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Diffstat (limited to 'contrib/llvm/lib/Analysis/BasicAliasAnalysis.cpp')
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diff --git a/contrib/llvm/lib/Analysis/BasicAliasAnalysis.cpp b/contrib/llvm/lib/Analysis/BasicAliasAnalysis.cpp new file mode 100644 index 0000000..cfe7a1c --- /dev/null +++ b/contrib/llvm/lib/Analysis/BasicAliasAnalysis.cpp @@ -0,0 +1,758 @@ +//===- BasicAliasAnalysis.cpp - Local Alias Analysis Impl -----------------===// +// +// The LLVM Compiler Infrastructure +// +// This file is distributed under the University of Illinois Open Source +// License. See LICENSE.TXT for details. +// +//===----------------------------------------------------------------------===// +// +// This file defines the default implementation of the Alias Analysis interface +// that simply implements a few identities (two different globals cannot alias, +// etc), but otherwise does no analysis. +// +//===----------------------------------------------------------------------===// + +#include "llvm/Analysis/AliasAnalysis.h" +#include "llvm/Analysis/Passes.h" +#include "llvm/Constants.h" +#include "llvm/DerivedTypes.h" +#include "llvm/Function.h" +#include "llvm/GlobalVariable.h" +#include "llvm/Instructions.h" +#include "llvm/IntrinsicInst.h" +#include "llvm/Operator.h" +#include "llvm/Pass.h" +#include "llvm/Analysis/CaptureTracking.h" +#include "llvm/Analysis/MemoryBuiltins.h" +#include "llvm/Analysis/ValueTracking.h" +#include "llvm/Target/TargetData.h" +#include "llvm/ADT/SmallPtrSet.h" +#include "llvm/ADT/SmallVector.h" +#include "llvm/Support/ErrorHandling.h" +#include <algorithm> +using namespace llvm; + +//===----------------------------------------------------------------------===// +// Useful predicates +//===----------------------------------------------------------------------===// + +/// isKnownNonNull - Return true if we know that the specified value is never +/// null. +static bool isKnownNonNull(const Value *V) { + // Alloca never returns null, malloc might. + if (isa<AllocaInst>(V)) return true; + + // A byval argument is never null. + if (const Argument *A = dyn_cast<Argument>(V)) + return A->hasByValAttr(); + + // Global values are not null unless extern weak. + if (const GlobalValue *GV = dyn_cast<GlobalValue>(V)) + return !GV->hasExternalWeakLinkage(); + return false; +} + +/// isNonEscapingLocalObject - Return true if the pointer is to a function-local +/// object that never escapes from the function. +static bool isNonEscapingLocalObject(const Value *V) { + // If this is a local allocation, check to see if it escapes. + if (isa<AllocaInst>(V) || isNoAliasCall(V)) + // Set StoreCaptures to True so that we can assume in our callers that the + // pointer is not the result of a load instruction. Currently + // PointerMayBeCaptured doesn't have any special analysis for the + // StoreCaptures=false case; if it did, our callers could be refined to be + // more precise. + return !PointerMayBeCaptured(V, false, /*StoreCaptures=*/true); + + // If this is an argument that corresponds to a byval or noalias argument, + // then it has not escaped before entering the function. Check if it escapes + // inside the function. + if (const Argument *A = dyn_cast<Argument>(V)) + if (A->hasByValAttr() || A->hasNoAliasAttr()) { + // Don't bother analyzing arguments already known not to escape. + if (A->hasNoCaptureAttr()) + return true; + return !PointerMayBeCaptured(V, false, /*StoreCaptures=*/true); + } + return false; +} + + +/// isObjectSmallerThan - Return true