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llvm-project/llvm/lib/Analysis/BasicAliasAnalysis.cpp

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//===- 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 (!isa<PointerType>((*CI)->getType()) ||
!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) {
// 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 (!isa<PointerType>(V1->getType()) || !isa<PointerType>(V2->getType()))
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)
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