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

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//===- BasicAliasAnalysis.cpp - Stateless 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 primary stateless implementation of the
// Alias Analysis interface that implements identities (two different
// globals cannot alias, etc), but does no stateful analysis.
//
//===----------------------------------------------------------------------===//
#include "llvm/Analysis/Passes.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/Analysis/AliasAnalysis.h"
#include "llvm/Analysis/AssumptionCache.h"
#include "llvm/Analysis/CFG.h"
#include "llvm/Analysis/CaptureTracking.h"
#include "llvm/Analysis/InstructionSimplify.h"
#include "llvm/Analysis/LoopInfo.h"
#include "llvm/Analysis/MemoryBuiltins.h"
#include "llvm/Analysis/TargetLibraryInfo.h"
#include "llvm/Analysis/ValueTracking.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/DataLayout.h"
#include "llvm/IR/DerivedTypes.h"
#include "llvm/IR/Dominators.h"
#include "llvm/IR/Function.h"
#include "llvm/IR/GetElementPtrTypeIterator.h"
#include "llvm/IR/GlobalAlias.h"
#include "llvm/IR/GlobalVariable.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/IntrinsicInst.h"
#include "llvm/IR/LLVMContext.h"
#include "llvm/IR/Operator.h"
#include "llvm/Pass.h"
#include "llvm/Support/ErrorHandling.h"
#include <algorithm>
using namespace llvm;
/// Cutoff after which to stop analysing a set of phi nodes potentially involved
/// in a cycle. Because we are analysing 'through' phi nodes we need to be
/// careful with value equivalence. We use reachability to make sure a value
/// cannot be involved in a cycle.
const unsigned MaxNumPhiBBsValueReachabilityCheck = 20;
// The max limit of the search depth in DecomposeGEPExpression() and
// GetUnderlyingObject(), both functions need to use the same search
// depth otherwise the algorithm in aliasGEP will assert.
static const unsigned MaxLookupSearchDepth = 6;
//===----------------------------------------------------------------------===//
// Useful predicates
//===----------------------------------------------------------------------===//
/// 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())
// Note even if the argument is marked nocapture we still need to check
// for copies made inside the function. The nocapture attribute only
// specifies that there are no copies made that outlive the function.
return !PointerMayBeCaptured(V, false, /*StoreCaptures=*/true);
return false;
}
/// isEscapeSource - Return true if the pointer is one which would have
/// been considered an escape by isNonEscapingLocalObject.
static bool isEscapeSource(const Value *V) {
if (isa<CallInst>(V) || isa<InvokeInst>(V) || isa<Argument>(V))
return true;
// The load case works because isNonEscapingLocalObject considers all
// stores to be escapes (it passes true for the StoreCaptures argument
// to PointerMayBeCaptured).
if (isa<LoadInst>(V))
return true;
return false;
}
/// getObjectSize - Return the size of the object specified by V, or
/// UnknownSize if unknown.
static uint64_t getObjectSize(const Value *V, const DataLayout &DL,
const TargetLibraryInfo &TLI,
bool RoundToAlign = false) {
uint64_t Size;
if (getObjectSize(V, Size, DL, &TLI, RoundToAlign))
return Size;
return AliasAnalysis::UnknownSize;
}
/// isObjectSmallerThan - Return true if we can prove that the object specified
/// by V is smaller than Size.
static bool isObjectSmallerThan(const Value *V, uint64_t Size,
const DataLayout &DL,
const TargetLibraryInfo &TLI) {
// Note that the meanings of the "object" are slightly different in the
// following contexts:
// c1: llvm::getObjectSize()
// c2: llvm.objectsize() intrinsic
// c3: isObjectSmallerThan()
// c1 and c2 share the same meaning; however, the meaning of "object" in c3
// refers to the "entire object".
//
// Consider this example:
// char *p = (char*)malloc(100)
// char *q = p+80;
//
// In the context of c1 and c2, the "object" pointed by q refers to the
// stretch of memory of q[0:19]. So, getObjectSize(q) should return 20.
//
// However, in the context of c3, the "object" refers to the chunk of memory
// being allocated. So, the "object" has 100 bytes, and q points to the middle
// the "object". In case q is passed to isObjectSmallerThan() as the 1st
// parameter, before the llvm::getObjectSize() is called to get the size of
// entire object, we should:
// - either rewind the pointer q to the base-address of the object in
// question (in this case rewind to p), or
// - just give up. It is up to caller to make sure the pointer is pointing
// to the base address the object.
//
// We go for 2nd option for simplicity.
if (!isIdentifiedObject(V))
return false;
// This function needs to use the aligned object size because we allow
// reads a bit past the end given sufficient alignment.
uint64_t ObjectSize = getObjectSize(V, DL, TLI, /*RoundToAlign*/true);
return ObjectSize != AliasAnalysis::UnknownSize && ObjectSize < Size;
}
/// isObjectSize - Return true if we can prove that the object specified
/// by V has size Size.
static bool isObjectSize(const Value *V, uint64_t Size,
const DataLayout &DL, const TargetLibraryInfo &TLI) {
uint64_t ObjectSize = getObjectSize(V, DL, TLI);
return ObjectSize != AliasAnalysis::UnknownSize && ObjectSize == Size;
}
//===----------------------------------------------------------------------===//
// GetElementPtr Instruction Decomposition and Analysis
//===----------------------------------------------------------------------===//
namespace {
enum ExtensionKind {
EK_NotExtended,
EK_SignExt,
EK_ZeroExt
};
struct VariableGEPIndex {
const Value *V;
ExtensionKind Extension;
int64_t Scale;
bool operator==(const VariableGEPIndex &Other) const {
return V == Other.V && Extension == Other.Extension &&
Scale == Other.Scale;
}
bool operator!=(const VariableGEPIndex &Other) const {
return !operator==(Other);
}
};
}
/// GetLinearExpression - Analyze the specified value as a linear expression:
/// "A*V + B", where A and B are constant integers. Return the scale and offset
/// values as APInts and return V as a Value*, and return whether we looked
/// through any sign or zero extends. The incoming Value is known to have
/// IntegerType and it may already be sign or zero extended.
///
/// Note that this looks through extends, so the high bits may not be
/// represented in the result.
static Value *GetLinearExpression(Value *V, APInt &Scale, APInt &Offset,
ExtensionKind &Extension,
const DataLayout &DL, unsigned Depth,
AssumptionCache *AC, DominatorTree *DT) {
assert(V->getType()->isIntegerTy() && "Not an integer value");
// Limit our recursion depth.
if (Depth == 6) {
Scale = 1;
Offset = 0;
return V;
}
if (ConstantInt *Const = dyn_cast<ConstantInt>(V)) {
// if it's a constant, just convert it to an offset
// and remove the variable.
