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e597ed0112
llvm-mirror/lib/Analysis/BasicAliasAnalysis.cpp
Chandler Carruth e597ed0112 [AA] Hoist the logic to reformulate various AA queries in terms of other 
parts of the AA interface out of the base class of every single AA
result object.

Because this logic reformulates the query in terms of some other aspect
of the API, it would easily cause O(n^2) query patterns in alias
analysis. These could in turn be magnified further based on the number
of call arguments, and then further based on the number of AA queries
made for a particular call. This ended up causing problems for Rust that
were actually noticable enough to get a bug (PR26564) and probably other
places as well.

When originally re-working the AA infrastructure, the desire was to
regularize the pattern of refinement without losing any generality.
While I think it was successful, that is clearly proving to be too
costly. And the cost is needless: we gain no actual improvement for this
generality of making a direct query to tbaa actually be able to
re-use some other alias analysis's refinement logic for one of the other
APIs, or some such. In short, this is entirely wasted work.

To the extent possible, delegation to other API surfaces should be done
at the aggregation layer so that we can avoid re-walking the
aggregation. In fact, this significantly simplifies the logic as we no
longer need to smuggle the aggregation layer into each alias analysis
(or the TargetLibraryInfo into each alias analysis just so we can form
argument memory locations!).

However, we also have some delegation logic inside of BasicAA and some
of it even makes sense. When the delegation logic is baking in specific
knowledge of aliasing properties of the LLVM IR, as opposed to simply
reformulating the query to utilize a different alias analysis interface
entry point, it makes a lot of sense to restrict that logic to
a different layer such as BasicAA. So one aspect of the delegation that
was in every AA base class is that when we don't have operand bundles,
we re-use function AA results as a fallback for callsite alias results.
This relies on the IR properties of calls and functions w.r.t. aliasing,
and so seems a better fit to BasicAA. I've lifted the logic up to that
point where it seems to be a natural fit. This still does a bit of
redundant work (we query function attributes twice, once via the
callsite and once via the function AA query) but it is *exactly* twice
here, no more.

The end result is that all of the delegation logic is hoisted out of the
base class and into either the aggregation layer when it is a pure
retargeting to a different API surface, or into BasicAA when it relies
on the IR's aliasing properties. This should fix the quadratic query
pattern reported in PR26564, although I don't have a stand-alone test
case to reproduce it.

It also seems general goodness. Now the numerous AAs that don't need
target library info don't carry it around and depend on it. I think
I can even rip out the general access to the aggregation layer and only
expose that in BasicAA as it is the only place where we re-query in that
manner.

However, this is a non-trivial change to the AA infrastructure so I want
to get some additional eyes on this before it lands. Sadly, it can't
wait long because we should really cherry pick this into 3.8 if we're
going to go this route.

Differential Revision: http://reviews.llvm.org/D17329

llvm-svn: 262490
2016-03-02 15:56:53 +00:00

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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/BasicAliasAnalysis.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Analysis/AliasAnalysis.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/ValueTracking.h"
#include "llvm/Analysis/AssumptionCache.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/DataLayout.h"
#include "llvm/IR/DerivedTypes.h"
#include "llvm/IR/Dominators.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>
#define DEBUG_TYPE "basicaa"
using namespace llvm;
/// Enable analysis of recursive PHI nodes.
static cl::opt<bool> EnableRecPhiAnalysis("basicaa-recphi", cl::Hidden,
cl::init(false));
/// SearchLimitReached / SearchTimes shows how often the limit of
/// to decompose GEPs is reached. It will affect the precision
/// of basic alias analysis.
STATISTIC(SearchLimitReached, "Number of times the limit to "
"decompose GEPs is reached");
STATISTIC(SearchTimes, "Number of times a GEP is decomposed");
/// 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
//===----------------------------------------------------------------------===//
/// Returns 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;
}
/// Returns 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;
}
/// Returns 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 MemoryLocation::UnknownSize;
}
/// Returns 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 != MemoryLocation::UnknownSize && ObjectSize < Size;
}
/// Returns 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 != MemoryLocation::UnknownSize && ObjectSize == Size;
}
//===----------------------------------------------------------------------===//
// GetElementPtr Instruction Decomposition and Analysis
//===----------------------------------------------------------------------===//
/// Analyzes the specified value as a linear expression: "A*V + B", where A and
/// B are constant integers.
