| //===- Loads.cpp - Local load analysis ------------------------------------===// | |
| // | |
| // The LLVM Compiler Infrastructure | |
| // | |
| // This file is distributed under the University of Illinois Open Source | |
| // License. See LICENSE.TXT for details. | |
| // | |
| //===----------------------------------------------------------------------===// | |
| // | |
| // This file defines simple local analyses for load instructions. | |
| // | |
| //===----------------------------------------------------------------------===// | |
| #include "llvm/Analysis/Loads.h" | |
| #include "llvm/Analysis/AliasAnalysis.h" | |
| #include "llvm/Analysis/ValueTracking.h" | |
| #include "llvm/IR/DataLayout.h" | |
| #include "llvm/IR/GlobalAlias.h" | |
| #include "llvm/IR/GlobalVariable.h" | |
| #include "llvm/IR/IntrinsicInst.h" | |
| #include "llvm/IR/LLVMContext.h" | |
| #include "llvm/IR/Module.h" | |
| #include "llvm/IR/Operator.h" | |
| #include "llvm/IR/Statepoint.h" | |
| using namespace llvm; | |
| static bool isAligned(const Value *Base, const APInt &Offset, unsigned Align, | |
| const DataLayout &DL) { | |
| APInt BaseAlign(Offset.getBitWidth(), Base->getPointerAlignment(DL)); | |
| if (!BaseAlign) { | |
| Type *Ty = Base->getType()->getPointerElementType(); | |
| if (!Ty->isSized()) | |
| return false; | |
| BaseAlign = DL.getABITypeAlignment(Ty); | |
| } | |
| APInt Alignment(Offset.getBitWidth(), Align); | |
| assert(Alignment.isPowerOf2() && "must be a power of 2!"); | |
| return BaseAlign.uge(Alignment) && !(Offset & (Alignment-1)); | |
| } | |
| static bool isAligned(const Value *Base, unsigned Align, const DataLayout &DL) { | |
| Type *Ty = Base->getType(); | |
| assert(Ty->isSized() && "must be sized"); | |
| APInt Offset(DL.getTypeStoreSizeInBits(Ty), 0); | |
| return isAligned(Base, Offset, Align, DL); | |
| } | |
| /// Test if V is always a pointer to allocated and suitably aligned memory for | |
| /// a simple load or store. | |
| static bool isDereferenceableAndAlignedPointer( | |
| const Value *V, unsigned Align, const APInt &Size, const DataLayout &DL, | |
| const Instruction *CtxI, const DominatorTree *DT, | |
| SmallPtrSetImpl<const Value *> &Visited) { | |
| // Already visited? Bail out, we've likely hit unreachable code. | |
| if (!Visited.insert(V).second) | |
| return false; | |
| // Note that it is not safe to speculate into a malloc'd region because | |
| // malloc may return null. | |
| // bitcast instructions are no-ops as far as dereferenceability is concerned. | |
| if (const BitCastOperator *BC = dyn_cast<BitCastOperator>(V)) | |
| return isDereferenceableAndAlignedPointer(BC->getOperand(0), Align, Size, | |
| DL, CtxI, DT, Visited); | |
| bool CheckForNonNull = false; | |
| APInt KnownDerefBytes(Size.getBitWidth(), | |
| V->getPointerDereferenceableBytes(DL, CheckForNonNull)); | |
| if (KnownDerefBytes.getBoolValue()) { | |
| if (KnownDerefBytes.uge(Size)) | |
| if (!CheckForNonNull || isKnownNonZero(V, DL, 0, nullptr, CtxI, DT)) | |
| return isAligned(V, Align, DL); | |
| } | |
| // For GEPs, determine if the indexing lands within the allocated object. | |
| if (const GEPOperator *GEP = dyn_cast<GEPOperator>(V)) { | |
| const Value *Base = GEP->getPointerOperand(); | |
| APInt Offset(DL.getIndexTypeSizeInBits(GEP->getType()), 0); | |
| if (!GEP->accumulateConstantOffset(DL, Offset) || Offset.isNegative() || | |