Offset += Const->getValue();
assert(Scale == 0 && "Constant values don't have a scale");
return V;
}
if (BinaryOperator *BOp = dyn_cast<BinaryOperator>(V)) {
if (ConstantInt *RHSC = dyn_cast<ConstantInt>(BOp->getOperand(1))) {
switch (BOp->getOpcode()) {
default: break;
case Instruction::Or:
// X|C == X+C if all the bits in C are unset in X. Otherwise we can't
// analyze it.
if (!MaskedValueIsZero(BOp->getOperand(0), RHSC->getValue(), DL, 0, AC,
BOp, DT))
break;
// FALL THROUGH.
case Instruction::Add:
V = GetLinearExpression(BOp->getOperand(0), Scale, Offset, Extension,
DL, Depth + 1, AC, DT);
Offset += RHSC->getValue();
return V;
case Instruction::Mul:
V = GetLinearExpression(BOp->getOperand(0), Scale, Offset, Extension,
DL, Depth + 1, AC, DT);
Offset *= RHSC->getValue();
Scale *= RHSC->getValue();
return V;
case Instruction::Shl:
V = GetLinearExpression(BOp->getOperand(0), Scale, Offset, Extension,
DL, Depth + 1, AC, DT);
Offset <<= RHSC->getValue().getLimitedValue();
Scale <<= RHSC->getValue().getLimitedValue();
return V;
}
}
}
// Since GEP indices are sign extended anyway, we don't care about the high
// bits of a sign or zero extended value - just scales and offsets. The
// extensions have to be consistent though.
if ((isa<SExtInst>(V) && Extension != EK_ZeroExt) ||
(isa<ZExtInst>(V) && Extension != EK_SignExt)) {
Value *CastOp = cast<CastInst>(V)->getOperand(0);
unsigned OldWidth = Scale.getBitWidth();
unsigned SmallWidth = CastOp->getType()->getPrimitiveSizeInBits();
Scale = Scale.trunc(SmallWidth);
Offset = Offset.trunc(SmallWidth);
Extension = isa<SExtInst>(V) ? EK_SignExt : EK_ZeroExt;
Value *Result = GetLinearExpression(CastOp, Scale, Offset, Extension, DL,
Depth + 1, AC, DT);
Scale = Scale.zext(OldWidth);
// We have to sign-extend even if Extension == EK_ZeroExt as we can't
// decompose a sign extension (i.e. zext(x - 1) != zext(x) - zext(-1)).
Offset = Offset.sext(OldWidth);
return Result;
}
Scale = 1;
Offset = 0;
return V;
}
/// DecomposeGEPExpression - If V is a symbolic pointer expression, decompose it
/// into a base pointer with a constant offset and a number of scaled symbolic
/// offsets.
///
/// The scaled symbolic offsets (represented by pairs of a Value* and a scale in
/// the VarIndices vector) are Value*'s that are known to be scaled by the
/// specified amount, but which may have other unrepresented high bits. As such,
/// the gep cannot necessarily be reconstructed from its decomposed form.
///
/// When DataLayout is around, this function is capable of analyzing everything
/// that GetUnderlyingObject can look through. To be able to do that
/// GetUnderlyingObject and DecomposeGEPExpression must use the same search
/// depth (MaxLookupSearchDepth).
/// When DataLayout not is around, it just looks through pointer casts.
///
static const Value *
DecomposeGEPExpression(const Value *V, int64_t &BaseOffs,
SmallVectorImpl<VariableGEPIndex> &VarIndices,
bool &MaxLookupReached, const DataLayout &DL,
AssumptionCache *AC, DominatorTree *DT) {
// Limit recursion depth to limit compile time in crazy cases.
unsigned MaxLookup = MaxLookupSearchDepth;
MaxLookupReached = false;
BaseOffs = 0;
do {
// See if this is a bitcast or GEP.
const Operator *Op = dyn_cast<Operator>(V);
if (!Op) {
// The only non-operator case we can handle are GlobalAliases.
if (const GlobalAlias *GA = dyn_cast<GlobalAlias>(V)) {
if (!GA->mayBeOverridden()) {
V = GA->getAliasee();
continue;
}
}
return V;
}
if (Op->getOpcode() == Instruction::BitCast ||
Op->getOpcode() == Instruction::AddrSpaceCast) {
V = Op->getOperand(0);
continue;
}
const GEPOperator *GEPOp = dyn_cast<GEPOperator>(Op);
if (!GEPOp) {
// If it's not a GEP, hand it off to SimplifyInstruction to see if it
// can come up with something. This matches what GetUnderlyingObject does.
if (const Instruction *I = dyn_cast<Instruction>(V))
// TODO: Get a DominatorTree and AssumptionCache and use them here
// (these are both now available in this function, but this should be
// updated when GetUnderlyingObject is updated). TLI should be
// provided also.
if (const Value *Simplified =
SimplifyInstruction(const_cast<Instruction *>(I), DL)) {
V = Simplified;
continue;
}
return V;
}
// Don't attempt to analyze GEPs over unsized objects.
if (!GEPOp->getOperand(0)->getType()->getPointerElementType()->isSized())
return V;
unsigned AS = GEPOp->getPointerAddressSpace();
// Walk the indices of the GEP, accumulating them into BaseOff/VarIndices.
gep_type_iterator GTI = gep_type_begin(GEPOp);
for (User::const_op_iterator I = GEPOp->op_begin()+1,
E = GEPOp->op_end(); I != E; ++I) {
Value *Index = *I;
// Compute the (potentially symbolic) offset in bytes for this index.
if (StructType *STy = dyn_cast<StructType>(*GTI++)) {
// For a struct, add the member offset.
unsigned FieldNo = cast<ConstantInt>(Index)->getZExtValue();
if (FieldNo == 0) continue;
BaseOffs += DL.getStructLayout(STy)->getElementOffset(FieldNo);
continue;
}
// For an array/pointer, add the element offset, explicitly scaled.
if (ConstantInt *CIdx = dyn_cast<ConstantInt>(Index)) {
if (CIdx->isZero()) continue;
BaseOffs += DL.getTypeAllocSize(*GTI) * CIdx->getSExtValue();
continue;
}
uint64_t Scale = DL.getTypeAllocSize(*GTI);
ExtensionKind Extension = EK_NotExtended;
// If the integer type is smaller than the pointer size, it is implicitly
// sign extended to pointer size.
unsigned Width = Index->getType()->getIntegerBitWidth();
if (DL.getPointerSizeInBits(AS) > Width)
Extension = EK_SignExt;
// Use GetLinearExpression to decompose the index into a C1*V+C2 form.
APInt IndexScale(Width, 0), IndexOffset(Width, 0);
Index = GetLinearExpression(Index, IndexScale, IndexOffset, Extension, DL,
0, AC, DT);
// The GEP index scale ("Scale") scales C1*V+C2, yielding (C1*V+C2)*Scale.
// This gives us an aggregate computation of (C1*Scale)*V + C2*Scale.