if we can prove that the object specified +/// by V is smaller than Size. +static bool isObjectSmallerThan(const Value *V, unsigned Size, + const TargetData &TD) { + const Type *AccessTy; + if (const GlobalVariable *GV = dyn_cast<GlobalVariable>(V)) { + AccessTy = GV->getType()->getElementType(); + } else if (const AllocaInst *AI = dyn_cast<AllocaInst>(V)) { + if (!AI->isArrayAllocation()) + AccessTy = AI->getType()->getElementType(); + else + return false; + } else if (const CallInst* CI = extractMallocCall(V)) { + if (!isArrayMalloc(V, &TD)) + // The size is the argument to the malloc call. + if (const ConstantInt* C = dyn_cast<ConstantInt>(CI->getOperand(1))) + return (C->getZExtValue() < Size); + return false; + } else if (const Argument *A = dyn_cast<Argument>(V)) { + if (A->hasByValAttr()) + AccessTy = cast<PointerType>(A->getType())->getElementType(); + else + return false; + } else { + return false; + } + + if (AccessTy->isSized()) + return TD.getTypeAllocSize(AccessTy) < Size; + return false; +} + +//===----------------------------------------------------------------------===// +// NoAA Pass +//===----------------------------------------------------------------------===// + +namespace { + /// NoAA - This class implements the -no-aa pass, which always returns "I + /// don't know" for alias queries. NoAA is unlike other alias analysis + /// implementations, in that it does not chain to a previous analysis. As + /// such it doesn't follow many of the rules that other alias analyses must. + /// + struct NoAA : public ImmutablePass, public AliasAnalysis { + static char ID; // Class identification, replacement for typeinfo + NoAA() : ImmutablePass(&ID) {} + explicit NoAA(void *PID) : ImmutablePass(PID) { } + + virtual void getAnalysisUsage(AnalysisUsage &AU) const { + } + + virtual void initializePass() { + TD = getAnalysisIfAvailable<TargetData>(); + } + + virtual AliasResult alias(const Value *V1, unsigned V1Size, + const Value *V2, unsigned V2Size) { + return MayAlias; + } + + virtual void getArgumentAccesses(Function *F, CallSite CS, + std::vector<PointerAccessInfo> &Info) { + llvm_unreachable("This method may not be called on this function!"); + } + + virtual bool pointsToConstantMemory(const Value *P) { return false; } + virtual ModRefResult getModRefInfo(CallSite CS, Value *P, unsigned Size) { + return ModRef; + } + virtual ModRefResult getModRefInfo(CallSite CS1, CallSite CS2) { + return ModRef; + } + + virtual void deleteValue(Value *V) {} + virtual void copyValue(Value *From, Value *To) {} + + /// getAdjustedAnalysisPointer - This method is used when a pass implements + /// an analysis interface through multiple inheritance. If needed, it should + /// override this to adjust the this pointer as needed for the specified pass + /// info. + virtual void *getAdjustedAnalysisPointer(const PassInfo *PI) { + if (PI->isPassID(&AliasAnalysis::ID)) + return (AliasAnalysis*)this; + return this; + } + }; +} // End of anonymous namespace + +// Register this pass... +char NoAA::ID = 0; +static RegisterPass<NoAA> +U("no-aa", "No Alias Analysis (always returns 'may' alias)", true, true); + +// Declare that we implement the AliasAnalysis interface +static RegisterAnalysisGroup<AliasAnalysis> V(U); + +ImmutablePass *llvm::createNoAAPass() { return new NoAA(); } + +//===----------------------------------------------------------------------===// +// BasicAA Pass +//===----------------------------------------------------------------------===// + +namespace { + /// BasicAliasAnalysis - This is the default alias analysis