| !Offset.urem(APInt(Offset.getBitWidth(), Align)).isMinValue()) | |
| return false; | |
| // If the base pointer is dereferenceable for Offset+Size bytes, then the | |
| // GEP (== Base + Offset) is dereferenceable for Size bytes. If the base | |
| // pointer is aligned to Align bytes, and the Offset is divisible by Align | |
| // then the GEP (== Base + Offset == k_0 * Align + k_1 * Align) is also | |
| // aligned to Align bytes. | |
| // Offset and Size may have different bit widths if we have visited an | |
| // addrspacecast, so we can't do arithmetic directly on the APInt values. | |
| return isDereferenceableAndAlignedPointer( | |
| Base, Align, Offset + Size.sextOrTrunc(Offset.getBitWidth()), | |
| DL, CtxI, DT, Visited); | |
| } | |
| // For gc.relocate, look through relocations | |
| if (const GCRelocateInst *RelocateInst = dyn_cast<GCRelocateInst>(V)) | |
| return isDereferenceableAndAlignedPointer( | |
| RelocateInst->getDerivedPtr(), Align, Size, DL, CtxI, DT, Visited); | |
| if (const AddrSpaceCastInst *ASC = dyn_cast<AddrSpaceCastInst>(V)) | |
| return isDereferenceableAndAlignedPointer(ASC->getOperand(0), Align, Size, | |
| DL, CtxI, DT, Visited); | |
| if (auto CS = ImmutableCallSite(V)) | |
| if (auto *RP = getArgumentAliasingToReturnedPointer(CS)) | |
| return isDereferenceableAndAlignedPointer(RP, Align, Size, DL, CtxI, DT, | |
| Visited); | |
| // If we don't know, assume the worst. | |
| return false; | |
| } | |
| bool llvm::isDereferenceableAndAlignedPointer(const Value *V, unsigned Align, | |
| const APInt &Size, | |
| const DataLayout &DL, | |
| const Instruction *CtxI, | |
| const DominatorTree *DT) { | |
| SmallPtrSet<const Value *, 32> Visited; | |
| return ::isDereferenceableAndAlignedPointer(V, Align, Size, DL, CtxI, DT, | |
| Visited); | |
| } | |
| bool llvm::isDereferenceableAndAlignedPointer(const Value *V, unsigned Align, | |
| const DataLayout &DL, | |
| const Instruction *CtxI, | |
| const DominatorTree *DT) { | |
| // When dereferenceability information is provided by a dereferenceable | |
| // attribute, we know exactly how many bytes are dereferenceable. If we can | |
| // determine the exact offset to the attributed variable, we can use that | |
| // information here. | |
| Type *VTy = V->getType(); | |
| Type *Ty = VTy->getPointerElementType(); | |
| // Require ABI alignment for loads without alignment specification | |
| if (Align == 0) | |
| Align = DL.getABITypeAlignment(Ty); | |
| if (!Ty->isSized()) | |
| return false; | |
| SmallPtrSet<const Value *, 32> Visited; | |
| return ::isDereferenceableAndAlignedPointer( | |
| V, Align, APInt(DL.getIndexTypeSizeInBits(VTy), DL.getTypeStoreSize(Ty)), DL, | |
| CtxI, DT, Visited); | |
| } | |
| bool llvm::isDereferenceablePointer(const Value *V, const DataLayout &DL, | |
| const Instruction *CtxI, | |
| const DominatorTree *DT) { | |
| return isDereferenceableAndAlignedPointer(V, 1, DL, CtxI, DT); | |
| } | |
| /// Test if A and B will obviously have the same value. | |
| /// | |
| /// This includes recognizing that %t0 and %t1 will have the same | |
| /// value in code like this: | |
| /// \code | |
| /// %t0 = getelementptr \@a, 0, 3 | |
| /// store i32 0, i32* %t0 | |
| /// %t1 = getelementptr \@a, 0, 3 | |
| /// %t2 = load i32* %t1 | |
| /// \endcode | |
| /// | |
| static bool AreEquivalentAddressValues(const Value *A, const Value *B) { | |
| // Test if the values are trivially equivalent. | |