BaseOffs += IndexOffset.getSExtValue()*Scale;
Scale *= IndexScale.getSExtValue();
// If we already had an occurrence of this index variable, merge this
// scale into it. For example, we want to handle:
// A[x][x] -> x*16 + x*4 -> x*20
// This also ensures that 'x' only appears in the index list once.
for (unsigned i = 0, e = VarIndices.size(); i != e; ++i) {
if (VarIndices[i].V == Index &&
VarIndices[i].Extension == Extension) {
Scale += VarIndices[i].Scale;
VarIndices.erase(VarIndices.begin()+i);
break;
}
}
// Make sure that we have a scale that makes sense for this target's
// pointer size.
if (unsigned ShiftBits = 64 - DL.getPointerSizeInBits(AS)) {
Scale <<= ShiftBits;
Scale = (int64_t)Scale >> ShiftBits;
}
if (Scale) {
VariableGEPIndex Entry = {Index, Extension,
static_cast<int64_t>(Scale)};
VarIndices.push_back(Entry);
}
}
// Analyze the base pointer next.
V = GEPOp->getOperand(0);
} while (--MaxLookup);
// If the chain of expressions is too deep, just return early.
MaxLookupReached = true;
return V;
}
//===----------------------------------------------------------------------===//
// BasicAliasAnalysis Pass
//===----------------------------------------------------------------------===//
#ifndef NDEBUG
static const Function *getParent(const Value *V) {
if (const Instruction *inst = dyn_cast<Instruction>(V))
return inst->getParent()->getParent();
if (const Argument *arg = dyn_cast<Argument>(V))
return arg->getParent();
return nullptr;
}
static bool notDifferentParent(const Value *O1, const Value *O2) {
const Function *F1 = getParent(O1);
const Function *F2 = getParent(O2);
return !F1 || !F2 || F1 == F2;
}
#endif
namespace {
/// BasicAliasAnalysis - This is the primary alias analysis implementation.
struct BasicAliasAnalysis : public ImmutablePass, public AliasAnalysis {
static char ID; // Class identification, replacement for typeinfo
BasicAliasAnalysis() : ImmutablePass(ID) {
initializeBasicAliasAnalysisPass(*PassRegistry::getPassRegistry());
}
bool doInitialization(Module &M) override;
void getAnalysisUsage(AnalysisUsage &AU) const override {
AU.addRequired<AliasAnalysis>();
AU.addRequired<AssumptionCacheTracker>();
AU.addRequired<TargetLibraryInfoWrapperPass>();
}
AliasResult alias(const Location &LocA, const Location &LocB) override {
assert(AliasCache.empty() && "AliasCache must be cleared after use!");
assert(notDifferentParent(LocA.Ptr, LocB.Ptr) &&
"BasicAliasAnalysis doesn't support interprocedural queries.");
AliasResult Alias = aliasCheck(LocA.Ptr, LocA.Size, LocA.AATags,
LocB.Ptr, LocB.Size, LocB.AATags);
// AliasCache rarely has more than 1 or 2 elements, always use
// shrink_and_clear so it quickly returns to the inline capacity of the
// SmallDenseMap if it ever grows larger.
// FIXME: This should really be shrink_to_inline_capacity_and_clear().
AliasCache.shrink_and_clear();
VisitedPhiBBs.clear();
return Alias;
}
ModRefResult getModRefInfo(ImmutableCallSite CS,
const Location &Loc) override;
ModRefResult getModRefInfo(ImmutableCallSite CS1,
ImmutableCallSite CS2) override;
/// pointsToConstantMemory - Chase pointers until we find a (constant
/// global) or not.
bool pointsToConstantMemory(const Location &Loc, bool OrLocal) override;
/// Get the location associated with a pointer argument of a callsite.
Location getArgLocation(ImmutableCallSite CS, unsigned ArgIdx,
ModRefResult &Mask) override;
/// getModRefBehavior - Return the behavior when calling the given
/// call site.
ModRefBehavior getModRefBehavior(ImmutableCallSite CS) override;
/// getModRefBehavior - Return the behavior when calling the given function.
/// For use when the call site is not known.
ModRefBehavior getModRefBehavior(const Function *F) override;
/// 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.
void *getAdjustedAnalysisPointer(const void *ID) override {
if (ID == &AliasAnalysis::ID)
return (AliasAnalysis*)this;
return this;
}
private:
// AliasCache - Track alias queries to guard against recursion.
typedef std::pair<Location, Location> LocPair;
typedef SmallDenseMap<LocPair, AliasResult, 8> AliasCacheTy;
AliasCacheTy AliasCache;
/// \brief Track phi nodes we have visited. When interpret "Value" pointer
/// equality as value equality we need to make sure that the "Value" is not
/// part of a cycle. Otherwise, two uses could come from different
/// "iterations" of a cycle and see different values for the same "Value"
/// pointer.
/// The following example shows the problem:
/// %p = phi(%alloca1, %addr2)
/// %l = load %ptr
/// %addr1 = gep, %alloca2, 0, %l
/// %addr2 = gep %alloca2, 0, (%l + 1)
/// alias(%p, %addr1) -> MayAlias !
/// store %l, ...
SmallPtrSet<const BasicBlock*, 8> VisitedPhiBBs;
// Visited - Track instructions visited by pointsToConstantMemory.
SmallPtrSet<const Value*, 16> Visited;
/// \brief Check whether two Values can be considered equivalent.
///
/// In addition to pointer equivalence of \p V1 and \p V2 this checks
/// whether they can not be part of a cycle in the value graph by looking at
/// all visited phi nodes an making sure that the phis cannot reach the
/// value. We have to do this because we are looking through phi nodes (That
/// is we say noalias(V, phi(VA, VB)) if noalias(V, VA) and noalias(V, VB).
bool isValueEqualInPotentialCycles(const Value *V1, const Value *V2);
/// \brief 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.
void GetIndexDifference(SmallVectorImpl<VariableGEPIndex> &Dest,
const SmallVectorImpl<VariableGEPIndex> &Src);
// aliasGEP - Provide a bunch of ad-hoc rules to disambiguate a GEP
// instruction against another.
AliasResult aliasGEP(const GEPOperator *V1, uint64_t V1Size,
const AAMDNodes &V1AAInfo,
const Value *V2, uint64_t V2Size,
const AAMDNodes &V2AAInfo,
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, uint64_t PNSize,
const AAMDNodes &PNAAInfo,
const Value *V2, uint64_t V2Size,
const AAMDNodes &V2AAInfo);
/// aliasSelect - Disambiguate a Select instruction against another value.