implementation. + /// Because it doesn't chain to a previous alias analysis (like -no-aa), it + /// derives from the NoAA class. + struct BasicAliasAnalysis : public NoAA { + static char ID; // Class identification, replacement for typeinfo + BasicAliasAnalysis() : NoAA(&ID) {} + AliasResult alias(const Value *V1, unsigned V1Size, + const Value *V2, unsigned V2Size) { + assert(VisitedPHIs.empty() && "VisitedPHIs must be cleared after use!"); + AliasResult Alias = aliasCheck(V1, V1Size, V2, V2Size); + VisitedPHIs.clear(); + return Alias; + } + + ModRefResult getModRefInfo(CallSite CS, Value *P, unsigned Size); + ModRefResult getModRefInfo(CallSite CS1, CallSite CS2); + + /// pointsToConstantMemory - Chase pointers until we find a (constant + /// global) or not. + bool pointsToConstantMemory(const Value *P); + + /// getAdjustedAnalysisPointer - This method is used when a pass implements + /// an analysis interface through multiple inheritance. If needed, it should + /// override this to adjust the this pointer as needed for the specified pass + /// info. + virtual void *getAdjustedAnalysisPointer(const PassInfo *PI) { + if (PI->isPassID(&AliasAnalysis::ID)) + return (AliasAnalysis*)this; + return this; + } + + private: + // VisitedPHIs - Track PHI nodes visited by a aliasCheck() call. + SmallPtrSet<const Value*, 16> VisitedPHIs; + + // aliasGEP - Provide a bunch of ad-hoc rules to disambiguate a GEP + // instruction against another. + AliasResult aliasGEP(const GEPOperator *V1, unsigned V1Size, + const Value *V2, unsigned V2Size, + const Value *UnderlyingV1, const Value *UnderlyingV2); + + // aliasPHI - Provide a bunch of ad-hoc rules to disambiguate a PHI + // instruction against another. + AliasResult aliasPHI(const PHINode *PN, unsigned PNSize, + const Value *V2, unsigned V2Size); + + /// aliasSelect - Disambiguate a Select instruction against another value. + AliasResult aliasSelect(const SelectInst *SI, unsigned SISize, + const Value *V2, unsigned V2Size); + + AliasResult aliasCheck(const Value *V1, unsigned V1Size, + const Value *V2, unsigned V2Size); + }; +} // End of anonymous namespace + +// Register this pass... +char BasicAliasAnalysis::ID = 0; +static RegisterPass<BasicAliasAnalysis> +X("basicaa", "Basic Alias Analysis (default AA impl)", false, true); + +// Declare that we implement the AliasAnalysis interface +static RegisterAnalysisGroup<AliasAnalysis, true> Y(X); + +ImmutablePass *llvm::createBasicAliasAnalysisPass() { + return new BasicAliasAnalysis(); +} + + +/// pointsToConstantMemory - Chase pointers until we find a (constant +/// global) or not. +bool BasicAliasAnalysis::pointsToConstantMemory(const Value *P) { + if (const GlobalVariable *GV = + dyn_cast<GlobalVariable>(P->getUnderlyingObject())) + // Note: this doesn't require GV to be "ODR" because it isn't legal for a + // global to be marked constant in some modules and non-constant in others. + // GV may even be a declaration, not a definition. + return GV->isConstant(); + return false; +} + + +/// getModRefInfo - Check to see if the specified callsite can clobber the +/// specified memory object. Since we only look at local properties of this +/// function, we really can't say much about this query. We do, however, use +/// simple "address taken" analysis on local objects. +AliasAnalysis::ModRefResult +BasicAliasAnalysis::getModRefInfo(CallSite CS, Value *P, unsigned Size) { + const Value *Object = P->getUnderlyingObject(); + + // If this is a tail call and P points to a stack location, we know that + // the tail call cannot access or