| if (A == B) | |
| return true; | |
| // Test if the values come from identical arithmetic instructions. | |
| // Use isIdenticalToWhenDefined instead of isIdenticalTo because | |
| // this function is only used when one address use dominates the | |
| // other, which means that they'll always either have the same | |
| // value or one of them will have an undefined value. | |
| if (isa<BinaryOperator>(A) || isa<CastInst>(A) || isa<PHINode>(A) || | |
| isa<GetElementPtrInst>(A)) | |
| if (const Instruction *BI = dyn_cast<Instruction>(B)) | |
| if (cast<Instruction>(A)->isIdenticalToWhenDefined(BI)) | |
| return true; | |
| // Otherwise they may not be equivalent. | |
| return false; | |
| } | |
| /// Check if executing a load of this pointer value cannot trap. | |
| /// | |
| /// If DT and ScanFrom are specified this method performs context-sensitive | |
| /// analysis and returns true if it is safe to load immediately before ScanFrom. | |
| /// | |
| /// If it is not obviously safe to load from the specified pointer, we do | |
| /// a quick local scan of the basic block containing \c ScanFrom, to determine | |
| /// if the address is already accessed. | |
| /// | |
| /// This uses the pointee type to determine how many bytes need to be safe to | |
| /// load from the pointer. | |
| bool llvm::isSafeToLoadUnconditionally(Value *V, unsigned Align, | |
| const DataLayout &DL, | |
| Instruction *ScanFrom, | |
| const DominatorTree *DT) { | |
| // Zero alignment means that the load has the ABI alignment for the target | |
| if (Align == 0) | |
| Align = DL.getABITypeAlignment(V->getType()->getPointerElementType()); | |
| assert(isPowerOf2_32(Align)); | |
| // If DT is not specified we can't make context-sensitive query | |
| const Instruction* CtxI = DT ? ScanFrom : nullptr; | |
| if (isDereferenceableAndAlignedPointer(V, Align, DL, CtxI, DT)) | |
| return true; | |
| int64_t ByteOffset = 0; | |
| Value *Base = V; | |
| Base = GetPointerBaseWithConstantOffset(V, ByteOffset, DL); | |
| if (ByteOffset < 0) // out of bounds | |
| return false; | |
| Type *BaseType = nullptr; | |
| unsigned BaseAlign = 0; | |
| if (const AllocaInst *AI = dyn_cast<AllocaInst>(Base)) { | |
| // An alloca is safe to load from as load as it is suitably aligned. | |
| BaseType = AI->getAllocatedType(); | |
| BaseAlign = AI->getAlignment(); | |
| } else if (const GlobalVariable *GV = dyn_cast<GlobalVariable>(Base)) { | |
| // Global variables are not necessarily safe to load from if they are | |
| // interposed arbitrarily. Their size may change or they may be weak and | |
| // require a test to determine if they were in fact provided. | |
| if (!GV->isInterposable()) { | |
| BaseType = GV->getType()->getElementType(); | |
| BaseAlign = GV->getAlignment(); | |
| } | |
| } | |
| PointerType *AddrTy = cast<PointerType>(V->getType()); | |
| uint64_t LoadSize = DL.getTypeStoreSize(AddrTy->getElementType()); | |
| // If we found a base allocated type from either an alloca or global variable, | |
| // try to see if we are definitively within the allocated region. We need to | |
| // know the size of the base type and the loaded type to do anything in this | |
| // case. | |
| if (BaseType && BaseType->isSized()) { | |
| if (BaseAlign == 0) | |
| BaseAlign = DL.getPrefTypeAlignment(BaseType); | |
| if (Align <= BaseAlign) { | |
| // Check if the load is within the bounds of the underlying object. | |