AliasResult aliasSelect(const SelectInst *SI, uint64_t SISize,
const AAMDNodes &SIAAInfo,
const Value *V2, uint64_t V2Size,
const AAMDNodes &V2AAInfo);
AliasResult aliasCheck(const Value *V1, uint64_t V1Size,
AAMDNodes V1AATag,
const Value *V2, uint64_t V2Size,
AAMDNodes V2AATag);
};
} // End of anonymous namespace
// Register this pass...
char BasicAliasAnalysis::ID = 0;
INITIALIZE_AG_PASS_BEGIN(BasicAliasAnalysis, AliasAnalysis, "basicaa",
"Basic Alias Analysis (stateless AA impl)",
false, true, false)
INITIALIZE_PASS_DEPENDENCY(AssumptionCacheTracker)
INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfoWrapperPass)
INITIALIZE_AG_PASS_END(BasicAliasAnalysis, AliasAnalysis, "basicaa",
"Basic Alias Analysis (stateless AA impl)",
false, true, false)
ImmutablePass *llvm::createBasicAliasAnalysisPass() {
return new BasicAliasAnalysis();
}
/// pointsToConstantMemory - Returns whether the given pointer value
/// points to memory that is local to the function, with global constants being
/// considered local to all functions.
bool
BasicAliasAnalysis::pointsToConstantMemory(const Location &Loc, bool OrLocal) {
assert(Visited.empty() && "Visited must be cleared after use!");
unsigned MaxLookup = 8;
SmallVector<const Value *, 16> Worklist;
Worklist.push_back(Loc.Ptr);
do {
const Value *V = GetUnderlyingObject(Worklist.pop_back_val(), *DL);
if (!Visited.insert(V).second) {
Visited.clear();
return AliasAnalysis::pointsToConstantMemory(Loc, OrLocal);
}
// An alloca instruction defines local memory.
if (OrLocal && isa<AllocaInst>(V))
continue;
// A global constant counts as local memory for our purposes.
if (const GlobalVariable *GV = dyn_cast<GlobalVariable>(V)) {
// 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.
if (!GV->isConstant()) {
Visited.clear();
return AliasAnalysis::pointsToConstantMemory(Loc, OrLocal);
}
continue;
}
// If both select values point to local memory, then so does the select.
if (const SelectInst *SI = dyn_cast<SelectInst>(V)) {
Worklist.push_back(SI->getTrueValue());
Worklist.push_back(SI->getFalseValue());
continue;
}
// If all values incoming to a phi node point to local memory, then so does
// the phi.
if (const PHINode *PN = dyn_cast<PHINode>(V)) {
// Don't bother inspecting phi nodes with many operands.
if (PN->getNumIncomingValues() > MaxLookup) {
Visited.clear();
return AliasAnalysis::pointsToConstantMemory(Loc, OrLocal);
}
for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
Worklist.push_back(PN->getIncomingValue(i));
continue;
}
// Otherwise be conservative.
Visited.clear();
return AliasAnalysis::pointsToConstantMemory(Loc, OrLocal);
} while (!Worklist.empty() && --MaxLookup);
Visited.clear();
return Worklist.empty();
}
static bool isMemsetPattern16(const Function *MS,
const TargetLibraryInfo &TLI) {
if (TLI.has(LibFunc::memset_pattern16) &&
MS->getName() == "memset_pattern16") {
FunctionType *MemsetType = MS->getFunctionType();
if (!MemsetType->isVarArg() && MemsetType->getNumParams() == 3 &&
isa<PointerType>(MemsetType->getParamType(0)) &&
isa<PointerType>(MemsetType->getParamType(1)) &&
isa<IntegerType>(MemsetType->getParamType(2)))
return true;
}
return false;
}
/// getModRefBehavior - Return the behavior when calling the given call site.
AliasAnalysis::ModRefBehavior
BasicAliasAnalysis::getModRefBehavior(ImmutableCallSite CS) {
if (CS.doesNotAccessMemory())
// Can't do better than this.
return DoesNotAccessMemory;
ModRefBehavior Min = UnknownModRefBehavior;
// If the callsite knows it only reads memory, don't return worse
// than that.
if (CS.onlyReadsMemory())
Min = OnlyReadsMemory;
// The AliasAnalysis base class has some smarts, lets use them.
return ModRefBehavior(AliasAnalysis::getModRefBehavior(CS) & Min);
}
/// getModRefBehavior - Return the behavior when calling the given function.
/// For use when the call site is not known.
AliasAnalysis::ModRefBehavior
BasicAliasAnalysis::getModRefBehavior(const Function *F) {
// If the function declares it doesn't access memory, we can't do better.
if (F->doesNotAccessMemory())
return DoesNotAccessMemory;
// For intrinsics, we can check the table.
if (unsigned iid = F->getIntrinsicID()) {
#define GET_INTRINSIC_MODREF_BEHAVIOR
#include "llvm/IR/Intrinsics.gen"
#undef GET_INTRINSIC_MODREF_BEHAVIOR
}
ModRefBehavior Min = UnknownModRefBehavior;
// If the function declares it only reads memory, go with that.
if (F->onlyReadsMemory())
Min = OnlyReadsMemory;
const TargetLibraryInfo &TLI =
getAnalysis<TargetLibraryInfoWrapperPass>().getTLI();
if (isMemsetPattern16(F, TLI))
Min = OnlyAccessesArgumentPointees;
// Otherwise be conservative.
return ModRefBehavior(AliasAnalysis::getModRefBehavior(F) & Min);
}
AliasAnalysis::Location
BasicAliasAnalysis::getArgLocation(ImmutableCallSite CS, unsigned ArgIdx,
ModRefResult &Mask) {
Location Loc = AliasAnalysis::getArgLocation(CS, ArgIdx, Mask);
const TargetLibraryInfo &TLI =
getAnalysis<TargetLibraryInfoWrapperPass>().getTLI();
const IntrinsicInst *II = dyn_cast<IntrinsicInst>(CS.getInstruction());
if (II != nullptr)
switch (II->getIntrinsicID()) {
default: break;
case Intrinsic::memset:
case Intrinsic::memcpy:
case Intrinsic::memmove: {
assert((ArgIdx == 0 || ArgIdx == 1) &&
"Invalid argument index for memory intrinsic");
if (ConstantInt *LenCI = dyn_cast<ConstantInt>(II->getArgOperand(2)))
Loc.Size = LenCI->getZExtValue();
assert(Loc.Ptr == II->getArgOperand(ArgIdx) &&
"Memory intrinsic location pointer not argument?");
Mask = ArgIdx ? Ref : Mod;
break;
}
case Intrinsic::lifetime_start:
case Intrinsic::lifetime_end:
case Intrinsic::invariant_start: {
assert(ArgIdx == 1 && "Invalid argument index");
assert(Loc.Ptr == II->getArgOperand(ArgIdx) &&
"Intrinsic location pointer not argument?");
Loc.Size = cast<ConstantInt>(II->getArgOperand(0))->getZExtValue();
break;
}
case Intrinsic::invariant_end: {
assert(ArgIdx == 2 && "Invalid argument index");
assert(Loc.Ptr == II->getArgOperand(ArgIdx) &&
"Intrinsic location pointer not argument?");
Loc.Size = cast<ConstantInt>(II->getArgOperand(1))->getZExtValue();
break;
}
case Intrinsic::arm_neon_vld1: {
assert(ArgIdx == 0 && "Invalid argument index");
assert(Loc.Ptr == II->getArgOperand(ArgIdx) &&
"Intrinsic location pointer not argument?");
// LLVM's vld1 and vst1 intrinsics currently only support a single
// vector register.