modify the local stack. + // We cannot exclude byval arguments here; these belong to the caller of + // the current function not to the current function, and a tail callee + // may reference them. + if (isa<AllocaInst>(Object)) + if (CallInst *CI = dyn_cast<CallInst>(CS.getInstruction())) + if (CI->isTailCall()) + return NoModRef; + + // If the pointer is to a locally allocated object that does not escape, + // then the call can not mod/ref the pointer unless the call takes the pointer + // as an argument, and itself doesn't capture it. + if (!isa<Constant>(Object) && CS.getInstruction() != Object && + isNonEscapingLocalObject(Object)) { + bool PassedAsArg = false; + unsigned ArgNo = 0; + for (CallSite::arg_iterator CI = CS.arg_begin(), CE = CS.arg_end(); + CI != CE; ++CI, ++ArgNo) { + // Only look at the no-capture pointer arguments. + if (!(*CI)->getType()->isPointerTy() || + !CS.paramHasAttr(ArgNo+1, Attribute::NoCapture)) + continue; + + // If this is a no-capture pointer argument, see if we can tell that it + // is impossible to alias the pointer we're checking. If not, we have to + // assume that the call could touch the pointer, even though it doesn't + // escape. + if (!isNoAlias(cast<Value>(CI), ~0U, P, ~0U)) { + PassedAsArg = true; + break; + } + } + + if (!PassedAsArg) + return NoModRef; + } + + // Finally, handle specific knowledge of intrinsics. + IntrinsicInst *II = dyn_cast<IntrinsicInst>(CS.getInstruction()); + if (II == 0) + return AliasAnalysis::getModRefInfo(CS, P, Size); + + switch (II->getIntrinsicID()) { + default: break; + case Intrinsic::memcpy: + case Intrinsic::memmove: { + unsigned Len = ~0U; + if (ConstantInt *LenCI = dyn_cast<ConstantInt>(II->getOperand(3))) + Len = LenCI->getZExtValue(); + Value *Dest = II->getOperand(1); + Value *Src = II->getOperand(2); + if (isNoAlias(Dest, Len, P, Size)) { + if (isNoAlias(Src, Len, P, Size)) + return NoModRef; + return Ref; + } + break; + } + case Intrinsic::memset: + // Since memset is 'accesses arguments' only, the AliasAnalysis base class + // will handle it for the variable length case. + if (ConstantInt *LenCI = dyn_cast<ConstantInt>(II->getOperand(3))) { + unsigned Len = LenCI->getZExtValue(); + Value *Dest = II->getOperand(1); + if (isNoAlias(Dest, Len, P, Size)) + return NoModRef; + } + break; + case Intrinsic::atomic_cmp_swap: + case Intrinsic::atomic_swap: + case Intrinsic::atomic_load_add: + case Intrinsic::atomic_load_sub: + case Intrinsic::atomic_load_and: + case Intrinsic::atomic_load_nand: + case Intrinsic::atomic_load_or: + case Intrinsic::atomic_load_xor: + case Intrinsic::atomic_load_max: + case Intrinsic::atomic_load_min: + case Intrinsic::atomic_load_umax: + case Intrinsic::atomic_load_umin: + if (TD) { + Value *Op1 = II->getOperand(1); + unsigned Op1Size = TD->getTypeStoreSize(Op1->getType()); + if (isNoAlias(Op1, Op1Size, P, Size)) + return NoModRef; + } + break; + case Intrinsic::lifetime_start: + case Intrinsic::lifetime_end: + case Intrinsic::invariant_start: { + unsigned PtrSize = cast<ConstantInt>(II->getOperand(1))->getZExtValue(); + if (isNoAlias(II->getOperand(2), PtrSize, P, Size)) + return NoModRef; + break; + } + case Intrinsic::invariant_end: { + unsigned PtrSize = cast<ConstantInt>(II->getOperand(2))->getZExtValue(); + if (isNoAlias(II->getOperand(3), PtrSize, P, Size)) + return NoModRef; + break; + } + } + + // The AliasAnalysis base class has some smarts, lets use them. + return AliasAnalysis::getModRefInfo(CS, P, Size); +} + + +AliasAnalysis::ModRefResult +BasicAliasAnalysis::getModRefInfo(CallSite CS1, CallSite