///
/// Returns 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*/ const Value *BasicAAResult::GetLinearExpression(
const Value *V, APInt &Scale, APInt &Offset, unsigned &ZExtBits,
unsigned &SExtBits, const DataLayout &DL, unsigned Depth,
AssumptionCache *AC, DominatorTree *DT, bool &NSW, bool &NUW) {
assert(V->getType()->isIntegerTy() && "Not an integer value");
// Limit our recursion depth.
if (Depth == 6) {
Scale = 1;
Offset = 0;
return V;
}
if (const ConstantInt *Const = dyn_cast<ConstantInt>(V)) {
// If it's a constant, just convert it to an offset and remove the variable.
// If we've been called recursively, the Offset bit width will be greater
// than the constant's (the Offset's always as wide as the outermost call),
// so we'll zext here and process any extension in the isa<SExtInst> &
// isa<ZExtInst> cases below.
Offset += Const->getValue().zextOrSelf(Offset.getBitWidth());
assert(Scale == 0 && "Constant values don't have a scale");
return V;
}
if (const BinaryOperator *BOp = dyn_cast<BinaryOperator>(V)) {
if (ConstantInt *RHSC = dyn_cast<ConstantInt>(BOp->getOperand(1))) {
// If we've been called recursively, then Offset and Scale will be wider
// than the BOp operands. We'll always zext it here as we'll process sign
// extensions below (see the isa<SExtInst> / isa<ZExtInst> cases).
APInt RHS = RHSC->getValue().zextOrSelf(Offset.getBitWidth());
switch (BOp->getOpcode()) {
default:
// We don't understand this instruction, so we can't decompose it any
// further.
Scale = 1;
Offset = 0;
return V;
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)) {
Scale = 1;
Offset = 0;
return V;
}
// FALL THROUGH.
case Instruction::Add:
V = GetLinearExpression(BOp->getOperand(0), Scale, Offset, ZExtBits,
SExtBits, DL, Depth + 1, AC, DT, NSW, NUW);
Offset += RHS;
break;
case Instruction::Sub:
V = GetLinearExpression(BOp->getOperand(0), Scale, Offset, ZExtBits,
SExtBits, DL, Depth + 1, AC, DT, NSW, NUW);
Offset -= RHS;
break;
case Instruction::Mul:
V = GetLinearExpression(BOp->getOperand(0), Scale, Offset, ZExtBits,
SExtBits, DL, Depth + 1, AC, DT, NSW, NUW);
Offset *= RHS;
Scale *= RHS;
break;
case Instruction::Shl:
V = GetLinearExpression(BOp->getOperand(0), Scale, Offset, ZExtBits,
SExtBits, DL, Depth + 1, AC, DT, NSW, NUW);
Offset <<= RHS.getLimitedValue();
Scale <<= RHS.getLimitedValue();
// the semantics of nsw and nuw for left shifts don't match those of
// multiplications, so we won't propagate them.
NSW = NUW = false;
return V;
}
if (isa<OverflowingBinaryOperator>(BOp)) {
NUW &= BOp->hasNoUnsignedWrap();
NSW &= BOp->hasNoSignedWrap();
}
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) || isa<ZExtInst>(V)) {
Value *CastOp = cast<CastInst>(V)->getOperand(0);
unsigned NewWidth = V->getType()->getPrimitiveSizeInBits();
unsigned SmallWidth = CastOp->getType()->getPrimitiveSizeInBits();
unsigned OldZExtBits = ZExtBits, OldSExtBits = SExtBits;
const Value *Result =
GetLinearExpression(CastOp, Scale, Offset, ZExtBits, SExtBits, DL,
Depth + 1, AC, DT, NSW, NUW);
// zext(zext(%x)) == zext(%x), and similiarly for sext; we'll handle this
// by just incrementing the number of bits we've extended by.