if (DL)
Loc.Size = DL->getTypeStoreSize(II->getType());
break;
}
case Intrinsic::arm_neon_vst1: {
assert(ArgIdx == 0 && "Invalid argument index");
assert(Loc.Ptr == II->getArgOperand(ArgIdx) &&
"Intrinsic location pointer not argument?");
if (DL)
Loc.Size = DL->getTypeStoreSize(II->getArgOperand(1)->getType());
break;
}
}
// We can bound the aliasing properties of memset_pattern16 just as we can
// for memcpy/memset. This is particularly important because the
// LoopIdiomRecognizer likes to turn loops into calls to memset_pattern16
// whenever possible.
else if (CS.getCalledFunction() &&
isMemsetPattern16(CS.getCalledFunction(), TLI)) {
assert((ArgIdx == 0 || ArgIdx == 1) &&
"Invalid argument index for memset_pattern16");
if (ArgIdx == 1)
Loc.Size = 16;
else if (const ConstantInt *LenCI =
dyn_cast<ConstantInt>(CS.getArgument(2)))
Loc.Size = LenCI->getZExtValue();
assert(Loc.Ptr == CS.getArgument(ArgIdx) &&
"memset_pattern16 location pointer not argument?");
Mask = ArgIdx ? Ref : Mod;
}
// FIXME: Handle memset_pattern4 and memset_pattern8 also.
return Loc;
}
static bool isAssumeIntrinsic(ImmutableCallSite CS) {
const IntrinsicInst *II = dyn_cast<IntrinsicInst>(CS.getInstruction());
if (II && II->getIntrinsicID() == Intrinsic::assume)
return true;
return false;
}
bool BasicAliasAnalysis::doInitialization(Module &M) {
InitializeAliasAnalysis(this, &M.getDataLayout());
return true;
}
/// 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(ImmutableCallSite CS,
const Location &Loc) {
assert(notDifferentParent(CS.getInstruction(), Loc.Ptr) &&
"AliasAnalysis query involving multiple functions!");
const Value *Object = GetUnderlyingObject(Loc.Ptr, *DL);
// If this is a tail call and Loc.Ptr 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 (const 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 (ImmutableCallSite::arg_iterator CI = CS.arg_begin(), CE = CS.arg_end();
CI != CE; ++CI, ++ArgNo) {
// Only look at the no-capture or byval pointer arguments. If this
// pointer were passed to arguments that were neither of these, then it
// couldn't be no-capture.
if (!(*CI)->getType()->isPointerTy() ||
(!CS.doesNotCapture(ArgNo) && !CS.isByValArgument(ArgNo)))
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(Location(*CI), Location(Object))) {
PassedAsArg = true;
break;
}
}
if (!PassedAsArg)
return NoModRef;
}
// While the assume intrinsic is marked as arbitrarily writing so that
// proper control dependencies will be maintained, it never aliases any
// particular memory location.
if (isAssumeIntrinsic(CS))
return NoModRef;
// The AliasAnalysis base class has some smarts, lets use them.
return AliasAnalysis::getModRefInfo(CS, Loc);
}
AliasAnalysis::ModRefResult
BasicAliasAnalysis::getModRefInfo(ImmutableCallSite CS1,
ImmutableCallSite CS2) {
// While the assume intrinsic is marked as arbitrarily writing so that
// proper control dependencies will be maintained, it never aliases any
// particular memory location.
if (isAssumeIntrinsic(CS1) || isAssumeIntrinsic(CS2))
return NoModRef;
// The AliasAnalysis base class has some smarts, lets use them.
return AliasAnalysis::getModRefInfo(CS1, CS2);
}
/// \brief Provide ad-hoc rules to disambiguate accesses through two GEP
/// operators, both having the exact same pointer operand.
static AliasAnalysis::AliasResult
aliasSameBasePointerGEPs(const GEPOperator *GEP1, uint64_t V1Size,
const GEPOperator *GEP2, uint64_t V2Size,
const DataLayout &DL) {
assert(GEP1->getPointerOperand() == GEP2->getPointerOperand() &&
"Expected GEPs with the same pointer operand");
// Try to determine whether GEP1 and GEP2 index through arrays, into structs,
// such that the struct field accesses provably cannot alias.
// We also need at least two indices (the pointer, and the struct field).
if (GEP1->getNumIndices() != GEP2->getNumIndices() ||
GEP1->getNumIndices() < 2)
return AliasAnalysis::MayAlias;
// If we don't know the size of the accesses through both GEPs, we can't
// determine whether the struct fields accessed can't alias.
if (V1Size == AliasAnalysis::UnknownSize ||
V2Size == AliasAnalysis::UnknownSize)
return AliasAnalysis::MayAlias;
ConstantInt *C1 =
dyn_cast<ConstantInt>(GEP1->getOperand(GEP1->getNumOperands() - 1));
ConstantInt *C2 =
dyn_cast<ConstantInt>(GEP2->getOperand(GEP2->getNumOperands() - 1));
// If the last (struct) indices aren't constants, we can't say anything.
// If they're identical, the other indices might be also be dynamically
// equal, so the GEPs can alias.
if (!C1 || !C2 || C1 == C2)
return AliasAnalysis::MayAlias;
// Find the last-indexed type of the GEP, i.e., the type you'd get if
// you stripped the last index.
// On the way, look at each indexed type. If there's something other
// than an array, different indices can lead to different final types.
SmallVector<Value *, 8> IntermediateIndices;
// Insert the first index; we don't need to check the type indexed
// through it as it only drops the pointer indirection.
assert(GEP1->getNumIndices() > 1 && "Not enough GEP indices to examine");
IntermediateIndices.push_back(GEP1->getOperand(1));
// Insert all the remaining indices but the last one.
// Also, check that they all index through arrays.
for (unsigned i = 1, e = GEP1->getNumIndices() - 1; i != e; ++i) {
if (!isa<ArrayType>(GetElementPtrInst::getIndexedType(
GEP1->getSourceElementType(), IntermediateIndices)))
return AliasAnalysis::MayAlias;
IntermediateIndices.push_back(GEP1->getOperand(i + 1));
}
StructType *LastIndexedStruct =
dyn_cast<StructType>(GetElementPtrInst::getIndexedType(
GEP1->getSourceElementType(), IntermediateIndices));
if (!LastIndexedStruct)
return AliasAnalysis::MayAlias;
// We know that:
// - both GEPs begin indexing from the exact same pointer;
// - the last indices in both GEPs are constants, indexing into a struct;
// - said indices are different, hence, the pointed-to fields are different;
// - both GEPs only index through arrays prior to that.
//
// This lets us determine that the struct that GEP1 indexes into and the
// struct that GEP2 indexes into must either precisely overlap or be
// completely disjoint. Because they cannot partially overlap, indexing into
// different non-overlapping fields of the struct will never alias.
