https://github.com/epiqc/ScaffCC
Tip revision: 861c8b60980b0f4d36b101b45346a2e64b0fa390 authored by Pranav Gokhale on 05 February 2018, 05:29:11 UTC
update documentation and release notes
update documentation and release notes
Tip revision: 861c8b6
BasicAliasAnalysis.cpp
//===- 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/AliasAnalysis.h"
#include "llvm/Analysis/Passes.h"
#include "llvm/Constants.h"
#include "llvm/DerivedTypes.h"
#include "llvm/Function.h"
#include "llvm/GlobalAlias.h"
#include "llvm/GlobalVariable.h"
#include "llvm/Instructions.h"
#include "llvm/IntrinsicInst.h"
#include "llvm/LLVMContext.h"
#include "llvm/Operator.h"
#include "llvm/Pass.h"
#include "llvm/Analysis/CaptureTracking.h"
#include "llvm/Analysis/MemoryBuiltins.h"
#include "llvm/Analysis/InstructionSimplify.h"
#include "llvm/Analysis/ValueTracking.h"
#include "llvm/Target/TargetData.h"
#include "llvm/Target/TargetLibraryInfo.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/Support/ErrorHandling.h"
#include "llvm/Support/GetElementPtrTypeIterator.h"
#include <algorithm>
using namespace llvm;
//===----------------------------------------------------------------------===//
// 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()) {
// Don't bother analyzing arguments already known not to escape.
if (A->hasNoCaptureAttr())
return true;
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 TargetData &TD,
bool RoundToAlign = false) {
Type *AccessTy;
unsigned Align;
if (const GlobalVariable *GV = dyn_cast<GlobalVariable>(V)) {
if (!GV->hasDefinitiveInitializer())
return AliasAnalysis::UnknownSize;
AccessTy = GV->getType()->getElementType();
Align = GV->getAlignment();
} else if (const AllocaInst *AI = dyn_cast<AllocaInst>(V)) {
if (!AI->isArrayAllocation())
AccessTy = AI->getType()->getElementType();
else
return AliasAnalysis::UnknownSize;
Align = AI->getAlignment();
} else if (const CallInst* CI = extractMallocCall(V)) {
if (!RoundToAlign && !isArrayMalloc(V, &TD))
// The size is the argument to the malloc call.
if (const ConstantInt* C = dyn_cast<ConstantInt>(CI->getArgOperand(0)))
return C->getZExtValue();
return AliasAnalysis::UnknownSize;
} else if (const Argument *A = dyn_cast<Argument>(V)) {
if (A->hasByValAttr()) {
AccessTy = cast<PointerType>(A->getType())->getElementType();
Align = A->getParamAlignment();
} else {
return AliasAnalysis::UnknownSize;
}
} else {
return AliasAnalysis::UnknownSize;
}
if (!AccessTy->isSized())
return AliasAnalysis::UnknownSize;
uint64_t Size = TD.getTypeAllocSize(AccessTy);
// If there is an explicitly specified alignment, and we need to
// take alignment into account, round up the size. (If the alignment
// is implicit, getTypeAllocSize is sufficient.)
if (RoundToAlign && Align)
Size = RoundUpToAlignment(Size, Align);
return Size;
}
/// 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 TargetData &TD) {
// 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, TD, /*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 TargetData &TD) {
uint64_t ObjectSize = getObjectSize(V, TD);
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;
};
}
/// 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 TargetData &TD, unsigned Depth) {
assert(V->getType()->isIntegerTy() && "Not an integer value");
// Limit our recursion depth.
if (Depth == 6) {
Scale = 1;
Offset = 0;
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(), &TD))
break;
// FALL THROUGH.
case Instruction::Add:
V = GetLinearExpression(BOp->getOperand(0), Scale, Offset, Extension,
TD, Depth+1);
Offset += RHSC->getValue();
return V;
case Instruction::Mul:
V = GetLinearExpression(BOp->getOperand(0), Scale, Offset, Extension,
TD, Depth+1);
Offset *= RHSC->getValue();
Scale *= RHSC->getValue();
return V;
case Instruction::Shl:
V = GetLinearExpression(BOp->getOperand(0), Scale, Offset, Extension,
TD, Depth+1);
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,
TD, Depth+1);
Scale = Scale.zext(OldWidth);
Offset = Offset.zext(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 TargetData is around, this function is capable of analyzing everything
/// that GetUnderlyingObject can look through. When not, it just looks
/// through pointer casts.
