getClobberingMemoryAccess(Def); |
| return UnderlyingDef == ClobberDef; |
| } |
| } |
| return false; |
| } |
| }; |
| |
| bool eliminateDeadStoresMemorySSA(Function &F, AliasAnalysis &AA, |
| MemorySSA &MSSA, DominatorTree &DT, |
| PostDominatorTree &PDT, |
| const TargetLibraryInfo &TLI) { |
| bool MadeChange = false; |
| |
| DSEState State = DSEState::get(F, AA, MSSA, DT, PDT, TLI); |
| // For each store: |
| for (unsigned I = 0; I < State.MemDefs.size(); I++) { |
| MemoryDef *KillingDef = State.MemDefs[I]; |
| if (State.SkipStores.count(KillingDef)) |
| continue; |
| Instruction *SI = KillingDef->getMemoryInst(); |
| |
| Optional<MemoryLocation> MaybeSILoc; |
| if (State.isMemTerminatorInst(SI)) |
| MaybeSILoc = State.getLocForTerminator(SI).map( |
| [](const std::pair<MemoryLocation, bool> &P) { return P.first; }); |
| else |
| MaybeSILoc = State.getLocForWriteEx(SI); |
| |
| if (!MaybeSILoc) { |
| LLVM_DEBUG(dbgs() << "Failed to find analyzable write location for " |
| << *SI << "\n"); |
| continue; |
| } |
| MemoryLocation SILoc = *MaybeSILoc; |
| assert(SILoc.Ptr && "SILoc should not be null"); |
| const Value *SILocUnd = getUnderlyingObject(SILoc.Ptr); |
| |
| MemoryAccess *Current = KillingDef; |
| LLVM_DEBUG(dbgs() << "Trying to eliminate MemoryDefs killed by " |
| << *KillingDef << " (" << *SI << ")\n"); |
| |
| unsigned ScanLimit = MemorySSAScanLimit; |
| unsigned WalkerStepLimit = MemorySSAUpwardsStepLimit; |
| unsigned PartialLimit = MemorySSAPartialStoreLimit; |
| // Worklist of MemoryAccesses that may be killed by KillingDef. |
| SetVector<MemoryAccess *> ToCheck; |
| |
| if (SILocUnd) |
| ToCheck.insert(KillingDef->getDefiningAccess()); |
| |
| bool Shortend = false; |
| bool IsMemTerm = State.isMemTerminatorInst(SI); |
| // Check if MemoryAccesses in the worklist are killed by KillingDef. |
| for (unsigned I = 0; I < ToCheck.size(); I++) { |
| Current = ToCheck[I]; |
| if (State.SkipStores.count(Current)) |
| continue; |
| |
| Optional<MemoryAccess *> Next = State.getDomMemoryDef( |
| KillingDef, Current, SILoc, SILocUnd, ScanLimit, WalkerStepLimit, |
| IsMemTerm, PartialLimit); |
| |
| if (!Next) { |
| LLVM_DEBUG(dbgs() << " finished walk\n"); |
| continue; |
| } |
| |
| MemoryAccess *EarlierAccess = *Next; |
| LLVM_DEBUG(dbgs() << " Checking if we can kill " << *EarlierAccess); |
| if (isa<MemoryPhi>(EarlierAccess)) { |
| LLVM_DEBUG(dbgs() << "\n ... adding incoming values to worklist\n"); |
| for (Value *V : cast<MemoryPhi>(EarlierAccess)->incoming_values()) { |
| MemoryAccess *IncomingAccess = cast<MemoryAccess>(V); |
| BasicBlock *IncomingBlock = IncomingAccess->getBlock(); |
| BasicBlock *PhiBlock = EarlierAccess->getBlock(); |
| |
| // We only consider incoming MemoryAccesses that come before the |
| // MemoryPhi. Otherwise we could discover candidates that do not |
| // strictly dominate our starting def. |
| if (State.PostOrderNumbers[IncomingBlock] > |
| State.PostOrderNumbers[PhiBlock]) |
| ToCheck.insert(IncomingAccess); |
| } |
| continue; |
| } |
| auto *NextDef = cast<MemoryDef>(EarlierAccess); |
| Instruction *NI = NextDef->getMemoryInst(); |
| LLVM_DEBUG(dbgs() << " (" << *NI << ")\n"); |
| ToCheck.insert(NextDef->getDefiningAccess()); |
| NumGetDomMemoryDefPassed++; |
| |
| if (!DebugCounter::shouldExecute(MemorySSACounter)) |
| continue; |
| |
| MemoryLocation NILoc = *State.getLocForWriteEx(NI); |
| |
| if (IsMemTerm) { |