| if (ByteOffset + LoadSize <= DL.getTypeAllocSize(BaseType) && | |
| ((ByteOffset % Align) == 0)) | |
| return true; | |
| } | |
| } | |
| if (!ScanFrom) | |
| return false; | |
| // Otherwise, be a little bit aggressive by scanning the local block where we | |
| // want to check to see if the pointer is already being loaded or stored | |
| // from/to. If so, the previous load or store would have already trapped, | |
| // so there is no harm doing an extra load (also, CSE will later eliminate | |
| // the load entirely). | |
| BasicBlock::iterator BBI = ScanFrom->getIterator(), | |
| E = ScanFrom->getParent()->begin(); | |
| // We can at least always strip pointer casts even though we can't use the | |
| // base here. | |
| V = V->stripPointerCasts(); | |
| while (BBI != E) { | |
| --BBI; | |
| // If we see a free or a call which may write to memory (i.e. which might do | |
| // a free) the pointer could be marked invalid. | |
| if (isa<CallInst>(BBI) && BBI->mayWriteToMemory() && | |
| !isa<DbgInfoIntrinsic>(BBI)) | |
| return false; | |
| Value *AccessedPtr; | |
| unsigned AccessedAlign; | |
| if (LoadInst *LI = dyn_cast<LoadInst>(BBI)) { | |
| AccessedPtr = LI->getPointerOperand(); | |
| AccessedAlign = LI->getAlignment(); | |
| } else if (StoreInst *SI = dyn_cast<StoreInst>(BBI)) { | |
| AccessedPtr = SI->getPointerOperand(); | |
| AccessedAlign = SI->getAlignment(); | |
| } else | |
| continue; | |
| Type *AccessedTy = AccessedPtr->getType()->getPointerElementType(); | |
| if (AccessedAlign == 0) | |
| AccessedAlign = DL.getABITypeAlignment(AccessedTy); | |
| if (AccessedAlign < Align) | |
| continue; | |
| // Handle trivial cases. | |
| if (AccessedPtr == V) | |
| return true; | |
| if (AreEquivalentAddressValues(AccessedPtr->stripPointerCasts(), V) && | |
| LoadSize <= DL.getTypeStoreSize(AccessedTy)) | |
| return true; | |
| } | |
| return false; | |
| } | |
| /// DefMaxInstsToScan - the default number of maximum instructions | |
| /// to scan in the block, used by FindAvailableLoadedValue(). | |
| /// FindAvailableLoadedValue() was introduced in r60148, to improve jump | |
| /// threading in part by eliminating partially redundant loads. | |
| /// At that point, the value of MaxInstsToScan was already set to '6' | |
| /// without documented explanation. | |
| cl::opt<unsigned> | |
| llvm::DefMaxInstsToScan("available-load-scan-limit", cl::init(6), cl::Hidden, | |
| cl::desc("Use this to specify the default maximum number of instructions " | |
| "to scan backward from a given instruction, when searching for " | |
| "available loaded value")); | |
| Value *llvm::FindAvailableLoadedValue(LoadInst *Load, | |
| BasicBlock *ScanBB, | |
| BasicBlock::iterator &ScanFrom, | |
| unsigned MaxInstsToScan, | |
| AliasAnalysis *AA, bool *IsLoad, | |
| unsigned *NumScanedInst) { | |
| // Don't CSE load that is volatile or anything stronger than unordered. | |
| if (!Load->isUnordered()) | |
| return nullptr; | |
| return FindAvailablePtrLoadStore( | |
| Load->getPointerOperand(), Load->getType(), Load->isAtomic(), ScanBB, | |
| ScanFrom, MaxInstsToScan, AA, IsLoad, NumScanedInst); | |
| } | |
| Value *llvm::FindAvailablePtrLoadStore(Value *Ptr, Type *AccessTy, | |
| bool AtLeastAtomic, BasicBlock *ScanBB, | |
| BasicBlock::iterator &ScanFrom, | |
| unsigned MaxInstsToScan, | |
| AliasAnalysis *AA, bool *IsLoadCSE, | |
| unsigned *NumScanedInst) { | |