unsigned ExtendedBy = NewWidth - SmallWidth;
if (isa<SExtInst>(V) && ZExtBits == 0) {
// sext(sext(%x, a), b) == sext(%x, a + b)
if (NSW) {
// We haven't sign-wrapped, so it's valid to decompose sext(%x + c)
// into sext(%x) + sext(c). We'll sext the Offset ourselves:
unsigned OldWidth = Offset.getBitWidth();
Offset = Offset.trunc(SmallWidth).sext(NewWidth).zextOrSelf(OldWidth);
} else {
// We may have signed-wrapped, so don't decompose sext(%x + c) into
// sext(%x) + sext(c)
Scale = 1;
Offset = 0;
Result = CastOp;
ZExtBits = OldZExtBits;
SExtBits = OldSExtBits;
}
SExtBits += ExtendedBy;
} else {
// sext(zext(%x, a), b) = zext(zext(%x, a), b) = zext(%x, a + b)
if (!NUW) {
// We may have unsigned-wrapped, so don't decompose zext(%x + c) into
// zext(%x) + zext(c)
Scale = 1;
Offset = 0;
Result = CastOp;
ZExtBits = OldZExtBits;
SExtBits = OldSExtBits;
}
ZExtBits += ExtendedBy;
}
return Result;
}
Scale = 1;
Offset = 0;
return V;
}
/// To ensure a pointer offset fits in an integer of size PointerSize
/// (in bits) when that size is smaller than 64. This is an issue in
/// particular for 32b programs with negative indices that rely on two's
/// complement wrap-arounds for precise alias information.
static int64_t adjustToPointerSize(int64_t Offset, unsigned PointerSize) {
assert(PointerSize <= 64 && "Invalid PointerSize!");
unsigned ShiftBits = 64 - PointerSize;
return (int64_t)((uint64_t)Offset << ShiftBits) >> ShiftBits;
}
/// 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 *BasicAAResult::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;
SearchTimes++;
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->getSourceElementType()->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);
unsigned PointerSize = DL.getPointerSizeInBits(AS);
for (User::const_op_iterator I = GEPOp->op_begin() + 1, E = GEPOp->op_end();
I != E; ++I) {
const 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 (const ConstantInt *CIdx = dyn_cast<ConstantInt>(Index)) {
if (CIdx->isZero())
continue;
BaseOffs += DL.getTypeAllocSize(*GTI) * CIdx->getSExtValue();
continue;
}
uint64_t Scale = DL.getTypeAllocSize(*GTI);
unsigned ZExtBits = 0, SExtBits = 0;
// If the integer type is smaller than the pointer size, it is implicitly
// sign extended to pointer size.
unsigned Width = Index->getType()->getIntegerBitWidth();
if (PointerSize > Width)
SExtBits += PointerSize - Width;
// Use GetLinearExpression to decompose the index into a C1*V+C2 form.
APInt IndexScale(Width, 0), IndexOffset(Width, 0);
bool NSW = true, NUW = true;
Index = GetLinearExpression(Index, IndexScale, IndexOffset, ZExtBits,
SExtBits, DL, 0, AC, DT, NSW, NUW);
// 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].ZExtBits == ZExtBits &&
VarIndices[i].SExtBits == SExtBits) {
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.
Scale = adjustToPointerSize(Scale, PointerSize);
if (Scale) {
VariableGEPIndex Entry = {Index, ZExtBits, SExtBits,
static_cast<int64_t>(Scale)};
VarIndices.push_back(Entry);
}
}
// Take care of wrap-arounds
BaseOffs = adjustToPointerSize(BaseOffs, PointerSize);
// Analyze the base pointer next.
V = GEPOp->getOperand(0);
} while (--MaxLookup);
// If the chain of expressions is too deep, just return early.