// Therefore, the only remaining thing needed to show that both GEPs can't
// alias is that the fields are not overlapping.
const StructLayout *SL = DL.getStructLayout(LastIndexedStruct);
const uint64_t StructSize = SL->getSizeInBytes();
const uint64_t V1Off = SL->getElementOffset(C1->getZExtValue());
const uint64_t V2Off = SL->getElementOffset(C2->getZExtValue());
auto EltsDontOverlap = [StructSize](uint64_t V1Off, uint64_t V1Size,
uint64_t V2Off, uint64_t V2Size) {
return V1Off < V2Off && V1Off + V1Size <= V2Off &&
((V2Off + V2Size <= StructSize) ||
(V2Off + V2Size - StructSize <= V1Off));
};
if (EltsDontOverlap(V1Off, V1Size, V2Off, V2Size) ||
EltsDontOverlap(V2Off, V2Size, V1Off, V1Size))
return AliasAnalysis::NoAlias;
return AliasAnalysis::MayAlias;
}
/// 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 GetUnderlyingObject(GEP1, DL),
/// UnderlyingV2 is the same for V2.
///
AliasAnalysis::AliasResult
BasicAliasAnalysis::aliasGEP(const GEPOperator *GEP1, uint64_t V1Size,
const AAMDNodes &V1AAInfo,
const Value *V2, uint64_t V2Size,
const AAMDNodes &V2AAInfo,
const Value *UnderlyingV1,
const Value *UnderlyingV2) {
int64_t GEP1BaseOffset;
bool GEP1MaxLookupReached;
SmallVector<VariableGEPIndex, 4> GEP1VariableIndices;
// We have to get two AssumptionCaches here because GEP1 and V2 may be from
// different functions.
// FIXME: This really doesn't make any sense. We get a dominator tree below
// that can only refer to a single function. But this function (aliasGEP) is
// a method on an immutable pass that can be called when there *isn't*
// a single function. The old pass management layer makes this "work", but
// this isn't really a clean solution.
AssumptionCacheTracker &ACT = getAnalysis<AssumptionCacheTracker>();
AssumptionCache *AC1 = nullptr, *AC2 = nullptr;
if (auto *GEP1I = dyn_cast<Instruction>(GEP1))
AC1 = &ACT.getAssumptionCache(
const_cast<Function &>(*GEP1I->getParent()->getParent()));
if (auto *I2 = dyn_cast<Instruction>(V2))
AC2 = &ACT.getAssumptionCache(
const_cast<Function &>(*I2->getParent()->getParent()));
DominatorTreeWrapperPass *DTWP =
getAnalysisIfAvailable<DominatorTreeWrapperPass>();
DominatorTree *DT = DTWP ? &DTWP->getDomTree() : nullptr;
// If we have two gep instructions with must-alias or not-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, UnknownSize, AAMDNodes(),
UnderlyingV2, UnknownSize, AAMDNodes());
// Check for geps of non-aliasing underlying pointers where the offsets are
// identical.
if ((BaseAlias == MayAlias) && V1Size == V2Size) {
// Do the base pointers alias assuming type and size.
AliasResult PreciseBaseAlias = aliasCheck(UnderlyingV1, V1Size,
V1AAInfo, UnderlyingV2,
V2Size, V2AAInfo);
if (PreciseBaseAlias == NoAlias) {
// See if the computed offset from the common pointer tells us about the
// relation of the resulting pointer.
int64_t GEP2BaseOffset;
bool GEP2MaxLookupReached;
SmallVector<VariableGEPIndex, 4> GEP2VariableIndices;
const Value *GEP2BasePtr =
DecomposeGEPExpression(GEP2, GEP2BaseOffset, GEP2VariableIndices,
GEP2MaxLookupReached, *DL, AC2, DT);
const Value *GEP1BasePtr =
DecomposeGEPExpression(GEP1, GEP1BaseOffset, GEP1VariableIndices,
GEP1MaxLookupReached, *DL, AC1, DT);
// DecomposeGEPExpression and GetUnderlyingObject should return the
// same result except when DecomposeGEPExpression has no DataLayout.
if (GEP1BasePtr != UnderlyingV1 || GEP2BasePtr != UnderlyingV2) {
assert(!DL &&
"DecomposeGEPExpression and GetUnderlyingObject disagree!");
return MayAlias;
}
// If the max search depth is reached the result is undefined
if (GEP2MaxLookupReached || GEP1MaxLookupReached)
return MayAlias;
// Same offsets.
if (GEP1BaseOffset == GEP2BaseOffset &&
GEP1VariableIndices == GEP2VariableIndices)
return NoAlias;
GEP1VariableIndices.clear();
}
}
// 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,
GEP1MaxLookupReached, *DL, AC1, DT);
int64_t GEP2BaseOffset;
bool GEP2MaxLookupReached;
SmallVector<VariableGEPIndex, 4> GEP2VariableIndices;
const Value *GEP2BasePtr =
DecomposeGEPExpression(GEP2, GEP2BaseOffset, GEP2VariableIndices,
GEP2MaxLookupReached, *DL, AC2, DT);
// DecomposeGEPExpression and GetUnderlyingObject should return the
// same result except when DecomposeGEPExpression has no DataLayout.
if (GEP1BasePtr != UnderlyingV1 || GEP2BasePtr != UnderlyingV2) {
assert(!DL &&
"DecomposeGEPExpression and GetUnderlyingObject disagree!");
return MayAlias;
}
// If we know the two GEPs are based off of the exact same pointer (and not
// just the same underlying object), see if that tells us anything about
// the resulting pointers.
if (DL && GEP1->getPointerOperand() == GEP2->getPointerOperand()) {
AliasResult R = aliasSameBasePointerGEPs(GEP1, V1Size, GEP2, V2Size, *DL);
// If we couldn't find anything interesting, don't abandon just yet.
if (R != MayAlias)
return R;
}
// If the max search depth is reached the result is undefined
if (GEP2MaxLookupReached || GEP1MaxLookupReached)
return MayAlias;
// Subtract the GEP2 pointer from the GEP1 pointer to find out their
// symbolic difference.
GEP1BaseOffset -= GEP2BaseOffset;
GetIndexDifference(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 == UnknownSize && V2Size == UnknownSize)
return MayAlias;
AliasResult R = aliasCheck(UnderlyingV1, UnknownSize, AAMDNodes(),
V2, V2Size, V2AAInfo);
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,
GEP1MaxLookupReached, *DL, AC1, DT);
// DecomposeGEPExpression and GetUnderlyingObject should return the
// same result except when DecomposeGEPExpression has no DataLayout.
if (GEP1BasePtr != UnderlyingV1) {
assert(!DL &&
"DecomposeGEPExpression and GetUnderlyingObject disagree!");
return MayAlias;
}
// If the max search depth is reached the result is undefined
if (GEP1MaxLookupReached)
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 there is a constant difference between the pointers, but the difference
// is less than the size of the associated memory object, then we know
// that the objects are partially overlapping. If the difference is
// greater, we know they do not overlap.
if (GEP1BaseOffset != 0 && GEP1VariableIndices.empty()) {
if (GEP1BaseOffset >= 0) {
if (V2Size != UnknownSize) {
if ((uint64_t)GEP1BaseOffset < V2Size)
return PartialAlias;
return NoAlias;
}
} else {
// We have the situation where:
// + +
// | BaseOffset |
// ---------------->|
// |-->V1Size |-------> V2Size
// GEP1 V2
// We need to know that V2Size is not unknown, otherwise we might have
// stripped a gep with negative index ('gep <ptr>, -1, ...).
if (V1Size != UnknownSize && V2Size != UnknownSize) {
if (-(uint64_t)GEP1BaseOffset < V1Size)
return PartialAlias;
return NoAlias;
}
}
}
if (!GEP1VariableIndices.empty()) {
uint64_t Modulo = 0;
bool AllPositive = true;
for (unsigned i = 0, e = GEP1VariableIndices.size(); i != e; ++i) {
// Try to distinguish something like &A[i][1] against &A[42][0].