| if (MaxInstsToScan == 0) | |
| MaxInstsToScan = ~0U; | |
| const DataLayout &DL = ScanBB->getModule()->getDataLayout(); | |
| // Try to get the store size for the type. | |
| uint64_t AccessSize = DL.getTypeStoreSize(AccessTy); | |
| Value *StrippedPtr = Ptr->stripPointerCasts(); | |
| while (ScanFrom != ScanBB->begin()) { | |
| // We must ignore debug info directives when counting (otherwise they | |
| // would affect codegen). | |
| Instruction *Inst = &*--ScanFrom; | |
| if (isa<DbgInfoIntrinsic>(Inst)) | |
| continue; | |
| // Restore ScanFrom to expected value in case next test succeeds | |
| ScanFrom++; | |
| if (NumScanedInst) | |
| ++(*NumScanedInst); | |
| // Don't scan huge blocks. | |
| if (MaxInstsToScan-- == 0) | |
| return nullptr; | |
| --ScanFrom; | |
| // If this is a load of Ptr, the loaded value is available. | |
| // (This is true even if the load is volatile or atomic, although | |
| // those cases are unlikely.) | |
| if (LoadInst *LI = dyn_cast<LoadInst>(Inst)) | |
| if (AreEquivalentAddressValues( | |
| LI->getPointerOperand()->stripPointerCasts(), StrippedPtr) && | |
| CastInst::isBitOrNoopPointerCastable(LI->getType(), AccessTy, DL)) { | |
| // We can value forward from an atomic to a non-atomic, but not the | |
| // other way around. | |
| if (LI->isAtomic() < AtLeastAtomic) | |
| return nullptr; | |
| if (IsLoadCSE) | |
| *IsLoadCSE = true; | |
| return LI; | |
| } | |
| if (StoreInst *SI = dyn_cast<StoreInst>(Inst)) { | |
| Value *StorePtr = SI->getPointerOperand()->stripPointerCasts(); | |
| // If this is a store through Ptr, the value is available! | |
| // (This is true even if the store is volatile or atomic, although | |
| // those cases are unlikely.) | |
| if (AreEquivalentAddressValues(StorePtr, StrippedPtr) && | |
| CastInst::isBitOrNoopPointerCastable(SI->getValueOperand()->getType(), | |
| AccessTy, DL)) { | |
| // We can value forward from an atomic to a non-atomic, but not the | |
| // other way around. | |
| if (SI->isAtomic() < AtLeastAtomic) | |
| return nullptr; | |
| if (IsLoadCSE) | |
| *IsLoadCSE = false; | |
| return SI->getOperand(0); | |
| } | |
| // If both StrippedPtr and StorePtr reach all the way to an alloca or | |
| // global and they are different, ignore the store. This is a trivial form | |
| // of alias analysis that is important for reg2mem'd code. | |
| if ((isa<AllocaInst>(StrippedPtr) || isa<GlobalVariable>(StrippedPtr)) && | |
| (isa<AllocaInst>(StorePtr) || isa<GlobalVariable>(StorePtr)) && | |
| StrippedPtr != StorePtr) | |
| continue; | |
| // If we have alias analysis and it says the store won't modify the loaded | |
| // value, ignore the store. | |
| if (AA && !isModSet(AA->getModRefInfo(SI, StrippedPtr, AccessSize))) | |
| continue; | |
| // Otherwise the store that may or may not alias the pointer, bail out. | |
| ++ScanFrom; | |
| return nullptr; | |
| } | |
| // If this is some other instruction that may clobber Ptr, bail out. | |
| if (Inst->mayWriteToMemory()) { | |
| // If alias analysis claims that it really won't modify the load, | |
| // ignore it. | |
| if (AA && !isModSet(AA->getModRefInfo(Inst, StrippedPtr, AccessSize))) | |
| continue; | |
| // May modify the pointer, bail out. | |
| ++ScanFrom; | |
| return nullptr; | |
| } | |
| } | |
| // Got to the start of the block, we didn't find it, but are done for this | |
| // block. | |
| return nullptr; | |
| } |