MaxLookupReached = true;
SearchLimitReached++;
return V;
}
/// 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 BasicAAResult::pointsToConstantMemory(const MemoryLocation &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 AAResultBase::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 AAResultBase::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 AAResultBase::pointsToConstantMemory(Loc, OrLocal);
}
for (Value *IncValue : PN->incoming_values())
Worklist.push_back(IncValue);
continue;
}
// Otherwise be conservative.
Visited.clear();
return AAResultBase::pointsToConstantMemory(Loc, OrLocal);
} while (!Worklist.empty() && --MaxLookup);
Visited.clear();
return Worklist.empty();
}
// FIXME: This code is duplicated with MemoryLocation and should be hoisted to
// some common utility location.
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;
}
/// Returns the behavior when calling the given call site.
FunctionModRefBehavior BasicAAResult::getModRefBehavior(ImmutableCallSite CS) {
if (CS.doesNotAccessMemory())
// Can't do better than this.
return FMRB_DoesNotAccessMemory;
FunctionModRefBehavior Min = FMRB_UnknownModRefBehavior;
// If the callsite knows it only reads memory, don't return worse
// than that.
if (CS.onlyReadsMemory())
Min = FMRB_OnlyReadsMemory;
if (CS.onlyAccessesArgMemory())
Min = FunctionModRefBehavior(Min & FMRB_OnlyAccessesArgumentPointees);
// If CS has operand bundles then aliasing attributes from the function it
// calls do not directly apply to the CallSite. This can be made more
// precise in the future.
if (!CS.hasOperandBundles())
if (const Function *F = CS.getCalledFunction())
Min =
FunctionModRefBehavior(Min & getBestAAResults().getModRefBehavior(F));
return Min;
}
/// Returns the behavior when calling the given function. For use when the call
/// site is not known.
FunctionModRefBehavior BasicAAResult::getModRefBehavior(const Function *F) {
// If the function declares it doesn't access memory, we can't do better.
if (F->doesNotAccessMemory())
return FMRB_DoesNotAccessMemory;
FunctionModRefBehavior Min = FMRB_UnknownModRefBehavior;
// If the function declares it only reads memory, go with that.
if (F->onlyReadsMemory())
Min = FMRB_OnlyReadsMemory;
if (F->onlyAccessesArgMemory())
Min = FunctionModRefBehavior(Min & FMRB_OnlyAccessesArgumentPointees);
return Min;
}
/// Returns true if this is a writeonly (i.e Mod only) parameter. Currently,
/// we don't have a writeonly attribute, so this only knows about builtin
/// intrinsics and target library functions. We could consider adding a
/// writeonly attribute in the future and moving all of these facts to either
/// Intrinsics.td or InferFunctionAttr.cpp
static bool isWriteOnlyParam(ImmutableCallSite CS, unsigned ArgIdx,
const TargetLibraryInfo &TLI) {
if (const IntrinsicInst *II = dyn_cast<IntrinsicInst>(CS.getInstruction()))
switch (II->getIntrinsicID()) {
default:
break;
case Intrinsic::memset:
case Intrinsic::memcpy:
case Intrinsic::memmove:
// We don't currently have a writeonly attribute. All other properties
// of these intrinsics are nicely described via attributes in
// Intrinsics.td and handled generically.
if (ArgIdx == 0)
return true;
}
// 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. Note that all but the missing writeonly attribute are
// handled via InferFunctionAttr.
if (CS.getCalledFunction() && isMemsetPattern16(CS.getCalledFunction(), TLI))
if (ArgIdx == 0)
return true;
// TODO: memset_pattern4, memset_pattern8
// TODO: _chk variants
// TODO: strcmp, strcpy
return false;
}
ModRefInfo BasicAAResult::getArgModRefInfo(ImmutableCallSite CS,
unsigned ArgIdx) {
// Emulate the missing writeonly attribute by checking for known builtin
// intrinsics and target library functions.