///
static const Value *
DecomposeGEPExpression(const Value *V, int64_t &BaseOffs,
SmallVectorImpl<VariableGEPIndex> &VarIndices,
const TargetData *TD) {
// Limit recursion depth to limit compile time in crazy cases.
unsigned MaxLookup = 6;
BaseOffs = 0;
do {
// See if this is a bitcast or GEP.
const Operator *Op = dyn_cast<Operator>(V);
if (Op == 0) {
// 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) {
V = Op->getOperand(0);
continue;
}
const GEPOperator *GEPOp = dyn_cast<GEPOperator>(Op);
if (GEPOp == 0) {
// 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 use it here.
if (const Value *Simplified =
SimplifyInstruction(const_cast<Instruction *>(I), TD)) {
V = Simplified;
continue;
}
return V;
}
// Don't attempt to analyze GEPs over unsized objects.
if (!cast<PointerType>(GEPOp->getOperand(0)->getType())
->getElementType()->isSized())
return V;
// If we are lacking TargetData information, we can't compute the offets of
// elements computed by GEPs. However, we can handle bitcast equivalent
// GEPs.
if (TD == 0) {
if (!GEPOp->hasAllZeroIndices())
return V;
V = GEPOp->getOperand(0);
continue;
}
// 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 += TD->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 += TD->getTypeAllocSize(*GTI)*CIdx->getSExtValue();
continue;
}
uint64_t Scale = TD->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 = cast<IntegerType>(Index->getType())->getBitWidth();
if (TD->getPointerSizeInBits() > 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,
*TD, 0);
// 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-TD->getPointerSizeInBits()) {
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.
return V;
}
/// 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.
static void 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 (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);
}
}
}
//===----------------------------------------------------------------------===//
// 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 NULL;
}
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),
// AliasCache rarely has more than 1 or 2 elements,
// so start it off fairly small so that clear()
// doesn't have to tromp through 64 (the default)
// elements on each alias query. This really wants
// something like a SmallDenseMap.
AliasCache(8) {
initializeBasicAliasAnalysisPass(*PassRegistry::getPassRegistry());
}
virtual void initializePass() {
InitializeAliasAnalysis(this);
}
virtual void getAnalysisUsage(AnalysisUsage &AU) const {
AU.addRequired<AliasAnalysis>();
AU.addRequired<TargetLibraryInfo>();
}
virtual AliasResult alias(const Location &LocA,
const Location &LocB) {
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.TBAATag,
LocB.Ptr, LocB.Size, LocB.TBAATag);
AliasCache.clear();
return Alias;
}
virtual ModRefResult getModRefInfo(ImmutableCallSite CS,
const Location &Loc);
virtual ModRefResult getModRefInfo(ImmutableCallSite CS1,
ImmutableCallSite CS2) {
// The AliasAnalysis base class has some smarts, lets use them.
return AliasAnalysis::getModRefInfo(CS1, CS2);
}
/// pointsToConstantMemory - Chase pointers until we find a (constant
/// global) or not.
virtual bool pointsToConstantMemory(const Location &Loc, bool OrLocal);
/// getModRefBehavior - Return the behavior when calling the given
/// call site.
virtual ModRefBehavior getModRefBehavior(ImmutableCallSite CS);
/// getModRefBehavior - Return the behavior when calling the given function.
/// For use when the call site is not known.
virtual ModRefBehavior getModRefBehavior(const Function *F);
/// getAdjustedAnalysisPointer - This method is used when a pass implements
/// an analysis interface through multiple inheritance. If needed, it
/// should override this to adjust the this pointer as needed for the
/// specified pass info.
virtual void *getAdjustedAnalysisPointer(const void *ID) {
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 DenseMap<LocPair, AliasResult> AliasCacheTy;
AliasCacheTy AliasCache;
// Visited - Track instructions visited by pointsToConstantMemory.