// Grab the least significant bit set in any of the scales. We
// don't need std::abs here (even if the scale's negative) as we'll
// be ^'ing Modulo with itself later.
Modulo |= (uint64_t) GEP1VariableIndices[i].Scale;
if (AllPositive) {
// If the Value could change between cycles, then any reasoning about
// the Value this cycle may not hold in the next cycle. We'll just
// give up if we can't determine conditions that hold for every cycle:
const Value *V = GEP1VariableIndices[i].V;
bool SignKnownZero, SignKnownOne;
ComputeSignBit(const_cast<Value *>(V), SignKnownZero, SignKnownOne, *DL,
0, AC1, nullptr, DT);
// Zero-extension widens the variable, and so forces the sign
// bit to zero.
bool IsZExt = GEP1VariableIndices[i].Extension == EK_ZeroExt;
SignKnownZero |= IsZExt;
SignKnownOne &= !IsZExt;
// If the variable begins with a zero then we know it's
// positive, regardless of whether the value is signed or
// unsigned.
int64_t Scale = GEP1VariableIndices[i].Scale;
AllPositive =
(SignKnownZero && Scale >= 0) ||
(SignKnownOne && Scale < 0);
}
}
Modulo = Modulo ^ (Modulo & (Modulo - 1));
// We can compute the difference between the two addresses
// mod Modulo. Check whether that difference guarantees that the
// two locations do not alias.
uint64_t ModOffset = (uint64_t)GEP1BaseOffset & (Modulo - 1);
if (V1Size != UnknownSize && V2Size != UnknownSize &&
ModOffset >= V2Size && V1Size <= Modulo - ModOffset)
return NoAlias;
// If we know all the variables are positive, then GEP1 >= GEP1BasePtr.
// If GEP1BasePtr > V2 (GEP1BaseOffset > 0) then we know the pointers
// don't alias if V2Size can fit in the gap between V2 and GEP1BasePtr.
if (AllPositive && GEP1BaseOffset > 0 && V2Size <= (uint64_t) GEP1BaseOffset)
return NoAlias;
}
// Statically, we can see that the base objects are the same, but the
// pointers have dynamic offsets which we can't resolve. And none of our
// little tricks above worked.
//
// TODO: Returning PartialAlias instead of MayAlias is a mild hack; the
// practical effect of this is protecting TBAA in the case of dynamic
// indices into arrays of unions or malloc'd memory.
return PartialAlias;
}
static AliasAnalysis::AliasResult
MergeAliasResults(AliasAnalysis::AliasResult A, AliasAnalysis::AliasResult B) {
// If the results agree, take it.
if (A == B)
return A;
// A mix of PartialAlias and MustAlias is PartialAlias.
if ((A == AliasAnalysis::PartialAlias && B == AliasAnalysis::MustAlias) ||
(B == AliasAnalysis::PartialAlias && A == AliasAnalysis::MustAlias))
return AliasAnalysis::PartialAlias;
// Otherwise, we don't know anything.
return AliasAnalysis::MayAlias;
}
/// aliasSelect - Provide a bunch of ad-hoc rules to disambiguate a Select
/// instruction against another.
AliasAnalysis::AliasResult
BasicAliasAnalysis::aliasSelect(const SelectInst *SI, uint64_t SISize,
const AAMDNodes &SIAAInfo,
const Value *V2, uint64_t V2Size,
const AAMDNodes &V2AAInfo) {
// 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, SIAAInfo,
SI2->getTrueValue(), V2Size, V2AAInfo);
if (Alias == MayAlias)
return MayAlias;
AliasResult ThisAlias =
aliasCheck(SI->getFalseValue(), SISize, SIAAInfo,
SI2->getFalseValue(), V2Size, V2AAInfo);
return MergeAliasResults(ThisAlias, Alias);
}
// If both arms of the Select node NoAlias or MustAlias V2, then returns
// NoAlias / MustAlias. Otherwise, returns MayAlias.
AliasResult Alias =
aliasCheck(V2, V2Size, V2AAInfo, SI->getTrueValue(), SISize, SIAAInfo);
if (Alias == MayAlias)
return MayAlias;
AliasResult ThisAlias =
aliasCheck(V2, V2Size, V2AAInfo, SI->getFalseValue(), SISize, SIAAInfo);
return MergeAliasResults(ThisAlias, Alias);
}
// aliasPHI - Provide a bunch of ad-hoc rules to disambiguate a PHI instruction
// against another.
AliasAnalysis::AliasResult
BasicAliasAnalysis::aliasPHI(const PHINode *PN, uint64_t PNSize,
const AAMDNodes &PNAAInfo,
const Value *V2, uint64_t V2Size,
const AAMDNodes &V2AAInfo) {
// Track phi nodes we have visited. We use this information when we determine
// value equivalence.
VisitedPhiBBs.insert(PN->getParent());
// 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()) {
LocPair Locs(Location(PN, PNSize, PNAAInfo),
Location(V2, V2Size, V2AAInfo));
if (PN > V2)
std::swap(Locs.first, Locs.second);
// Analyse the PHIs' inputs under the assumption that the PHIs are
// NoAlias.
// If the PHIs are May/MustAlias there must be (recursively) an input
// operand from outside the PHIs' cycle that is MayAlias/MustAlias or
// there must be an operation on the PHIs within the PHIs' value cycle
// that causes a MayAlias.
// Pretend the phis do not alias.