if (isWriteOnlyParam(CS, ArgIdx, TLI))
return MRI_Mod;
if (CS.paramHasAttr(ArgIdx + 1, Attribute::ReadOnly))
return MRI_Ref;
if (CS.paramHasAttr(ArgIdx + 1, Attribute::ReadNone))
return MRI_NoModRef;
return AAResultBase::getArgModRefInfo(CS, ArgIdx);
}
static bool isAssumeIntrinsic(ImmutableCallSite CS) {
const IntrinsicInst *II = dyn_cast<IntrinsicInst>(CS.getInstruction());
return II && II->getIntrinsicID() == Intrinsic::assume;
}
#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
AliasResult BasicAAResult::alias(const MemoryLocation &LocA,
const MemoryLocation &LocB) {
assert(notDifferentParent(LocA.Ptr, LocB.Ptr) &&
"BasicAliasAnalysis doesn't support interprocedural queries.");
// If we have a directly cached entry for these locations, we have recursed
// through this once, so just return the cached results. Notably, when this
// happens, we don't clear the cache.
auto CacheIt = AliasCache.find(LocPair(LocA, LocB));
if (CacheIt != AliasCache.end())
return CacheIt->second;
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;
}
/// Checks 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.
ModRefInfo BasicAAResult::getModRefInfo(ImmutableCallSite CS,
const MemoryLocation &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 MRI_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 OperandNo = 0;
for (auto CI = CS.data_operands_begin(), CE = CS.data_operands_end();
CI != CE; ++CI, ++OperandNo) {
// 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(OperandNo) && !CS.isByValArgument(OperandNo)))
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.
AliasResult AR =
getBestAAResults().alias(MemoryLocation(*CI), MemoryLocation(Object));
if (AR) {
PassedAsArg = true;
break;
}
}
if (!PassedAsArg)
return MRI_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 MRI_NoModRef;
// The AAResultBase base class has some smarts, lets use them.
return AAResultBase::getModRefInfo(CS, Loc);
}
ModRefInfo BasicAAResult::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 MRI_NoModRef;
// The AAResultBase base class has some smarts, lets use them.
return AAResultBase::getModRefInfo(CS1, CS2);
}
/// Provide ad-hoc rules to disambiguate accesses through two GEP operators,
/// both having the exact same pointer operand.
static 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 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 == MemoryLocation::UnknownSize ||
V2Size == MemoryLocation::UnknownSize)
return 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 are constants and are equal, the other indices
// might be also be dynamically equal, so the GEPs can alias.
if (C1 && C2 && C1 == C2)
return 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 MayAlias;
IntermediateIndices.push_back(GEP1->getOperand(i + 1));
}
auto *Ty = GetElementPtrInst::getIndexedType(
GEP1->getSourceElementType(), IntermediateIndices);
StructType *LastIndexedStruct = dyn_cast<StructType>(Ty);
if (isa<SequentialType>(Ty)) {
// We know that:
// - both GEPs begin indexing from the exact same pointer;
// - the last indices in both GEPs are constants, indexing into a sequential
// type (array or pointer);
// - both GEPs only index through arrays prior to that.
//
// Because array indices greater than the number of elements are valid in
// GEPs, unless we know the intermediate indices are identical between
// GEP1 and GEP2 we cannot guarantee that the last indexed arrays don't
// partially overlap. We also need to check that the loaded size matches
// the element size, otherwise we could still have overlap.
const uint64_t ElementSize =
DL.getTypeStoreSize(cast<SequentialType>(Ty)->getElementType());
if (V1Size != ElementSize || V2Size != ElementSize)
return MayAlias;
for (unsigned i = 0, e = GEP1->getNumIndices() - 1; i != e; ++i)
if (GEP1->getOperand(i + 1) != GEP2->getOperand(i + 1))
return MayAlias;
// Now we know that the array/pointer that GEP1 indexes into and that
// that GEP2 indexes into must either precisely overlap or be disjoint.
// Because they cannot partially overlap and because fields in an array
// cannot overlap, if we can prove the final indices are different between
// GEP1 and GEP2, we can conclude GEP1 and GEP2 don't alias.