| const Value *NIUnd = getUnderlyingObject(NILoc.Ptr); |
| if (SILocUnd != NIUnd) |
| continue; |
| LLVM_DEBUG(dbgs() << "DSE: Remove Dead Store:\n DEAD: " << *NI |
| << "\n KILLER: " << *SI << '\n'); |
| State.deleteDeadInstruction(NI); |
| ++NumFastStores; |
| MadeChange = true; |
| } else { |
| // Check if NI overwrites SI. |
| int64_t InstWriteOffset, DepWriteOffset; |
| OverwriteResult OR = |
| isOverwrite(SI, NI, SILoc, NILoc, State.DL, TLI, DepWriteOffset, |
| InstWriteOffset, State.BatchAA, &F); |
| if (OR == OW_MaybePartial) { |
| auto Iter = State.IOLs.insert( |
| std::make_pair<BasicBlock *, InstOverlapIntervalsTy>( |
| NI->getParent(), InstOverlapIntervalsTy())); |
| auto &IOL = Iter.first->second; |
| OR = isPartialOverwrite(SILoc, NILoc, DepWriteOffset, InstWriteOffset, |
| NI, IOL); |
| } |
| |
| if (EnablePartialStoreMerging && OR == OW_PartialEarlierWithFullLater) { |
| auto *Earlier = dyn_cast<StoreInst>(NI); |
| auto *Later = dyn_cast<StoreInst>(SI); |
| // We are re-using tryToMergePartialOverlappingStores, which requires |
| // Earlier to domiante Later. |
| // TODO: implement tryToMergeParialOverlappingStores using MemorySSA. |
| if (Earlier && Later && DT.dominates(Earlier, Later)) { |
| if (Constant *Merged = tryToMergePartialOverlappingStores( |
| Earlier, Later, InstWriteOffset, DepWriteOffset, State.DL, |
| State.BatchAA, &DT)) { |
| |
| // Update stored value of earlier store to merged constant. |
| Earlier->setOperand(0, Merged); |
| ++NumModifiedStores; |
| MadeChange = true; |
| |
| Shortend = true; |
| // Remove later store and remove any outstanding overlap intervals |
| // for the updated store. |
| State.deleteDeadInstruction(Later); |
| auto I = State.IOLs.find(Earlier->getParent()); |
| if (I != State.IOLs.end()) |
| I->second.erase(Earlier); |
| break; |
| } |
| } |
| } |
| |
| if (OR == OW_Complete) { |
| LLVM_DEBUG(dbgs() << "DSE: Remove Dead Store:\n DEAD: " << *NI |
| << "\n KILLER: " << *SI << '\n'); |
| State.deleteDeadInstruction(NI); |
| ++NumFastStores; |
| MadeChange = true; |
| } |
| } |
| } |
| |
| // Check if the store is a no-op. |
| if (!Shortend && isRemovable(SI) && |
| State.storeIsNoop(KillingDef, SILoc, SILocUnd)) { |
| LLVM_DEBUG(dbgs() << "DSE: Remove No-Op Store:\n DEAD: " << *SI << '\n'); |
| State.deleteDeadInstruction(SI); |
| NumRedundantStores++; |
| MadeChange = true; |
| continue; |
| } |
| } |
| |
| if (EnablePartialOverwriteTracking) |
| for (auto &KV : State.IOLs) |
| MadeChange |= removePartiallyOverlappedStores(State.DL, KV.second, TLI); |
| |
| MadeChange |= State.eliminateDeadWritesAtEndOfFunction(); |
| return MadeChange; |
| } |
| } // end anonymous namespace |
| |
| //===----------------------------------------------------------------------===// |
| // DSE Pass |
| //===----------------------------------------------------------------------===// |
| PreservedAnalyses DSEPass::run(Function &F, FunctionAnalysisManager &AM) { |
| AliasAnalysis &AA = AM.getResult<AAManager>(F); |
| const TargetLibraryInfo &TLI = AM.getResult<TargetLibraryAnalysis>(F); |
| DominatorTree &DT = AM.getResult<DominatorTreeAnalysis>(F); |
| |
| bool Changed = false; |
| if (EnableMemorySSA) { |
| MemorySSA &MSSA = AM.getResult<MemorySSAAnalysis>(F).getMSSA(); |
| PostDominatorTree &PDT = AM.getResult<PostDominatorTreeAnalysis>(F); |
| |
| Changed = eliminateDeadStoresMemorySSA(F, AA, MSSA, DT, PDT, TLI); |