CS2) { + // If CS1 or CS2 are readnone, they don't interact. + ModRefBehavior CS1B = AliasAnalysis::getModRefBehavior(CS1); + if (CS1B == DoesNotAccessMemory) return NoModRef; + + ModRefBehavior CS2B = AliasAnalysis::getModRefBehavior(CS2); + if (CS2B == DoesNotAccessMemory) return NoModRef; + + // If they both only read from memory, just return ref. + if (CS1B == OnlyReadsMemory && CS2B == OnlyReadsMemory) + return Ref; + + // Otherwise, fall back to NoAA (mod+ref). + return NoAA::getModRefInfo(CS1, CS2); +} + +/// GetIndiceDifference - Dest and Src are the variable indices from two +/// decomposed GetElementPtr instructions GEP1 and GEP2 which have common base +/// pointers. Subtract the GEP2 indices from GEP1 to find the symbolic +/// difference between the two pointers. +static void GetIndiceDifference( + SmallVectorImpl<std::pair<const Value*, int64_t> > &Dest, + const SmallVectorImpl<std::pair<const Value*, int64_t> > &Src) { + if (Src.empty()) return; + + for (unsigned i = 0, e = Src.size(); i != e; ++i) { + const Value *V = Src[i].first; + int64_t Scale = Src[i].second; + + // Find V in Dest. This is N^2, but pointer indices almost never have more + // than a few variable indexes. + for (unsigned j = 0, e = Dest.size(); j != e; ++j) { + if (Dest[j].first != V) continue; + + // If we found it, subtract off Scale V's from the entry in Dest. If it + // goes to zero, remove the entry. + if (Dest[j].second != Scale) + Dest[j].second -= Scale; + else + Dest.erase(Dest.begin()+j); + Scale = 0; + break; + } + + // If we didn't consume this entry, add it to the end of the Dest list. + if (Scale) + Dest.push_back(std::make_pair(V, -Scale)); + } +} + +/// aliasGEP - Provide a bunch of ad-hoc rules to disambiguate a GEP instruction +/// against another pointer. We know that V1 is a GEP, but we don't know +/// anything about V2. UnderlyingV1 is GEP1->getUnderlyingObject(), +/// UnderlyingV2 is the same for V2. +/// +AliasAnalysis::AliasResult +BasicAliasAnalysis::aliasGEP(const GEPOperator *GEP1, unsigned V1Size, + const Value *V2, unsigned V2Size, + const Value *UnderlyingV1, + const Value *UnderlyingV2) { + int64_t GEP1BaseOffset; + SmallVector<std::pair<const Value*, int64_t>, 4> GEP1VariableIndices; + + // If we have two gep instructions with must-alias'ing base pointers, figure + // out if the indexes to the GEP tell us anything about the derived pointer. + if (const GEPOperator *GEP2 = dyn_cast<GEPOperator>(V2)) { + // Do the base pointers alias? + AliasResult BaseAlias = aliasCheck(UnderlyingV1, ~0U, UnderlyingV2, ~0U); + + // If we get a No or May, then return it immediately, no amount of analysis + // will improve this situation. + if (BaseAlias != MustAlias) return BaseAlias; + + // Otherwise, we have a MustAlias. Since the base pointers alias each other + // exactly, see if the computed offset from the common pointer tells us + // about the relation of the resulting pointer. + const Value *GEP1BasePtr = + DecomposeGEPExpression(GEP1, GEP1BaseOffset, GEP1VariableIndices, TD); + + int64_t GEP2BaseOffset; + SmallVector<std::pair<const Value*, int64_t>, 4> GEP2VariableIndices; + const Value *GEP2BasePtr = + DecomposeGEPExpression(GEP2, GEP2BaseOffset, GEP2VariableIndices, TD); + + // If DecomposeGEPExpression isn't able to look all the way through the + // addressing operation, we must not have TD and this is too complex for us + // to handle without it. + if (GEP1BasePtr != UnderlyingV1 || GEP2BasePtr != UnderlyingV2) { + assert(TD == 0 && + "DecomposeGEPExpression and getUnderlyingObject disagree!"); + return