// If the last indices are constants, we've already checked they don't
// equal each other so we can exit early.
if (C1 && C2)
return NoAlias;
if (isKnownNonEqual(GEP1->getOperand(GEP1->getNumOperands() - 1),
GEP2->getOperand(GEP2->getNumOperands() - 1),
DL))
return NoAlias;
return MayAlias;
} else if (!LastIndexedStruct || !C1 || !C2) {
return 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 NoAlias;
return MayAlias;
}
/// Provides 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.
AliasResult BasicAAResult::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;
// 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, MemoryLocation::UnknownSize, AAMDNodes(),
UnderlyingV2, MemoryLocation::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, &AC, DT);
const Value *GEP1BasePtr =
DecomposeGEPExpression(GEP1, GEP1BaseOffset, GEP1VariableIndices,
GEP1MaxLookupReached, DL, &AC, DT);
// DecomposeGEPExpression and GetUnderlyingObject should return the
// same result except when DecomposeGEPExpression has no DataLayout.
// FIXME: They always have a DataLayout, so this should become an
// assert.
if (GEP1BasePtr != UnderlyingV1 || GEP2BasePtr != UnderlyingV2) {
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, &AC, DT);
int64_t GEP2BaseOffset;
bool GEP2MaxLookupReached;
SmallVector<VariableGEPIndex, 4> GEP2VariableIndices;
const Value *GEP2BasePtr =
DecomposeGEPExpression(GEP2, GEP2BaseOffset, GEP2VariableIndices,
GEP2MaxLookupReached, DL, &AC, DT);
// DecomposeGEPExpression and GetUnderlyingObject should return the
// same result except when DecomposeGEPExpression has no DataLayout.
// FIXME: They always have a DataLayout, so this should become an assert.
if (GEP1BasePtr != UnderlyingV1 || GEP2BasePtr != UnderlyingV2) {
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 (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 == MemoryLocation::UnknownSize &&
V2Size == MemoryLocation::UnknownSize)
return MayAlias;
AliasResult R = aliasCheck(UnderlyingV1, MemoryLocation::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, &AC, DT);
// DecomposeGEPExpression and GetUnderlyingObject should return the
// same result except when DecomposeGEPExpression has no DataLayout.
// FIXME: They always have a DataLayout, so this should become an assert.
if (GEP1BasePtr != UnderlyingV1) {
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
// happens 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 != MemoryLocation::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 != MemoryLocation::UnknownSize &&
V2Size != MemoryLocation::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, &AC, nullptr, DT);
// Zero-extension widens the variable, and so forces the sign
// bit to zero.
bool IsZExt = GEP1VariableIndices[i].ZExtBits > 0 || isa<ZExtInst>(V);
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 != MemoryLocation::UnknownSize &&
V2Size != MemoryLocation::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;
if (constantOffsetHeuristic(GEP1VariableIndices, V1Size, V2Size,
GEP1BaseOffset, &AC, DT))
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 AliasResult MergeAliasResults(AliasResult A, AliasResult B) {
// If the results agree, take it.
if (A == B)
return A;
// A mix of PartialAlias and MustAlias is PartialAlias.
if ((A == PartialAlias && B == MustAlias) ||
(B == PartialAlias && A == MustAlias))
return PartialAlias;
// Otherwise, we don't know anything.
return MayAlias;
}
/// Provides a bunch of ad-hoc rules to disambiguate a Select instruction
/// against another.
AliasResult BasicAAResult::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);
}
/// Provide a bunch of ad-hoc rules to disambiguate a PHI instruction against
/// another.