AliasResult Alias = NoAlias;
assert(AliasCache.count(Locs) &&
"There must exist an entry for the phi node");
AliasResult OrigAliasResult = AliasCache[Locs];
AliasCache[Locs] = NoAlias;
for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
AliasResult ThisAlias =
aliasCheck(PN->getIncomingValue(i), PNSize, PNAAInfo,
PN2->getIncomingValueForBlock(PN->getIncomingBlock(i)),
V2Size, V2AAInfo);
Alias = MergeAliasResults(ThisAlias, Alias);
if (Alias == MayAlias)
break;
}
// Reset if speculation failed.
if (Alias != NoAlias)
AliasCache[Locs] = OrigAliasResult;
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).second)
V1Srcs.push_back(PV1);
}
AliasResult Alias = aliasCheck(V2, V2Size, V2AAInfo,
V1Srcs[0], PNSize, PNAAInfo);
// 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];
AliasResult ThisAlias = aliasCheck(V2, V2Size, V2AAInfo,
V, PNSize, PNAAInfo);
Alias = MergeAliasResults(ThisAlias, Alias);
if (Alias == MayAlias)
break;
}
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, uint64_t V1Size,
AAMDNodes V1AAInfo,
const Value *V2, uint64_t V2Size,
AAMDNodes V2AAInfo) {
// 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();
// If V1 or V2 is undef, the result is NoAlias because we can always pick a
// value for undef that aliases nothing in the program.
if (isa<UndefValue>(V1) || isa<UndefValue>(V2))
return NoAlias;
// Are we checking for alias of the same value?
// Because we look 'through' phi nodes we could look at "Value" pointers from
// different iterations. We must therefore make sure that this is not the
// case. The function isValueEqualInPotentialCycles ensures that this cannot
// happen by looking at the visited phi nodes and making sure they cannot
// reach the value.
if (isValueEqualInPotentialCycles(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 = GetUnderlyingObject(V1, *DL, MaxLookupSearchDepth);
const Value *O2 = GetUnderlyingObject(V2, *DL, MaxLookupSearchDepth);
// 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;
// Function arguments can't alias with things that are known to be
// unambigously identified at the function level.
if ((isa<Argument>(O1) && isIdentifiedFunctionLocal(O2)) ||
(isa<Argument>(O2) && isIdentifiedFunctionLocal(O1)))
return NoAlias;
// Most objects can't alias null.
if ((isa<ConstantPointerNull>(O2) && isKnownNonNull(O1)) ||
(isa<ConstantPointerNull>(O1) && isKnownNonNull(O2)))
return NoAlias;
// If one pointer is the result of a call/invoke or load and the other is a
// non-escaping local object within the same function, then we know the
// object couldn't escape to a point where the call could return it.
//
// Note that if the pointers are in different functions, there are a
// variety of complications. A call with a nocapture argument may still
// temporary store the nocapture argument's value in a temporary memory
// location if that memory location doesn't escape. Or it may pass a
// nocapture value to other functions as long as they don't capture it.
if (isEscapeSource(O1) && isNonEscapingLocalObject(O2))
return NoAlias;
if (isEscapeSource(O2) && isNonEscapingLocalObject(O1))
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 (DL)
if ((V1Size != UnknownSize && isObjectSmallerThan(O2, V1Size, *DL, *TLI)) ||
(V2Size != UnknownSize && isObjectSmallerThan(O1, V2Size, *DL, *TLI)))
return NoAlias;
// Check the cache before climbing up use-def chains. This also terminates
// otherwise infinitely recursive queries.
LocPair Locs(Location(V1, V1Size, V1AAInfo),
Location(V2, V2Size, V2AAInfo));
if (V1 > V2)
std::swap(Locs.first, Locs.second);
std::pair<AliasCacheTy::iterator, bool> Pair =
AliasCache.insert(std::make_pair(Locs, MayAlias));
if (!Pair.second)
return Pair.first->second;
// 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);
std::swap(V1AAInfo, V2AAInfo);
}
if (const GEPOperator *GV1 = dyn_cast<GEPOperator>(V1)) {
AliasResult Result = aliasGEP(GV1, V1Size, V1AAInfo, V2, V2Size, V2AAInfo, O1, O2);
if (Result != MayAlias) return AliasCache[Locs] = Result;
}
if (isa<PHINode>(V2) && !isa<PHINode>(V1)) {
std::swap(V1, V2);
std::swap(V1Size, V2Size);
std::swap(V1AAInfo, V2AAInfo);
}
if (const PHINode *PN = dyn_cast<PHINode>(V1)) {
AliasResult Result = aliasPHI(PN, V1Size, V1AAInfo,
V2, V2Size, V2AAInfo);
if (Result != MayAlias) return AliasCache[Locs] = Result;
}
if (isa<SelectInst>(V2) && !isa<SelectInst>(V1)) {
std::swap(V1, V2);
std::swap(V1Size, V2Size);
std::swap(V1AAInfo, V2AAInfo);
}
if (const SelectInst *S1 = dyn_cast<SelectInst>(V1)) {
AliasResult Result = aliasSelect(S1, V1Size, V1AAInfo,
V2, V2Size, V2AAInfo);
if (Result != MayAlias) return AliasCache[Locs] = Result;
}
// If both pointers are pointing into the same object and one of them
// accesses is accessing the entire object, then the accesses must
// overlap in some way.
if (DL && O1 == O2)
if ((V1Size != UnknownSize && isObjectSize(O1, V1Size, *DL, *TLI)) ||
(V2Size != UnknownSize && isObjectSize(O2, V2Size, *DL, *TLI)))
return AliasCache[Locs] = PartialAlias;
AliasResult Result =
AliasAnalysis::alias(Location(V1, V1Size, V1AAInfo),
Location(V2, V2Size, V2AAInfo));
return AliasCache[Locs] = Result;
}
bool BasicAliasAnalysis::isValueEqualInPotentialCycles(const Value *V,
const Value *V2) {
if (V != V2)
return false;
const Instruction *Inst = dyn_cast<Instruction>(V);
if (!Inst)
return true;
if (VisitedPhiBBs.empty())
return true;
if (VisitedPhiBBs.size() > MaxNumPhiBBsValueReachabilityCheck)
return false;
// Use dominance or loop info if available.
DominatorTreeWrapperPass *DTWP =
getAnalysisIfAvailable<DominatorTreeWrapperPass>();
DominatorTree *DT = DTWP ? &DTWP->getDomTree() : nullptr;
auto *LIWP = getAnalysisIfAvailable<LoopInfoWrapperPass>();
LoopInfo *LI = LIWP ? &LIWP->getLoopInfo() : nullptr;
// Make sure that the visited phis cannot reach the Value. This ensures that
// the Values cannot come from different iterations of a potential cycle the
// phi nodes could be involved in.
for (auto *P : VisitedPhiBBs)
if (isPotentiallyReachable(P->begin(), Inst, DT, LI))
return false;
return true;
}
/// GetIndexDifference - 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.
void BasicAliasAnalysis::GetIndexDifference(
SmallVectorImpl<VariableGEPIndex> &Dest,
const SmallVectorImpl<VariableGEPIndex> &Src) {
if (Src.empty())
return;
for (unsigned i = 0, e = Src.size(); i != e; ++i) {
const Value *V = Src[i].V;
ExtensionKind Extension = Src[i].Extension;
int64_t Scale = Src[i].Scale;
// 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 (!isValueEqualInPotentialCycles(Dest[j].V, V) ||
Dest[j].Extension != Extension)
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].Scale != Scale)
Dest[j].Scale -= 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) {
VariableGEPIndex Entry = { V, Extension, -Scale };
Dest.push_back(Entry);
}
}
}
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