MayAlias; + } + + // Subtract the GEP2 pointer from the GEP1 pointer to find out their + // symbolic difference. + GEP1BaseOffset -= GEP2BaseOffset; + GetIndiceDifference(GEP1VariableIndices, GEP2VariableIndices); + + } else { + // Check to see if these two pointers are related by the getelementptr + // instruction. If one pointer is a GEP with a non-zero index of the other + // pointer, we know they cannot alias. + + // If both accesses are unknown size, we can't do anything useful here. + if (V1Size == ~0U && V2Size == ~0U) + return MayAlias; + + AliasResult R = aliasCheck(UnderlyingV1, ~0U, V2, V2Size); + if (R != MustAlias) + // If V2 may alias GEP base pointer, conservatively returns MayAlias. + // If V2 is known not to alias GEP base pointer, then the two values + // cannot alias per GEP semantics: "A pointer value formed from a + // getelementptr instruction is associated with the addresses associated + // with the first operand of the getelementptr". + return R; + + const Value *GEP1BasePtr = + DecomposeGEPExpression(GEP1, GEP1BaseOffset, GEP1VariableIndices, TD); + + // If DecomposeGEPExpression isn't able to look all the way through the + // addressing operation, we must not have TD and this is too complex for us + // to handle without it. + if (GEP1BasePtr != UnderlyingV1) { + assert(TD == 0 && + "DecomposeGEPExpression and getUnderlyingObject disagree!"); + return MayAlias; + } + } + + // In the two GEP Case, if there is no difference in the offsets of the + // computed pointers, the resultant pointers are a must alias. This + // hapens when we have two lexically identical GEP's (for example). + // + // In the other case, if we have getelementptr <ptr>, 0, 0, 0, 0, ... and V2 + // must aliases the GEP, the end result is a must alias also. + if (GEP1BaseOffset == 0 && GEP1VariableIndices.empty()) + return MustAlias; + + // If we have a known constant offset, see if this offset is larger than the + // access size being queried. If so, and if no variable indices can remove + // pieces of this constant, then we know we have a no-alias. For example, + // &A[100] != &A. + + // In order to handle cases like &A[100][i] where i is an out of range + // subscript, we have to ignore all constant offset pieces that are a multiple + // of a scaled index. Do this by removing constant offsets that are a + // multiple of any of our variable indices. This allows us to transform + // things like &A[i][1] because i has a stride of (e.g.) 8 bytes but the 1 + // provides an offset of 4 bytes (assuming a <= 4 byte access). + for (unsigned i = 0, e = GEP1VariableIndices.size(); + i != e && GEP1BaseOffset;++i) + if (int64_t RemovedOffset = GEP1BaseOffset/GEP1VariableIndices[i].second) + GEP1BaseOffset -= RemovedOffset*GEP1VariableIndices[i].second; + + // If our known offset is bigger than the access size, we know we don't have + // an alias. + if (GEP1BaseOffset) { + if (GEP1BaseOffset >= (int64_t)V2Size || + GEP1BaseOffset <= -(int64_t)V1Size) + return NoAlias; + } + + return MayAlias; +} + +/// aliasSelect - Provide a bunch of ad-hoc rules to disambiguate a Select +/// instruction against another. +AliasAnalysis::AliasResult +BasicAliasAnalysis::aliasSelect(const SelectInst *SI, unsigned SISize, + const Value *V2, unsigned V2Size) { + // If the values are Selects with the same condition, we can do a more precise + // check: just check for aliases between the values on corresponding arms. + if (const SelectInst *SI2 = dyn_cast<SelectInst>(V2)) + if (SI->getCondition() == SI2->getCondition()) { + AliasResult Alias = + aliasCheck(SI->getTrueValue(), SISize, + SI2->getTrueValue(), V2Size); + if (Alias == MayAlias) + return MayAlias; + AliasResult ThisAlias = + aliasCheck(SI->getFalseValue(), SISize, + SI2->getFalseValue(), V2Size); + if (ThisAlias != Alias) + return MayAlias; + return Alias; + } + + // If both arms of the Select node NoAlias or MustAlias V2, then returns + // NoAlias / MustAlias. Otherwise, returns MayAlias. + AliasResult Alias = + aliasCheck(SI->getTrueValue(), SISize, V2, V2Size); + if (Alias == MayAlias) + return MayAlias; + AliasResult ThisAlias = + aliasCheck(SI->getFalseValue(), SISize, V2, V2Size); + if (ThisAlias != Alias) + return MayAlias; + return Alias; +} + +// aliasPHI - Provide a bunch of ad-hoc rules to disambiguate a PHI instruction +// against another. +AliasAnalysis::AliasResult +BasicAliasAnalysis::aliasPHI(const PHINode *PN, unsigned PNSize, + const Value *V2, unsigned V2Size) { + // The PHI node has already been visited, avoid recursion any further. + if (!VisitedPHIs.insert(PN)) + return MayAlias; + + // If the values are PHIs in the same block, we can do a more precise + // as well as efficient check: just check for aliases between the values + // on corresponding edges. + if (const PHINode *PN2 = dyn_cast<PHINode>(V2)) + if (PN2->getParent() == PN->getParent()) { + AliasResult Alias = + aliasCheck(PN->getIncomingValue(0), PNSize, + PN2->getIncomingValueForBlock(PN->getIncomingBlock(0)), + V2Size); + if (Alias == MayAlias) + return MayAlias; + for (unsigned i = 1, e = PN->getNumIncomingValues(); i != e; ++i) { + AliasResult ThisAlias = + aliasCheck(PN->getIncomingValue(i), PNSize, + PN2->getIncomingValueForBlock(PN->getIncomingBlock(i)), + V2Size); + if (ThisAlias != Alias) + return MayAlias; + } + return Alias; + } + + SmallPtrSet<Value*, 4> UniqueSrc; + SmallVector<Value*, 4> V1Srcs; + for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) { + Value *PV1 = PN->getIncomingValue(i); + if (isa<PHINode>(PV1)) + // If any of the source itself is a PHI, return MayAlias conservatively + // to avoid compile time explosion. The worst possible case is if both + // sides are PHI nodes. In which case, this is O(m x n) time where 'm' + // and 'n' are the number of PHI sources. + return MayAlias; + if (UniqueSrc.insert(PV1)) + V1Srcs.push_back(PV1); + } + + AliasResult Alias = aliasCheck(V2, V2Size, V1Srcs[0], PNSize); + // Early exit if the check of the first PHI source against V2 is MayAlias. + // Other results are not possible. + if (Alias == MayAlias) + return MayAlias; + + // If all sources of the PHI node NoAlias or MustAlias V2, then returns + // NoAlias / MustAlias. Otherwise, returns MayAlias. + for (unsigned i = 1, e = V1Srcs.size(); i != e; ++i) { + Value *V = V1Srcs[i]; + + // If V2 is a PHI, the recursive case will have been caught in the + // above aliasCheck call, so these subsequent calls to aliasCheck + // don't need to assume that V2 is being visited recursively. + VisitedPHIs.erase(V2); + + AliasResult ThisAlias = aliasCheck(V2, V2Size, V, PNSize); + if (ThisAlias != Alias || ThisAlias == MayAlias) + return MayAlias; + } + + return Alias; +} + +// aliasCheck - Provide a bunch of ad-hoc rules to disambiguate in common cases, +// such as array references. +// +AliasAnalysis::AliasResult +BasicAliasAnalysis::aliasCheck(const Value *V1, unsigned V1Size, + const Value *V2, unsigned V2Size) { + // If either of the memory references is empty, it doesn't matter what the + // pointer values are. + if (V1Size == 0 || V2Size == 0) + return NoAlias; + + // Strip off any casts if they exist. + V1 = V1->stripPointerCasts(); + V2 = V2->stripPointerCasts(); + + // Are we checking for alias of the same value? + if (V1 == V2) return MustAlias; + + if (!V1->getType()->isPointerTy() || !V2->getType()->isPointerTy()) + return NoAlias; // Scalars cannot alias each other + + // Figure out what objects these things are pointing to if we can. + const Value *O1 = V1->getUnderlyingObject(); + const Value *O2 = V2->getUnderlyingObject(); + + // Null values in the default address space don't point to any object, so they + // don't alias any other pointer. + if (const ConstantPointerNull *CPN = dyn_cast<ConstantPointerNull>(O1)) + if (CPN->getType()->getAddressSpace() == 0) + return NoAlias; + if (const ConstantPointerNull *CPN = dyn_cast<ConstantPointerNull>(O2)) + if (CPN->getType()->getAddressSpace() == 0) + return NoAlias; + + if (O1 != O2) { + // If V1/V2 point to two different objects we know that we have no alias. + if (isIdentifiedObject(O1) && isIdentifiedObject(O2)) + return NoAlias; + + // Constant pointers can't alias with non-const isIdentifiedObject objects. + if ((isa<Constant>(O1) && isIdentifiedObject(O2) && !isa<Constant>(O2)) || + (isa<Constant>(O2) && isIdentifiedObject(O1) && !isa<Constant>(O1))) + return NoAlias; + + // Arguments can't alias with local allocations or noalias calls. + if ((isa<Argument>(O1) && (isa<AllocaInst>(O2) || isNoAliasCall(O2))) || + (isa<Argument>(O2) && (isa<AllocaInst>(O1) || isNoAliasCall(O1)))) + return NoAlias; + + // Most objects can't alias null. + if ((isa<ConstantPointerNull>(V2) && isKnownNonNull(O1)) || + (isa<ConstantPointerNull>(V1) && isKnownNonNull(O2))) + return NoAlias; + } + + // If the size of one access is larger than the entire object on the other + // side, then we know such behavior is undefined and can assume no alias. + if (TD) + if ((V1Size != ~0U && isObjectSmallerThan(O2, V1Size, *TD)) || + (V2Size != ~0U && isObjectSmallerThan(O1, V2Size, *TD))) + return NoAlias; + + // If one pointer is the result of a call/invoke or load and the other is a + // non-escaping local object, then we know the object couldn't escape to a + // point where the call could return it. The load case works because + // isNonEscapingLocalObject considers all stores to be escapes (it + // passes true for the StoreCaptures argument to PointerMayBeCaptured). + if (O1 != O2) { + if ((isa<CallInst>(O1) || isa<InvokeInst>(O1) || isa<LoadInst>(O1) || + isa<Argument>(O1)) && + isNonEscapingLocalObject(O2)) + return NoAlias; + if ((isa<CallInst>(O2) || isa<InvokeInst>(O2) || isa<LoadInst>(O2) || + isa<Argument>(O2)) && + isNonEscapingLocalObject(O1)) + return NoAlias; + } + + // FIXME: This isn't aggressively handling alias(GEP, PHI) for example: if the + // GEP can't simplify, we don't even look at the PHI cases. + if (!isa<GEPOperator>(V1) && isa<GEPOperator>(V2)) { + std::swap(V1, V2); + std::swap(V1Size, V2Size); + std::swap(O1, O2); + } + if (const GEPOperator *GV1 = dyn_cast<GEPOperator>(V1)) + return aliasGEP(GV1, V1Size, V2, V2Size, O1, O2); + + if (isa<PHINode>(V2) && !isa<PHINode>(V1)) { + std::swap(V1, V2); + std::swap(V1Size, V2Size); + } + if (const PHINode *PN = dyn_cast<PHINode>(V1)) + return aliasPHI(PN, V1Size, V2, V2Size); + + if (isa<SelectInst>(V2) && !isa<SelectInst>(V1)) { + std::swap(V1, V2); + std::swap(V1Size, V2Size); + } + if (const SelectInst *S1 = dyn_cast<SelectInst>(V1)) + return aliasSelect(S1, V1Size, V2, V2Size); + + return MayAlias; +} + +// Make sure that anything that uses AliasAnalysis pulls in this file. +DEFINING_FILE_FOR(BasicAliasAnalysis) |