AliasResult BasicAAResult::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(MemoryLocation(PN, PNSize, PNAAInfo),
MemoryLocation(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;
bool isRecursive = false;
for (Value *PV1 : PN->incoming_values()) {
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 (EnableRecPhiAnalysis)
if (GEPOperator *PV1GEP = dyn_cast<GEPOperator>(PV1)) {
// Check whether the incoming value is a GEP that advances the pointer
// result of this PHI node (e.g. in a loop). If this is the case, we
// would recurse and always get a MayAlias. Handle this case specially
// below.
if (PV1GEP->getPointerOperand() == PN && PV1GEP->getNumIndices() == 1 &&
isa<ConstantInt>(PV1GEP->idx_begin())) {
isRecursive = true;
continue;
}
}
if (UniqueSrc.insert(PV1).second)
V1Srcs.push_back(PV1);
}
// If this PHI node is recursive, set the size of the accessed memory to
// unknown to represent all the possible values the GEP could advance the
// pointer to.
if (isRecursive)
PNSize = MemoryLocation::UnknownSize;
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;
}
/// Provides a bunch of ad-hoc rules to disambiguate in common cases, such as
/// array references.
AliasResult BasicAAResult::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 ((V1Size != MemoryLocation::UnknownSize &&
isObjectSmallerThan(O2, V1Size, DL, TLI)) ||
(V2Size != MemoryLocation::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(MemoryLocation(V1, V1Size, V1AAInfo),
MemoryLocation(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 the entire object, then the accesses must overlap in some way.
if (O1 == O2)
if ((V1Size != MemoryLocation::UnknownSize &&
isObjectSize(O1, V1Size, DL, TLI)) ||
(V2Size != MemoryLocation::UnknownSize &&
isObjectSize(O2, V2Size, DL, TLI)))
return AliasCache[Locs] = PartialAlias;
// Recurse back into the best AA results we have, potentially with refined
// memory locations. We have already ensured that BasicAA has a MayAlias
// cache result for these, so any recursion back into BasicAA won't loop.
AliasResult Result = getBestAAResults().alias(Locs.first, Locs.second);
return AliasCache[Locs] = Result;
}
/// 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 BasicAAResult::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;
// 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->front(), Inst, DT, LI))
return false;
return true;
}
/// Computes the symbolic difference between two de-composed GEPs.
///
/// Dest and Src are the variable indices from two decomposed GetElementPtr
/// instructions GEP1 and GEP2 which have common base pointers.
void BasicAAResult::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;
unsigned ZExtBits = Src[i].ZExtBits, SExtBits = Src[i].SExtBits;
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].ZExtBits != ZExtBits || Dest[j].SExtBits != SExtBits)
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, ZExtBits, SExtBits, -Scale};
Dest.push_back(Entry);
}
}
}
bool BasicAAResult::constantOffsetHeuristic(
const SmallVectorImpl<VariableGEPIndex> &VarIndices, uint64_t V1Size,
uint64_t V2Size, int64_t BaseOffset, AssumptionCache *AC,
DominatorTree *DT) {
if (VarIndices.size() != 2 || V1Size == MemoryLocation::UnknownSize ||
V2Size == MemoryLocation::UnknownSize)
return false;
const VariableGEPIndex &Var0 = VarIndices[0], &Var1 = VarIndices[1];
if (Var0.ZExtBits != Var1.ZExtBits || Var0.SExtBits != Var1.SExtBits ||
Var0.Scale != -Var1.Scale)
return false;
unsigned Width = Var1.V->getType()->getIntegerBitWidth();
// We'll strip off the Extensions of Var0 and Var1 and do another round
// of GetLinearExpression decomposition. In the example above, if Var0
// is zext(%x + 1) we should get V1 == %x and V1Offset == 1.
APInt V0Scale(Width, 0), V0Offset(Width, 0), V1Scale(Width, 0),
V1Offset(Width, 0);
bool NSW = true, NUW = true;
unsigned V0ZExtBits = 0, V0SExtBits = 0, V1ZExtBits = 0, V1SExtBits = 0;
const Value *V0 = GetLinearExpression(Var0.V, V0Scale, V0Offset, V0ZExtBits,
V0SExtBits, DL, 0, AC, DT, NSW, NUW);
NSW = true;
NUW = true;
const Value *V1 = GetLinearExpression(Var1.V, V1Scale, V1Offset, V1ZExtBits,
V1SExtBits, DL, 0, AC, DT, NSW, NUW);
if (V0Scale != V1Scale || V0ZExtBits != V1ZExtBits ||
V0SExtBits != V1SExtBits || !isValueEqualInPotentialCycles(V0, V1))
return false;
// We have a hit - Var0 and Var1 only differ by a constant offset!
