llvm-6502/lib/Analysis/BasicAliasAnalysis.cpp
Eric Christopher 551754c495 Revert 101465, it broke internal OpenGL testing.
Probably the best way to know that all getOperand() calls have been handled
is to replace that API instead of updating.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@101579 91177308-0d34-0410-b5e6-96231b3b80d8
2010-04-16 23:37:20 +00:00

759 lines
29 KiB
C++

//===- 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)