| //===- 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; |
| } |