// If we've been sext'ed then zext'd the maximum difference between Var0 and
// Var1 is possible to calculate, but we're just interested in the absolute
// minimum difference between the two. The minimum distance may occur due to
// wrapping; consider "add i3 %i, 5": if %i == 7 then 7 + 5 mod 8 == 4, and so
// the minimum distance between %i and %i + 5 is 3.
APInt MinDiff = V0Offset - V1Offset, Wrapped = -MinDiff;
MinDiff = APIntOps::umin(MinDiff, Wrapped);
uint64_t MinDiffBytes = MinDiff.getZExtValue() * std::abs(Var0.Scale);
// We can't definitely say whether GEP1 is before or after V2 due to wrapping
// arithmetic (i.e. for some values of GEP1 and V2 GEP1 < V2, and for other
// values GEP1 > V2). We'll therefore only declare NoAlias if both V1Size and
// V2Size can fit in the MinDiffBytes gap.
return V1Size + std::abs(BaseOffset) <= MinDiffBytes &&
V2Size + std::abs(BaseOffset) <= MinDiffBytes;
}
//===----------------------------------------------------------------------===//
// BasicAliasAnalysis Pass
//===----------------------------------------------------------------------===//
BasicAAResult BasicAA::run(Function &F, AnalysisManager<Function> *AM) {
return BasicAAResult(F.getParent()->getDataLayout(),
AM->getResult<TargetLibraryAnalysis>(F),
AM->getResult<AssumptionAnalysis>(F),
AM->getCachedResult<DominatorTreeAnalysis>(F),
AM->getCachedResult<LoopAnalysis>(F));
}
BasicAAWrapperPass::BasicAAWrapperPass() : FunctionPass(ID) {
initializeBasicAAWrapperPassPass(*PassRegistry::getPassRegistry());
}
char BasicAAWrapperPass::ID = 0;
void BasicAAWrapperPass::anchor() {}
INITIALIZE_PASS_BEGIN(BasicAAWrapperPass, "basicaa",
"Basic Alias Analysis (stateless AA impl)", true, true)
INITIALIZE_PASS_DEPENDENCY(AssumptionCacheTracker)
INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfoWrapperPass)
INITIALIZE_PASS_END(BasicAAWrapperPass, "basicaa",
"Basic Alias Analysis (stateless AA impl)", true, true)
FunctionPass *llvm::createBasicAAWrapperPass() {
return new BasicAAWrapperPass();
}
bool BasicAAWrapperPass::runOnFunction(Function &F) {
auto &ACT = getAnalysis<AssumptionCacheTracker>();
auto &TLIWP = getAnalysis<TargetLibraryInfoWrapperPass>();
auto *DTWP = getAnalysisIfAvailable<DominatorTreeWrapperPass>();
auto *LIWP = getAnalysisIfAvailable<LoopInfoWrapperPass>();
Result.reset(new BasicAAResult(F.getParent()->getDataLayout(), TLIWP.getTLI(),
ACT.getAssumptionCache(F),
DTWP ? &DTWP->getDomTree() : nullptr,
LIWP ? &LIWP->getLoopInfo() : nullptr));
return false;
}
void BasicAAWrapperPass::getAnalysisUsage(AnalysisUsage &AU) const {
AU.setPreservesAll();
AU.addRequired<AssumptionCacheTracker>();
AU.addRequired<TargetLibraryInfoWrapperPass>();
}
BasicAAResult llvm::createLegacyPMBasicAAResult(Pass &P, Function &F) {
return BasicAAResult(
F.getParent()->getDataLayout(),
P.getAnalysis<TargetLibraryInfoWrapperPass>().getTLI(),
P.getAnalysis<AssumptionCacheTracker>().getAssumptionCache(F));
}
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