| } else { |
| MemoryDependenceResults &MD = AM.getResult<MemoryDependenceAnalysis>(F); |
| |
| Changed = eliminateDeadStores(F, &AA, &MD, &DT, &TLI); |
| } |
| |
| #ifdef LLVM_ENABLE_STATS |
| if (AreStatisticsEnabled()) |
| for (auto &I : instructions(F)) |
| NumRemainingStores += isa<StoreInst>(&I); |
| #endif |
| |
| if (!Changed) |
| return PreservedAnalyses::all(); |
| |
| PreservedAnalyses PA; |
| PA.preserveSet<CFGAnalyses>(); |
| PA.preserve<GlobalsAA>(); |
| if (EnableMemorySSA) |
| PA.preserve<MemorySSAAnalysis>(); |
| else |
| PA.preserve<MemoryDependenceAnalysis>(); |
| return PA; |
| } |
| |
| namespace { |
| |
| /// A legacy pass for the legacy pass manager that wraps \c DSEPass. |
| class DSELegacyPass : public FunctionPass { |
| public: |
| static char ID; // Pass identification, replacement for typeid |
| |
| DSELegacyPass() : FunctionPass(ID) { |
| initializeDSELegacyPassPass(*PassRegistry::getPassRegistry()); |
| } |
| |
| bool runOnFunction(Function &F) override { |
| if (skipFunction(F)) |
| return false; |
| |
| AliasAnalysis &AA = getAnalysis<AAResultsWrapperPass>().getAAResults(); |
| DominatorTree &DT = getAnalysis<DominatorTreeWrapperPass>().getDomTree(); |
| const TargetLibraryInfo &TLI = |
| getAnalysis<TargetLibraryInfoWrapperPass>().getTLI(F); |
| |
| bool Changed = false; |
| if (EnableMemorySSA) { |
| MemorySSA &MSSA = getAnalysis<MemorySSAWrapperPass>().getMSSA(); |
| PostDominatorTree &PDT = |
| getAnalysis<PostDominatorTreeWrapperPass>().getPostDomTree(); |
| |
| Changed = eliminateDeadStoresMemorySSA(F, AA, MSSA, DT, PDT, TLI); |
| } else { |
| MemoryDependenceResults &MD = |
| getAnalysis<MemoryDependenceWrapperPass>().getMemDep(); |
| |
| Changed = eliminateDeadStores(F, &AA, &MD, &DT, &TLI); |
| } |
| |
| #ifdef LLVM_ENABLE_STATS |
| if (AreStatisticsEnabled()) |
| for (auto &I : instructions(F)) |
| NumRemainingStores += isa<StoreInst>(&I); |
| #endif |
| |
| return Changed; |
| } |
| |
| void getAnalysisUsage(AnalysisUsage &AU) const override { |
| AU.setPreservesCFG(); |
| AU.addRequired<AAResultsWrapperPass>(); |
| AU.addRequired<TargetLibraryInfoWrapperPass>(); |
| AU.addPreserved<GlobalsAAWrapperPass>(); |
| AU.addRequired<DominatorTreeWrapperPass>(); |
| AU.addPreserved<DominatorTreeWrapperPass>(); |
| |
| if (EnableMemorySSA) { |
| AU.addRequired<PostDominatorTreeWrapperPass>(); |
| AU.addRequired<MemorySSAWrapperPass>(); |
| AU.addPreserved<PostDominatorTreeWrapperPass>(); |
| AU.addPreserved<MemorySSAWrapperPass>(); |
| } else { |
| AU.addRequired<MemoryDependenceWrapperPass>(); |
| AU.addPreserved<MemoryDependenceWrapperPass>(); |
| } |
| } |
| }; |
| |
| } // end anonymous namespace |
| |
| char DSELegacyPass::ID = 0; |
| |
| INITIALIZE_PASS_BEGIN(DSELegacyPass, "dse", "Dead Store Elimination", false, |
| false) |
| INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass) |
| INITIALIZE_PASS_DEPENDENCY(PostDominatorTreeWrapperPass) |
| INITIALIZE_PASS_DEPENDENCY(AAResultsWrapperPass) |
| INITIALIZE_PASS_DEPENDENCY(GlobalsAAWrapperPass) |
| INITIALIZE_PASS_DEPENDENCY(MemorySSAWrapperPass) |
| INITIALIZE_PASS_DEPENDENCY(MemoryDependenceWrapperPass) |
| INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfoWrapperPass) |
| INITIALIZE_PASS_END(DSELegacyPass, "dse", "Dead Store Elimination", false, |
| false) |
| |
| FunctionPass *llvm::createDeadStoreEliminationPass() { |
| return new DSELegacyPass(); |
| } |