SmallPtrSet<const Value*, 16> Visited;
// aliasGEP - Provide a bunch of ad-hoc rules to disambiguate a GEP
// instruction against another.
AliasResult aliasGEP(const GEPOperator *V1, uint64_t V1Size,
const Value *V2, uint64_t V2Size,
const MDNode *V2TBAAInfo,
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 MDNode *PNTBAAInfo,
const Value *V2, uint64_t V2Size,
const MDNode *V2TBAAInfo);
/// aliasSelect - Disambiguate a Select instruction against another value.
AliasResult aliasSelect(const SelectInst *SI, uint64_t SISize,
const MDNode *SITBAAInfo,
const Value *V2, uint64_t V2Size,
const MDNode *V2TBAAInfo);
AliasResult aliasCheck(const Value *V1, uint64_t V1Size,
const MDNode *V1TBAATag,
const Value *V2, uint64_t V2Size,
const MDNode *V2TBAATag);
};
} // 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(TargetLibraryInfo)
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(), TD);
if (!Visited.insert(V)) {
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();
}
/// 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/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;
// Otherwise be conservative.
return ModRefBehavior(AliasAnalysis::getModRefBehavior(F) & Min);
}
/// 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, TD);
// 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;
}
const TargetLibraryInfo &TLI = getAnalysis<TargetLibraryInfo>();
ModRefResult Min = ModRef;
// Finally, handle specific knowledge of intrinsics.
const IntrinsicInst *II = dyn_cast<IntrinsicInst>(CS.getInstruction());
if (II != 0)
switch (II->getIntrinsicID()) {
default: break;
case Intrinsic::memcpy:
case Intrinsic::memmove: {
uint64_t Len = UnknownSize;
if (ConstantInt *LenCI = dyn_cast<ConstantInt>(II->getArgOperand(2)))
Len = LenCI->getZExtValue();
Value *Dest = II->getArgOperand(0);
Value *Src = II->getArgOperand(1);
// If it can't overlap the source dest, then it doesn't modref the loc.
if (isNoAlias(Location(Dest, Len), Loc)) {
if (isNoAlias(Location(Src, Len), Loc))
return NoModRef;
// If it can't overlap the dest, then worst case it reads the loc.
Min = Ref;
} else if (isNoAlias(Location(Src, Len), Loc)) {
// If it can't overlap the source, then worst case it mutates the loc.
Min = Mod;
}
break;
}
case Intrinsic::memset:
// Since memset is 'accesses arguments' only, the AliasAnalysis base class
// will handle it for the variable length case.
if (ConstantInt *LenCI = dyn_cast<ConstantInt>(II->getArgOperand(2))) {
uint64_t Len = LenCI->getZExtValue();
Value *Dest = II->getArgOperand(0);
if (isNoAlias(Location(Dest, Len), Loc))
return NoModRef;
}
// We know that memset doesn't load anything.
Min = Mod;
break;
case Intrinsic::lifetime_start:
case Intrinsic::lifetime_end:
case Intrinsic::invariant_start: {
uint64_t PtrSize =
cast<ConstantInt>(II->getArgOperand(0))->getZExtValue();
if (isNoAlias(Location(II->getArgOperand(1),
PtrSize,
II->getMetadata(LLVMContext::MD_tbaa)),
Loc))
return NoModRef;
break;
}
case Intrinsic::invariant_end: {
uint64_t PtrSize =
cast<ConstantInt>(II->getArgOperand(1))->getZExtValue();
if (isNoAlias(Location(II->getArgOperand(2),
PtrSize,
II->getMetadata(LLVMContext::MD_tbaa)),
Loc))
return NoModRef;
break;
}
case Intrinsic::arm_neon_vld1: {
// LLVM's vld1 and vst1 intrinsics currently only support a single
// vector register.
uint64_t Size =
TD ? TD->getTypeStoreSize(II->getType()) : UnknownSize;
if (isNoAlias(Location(II->getArgOperand(0), Size,
II->getMetadata(LLVMContext::MD_tbaa)),
Loc))
return NoModRef;
break;
}
case Intrinsic::arm_neon_vst1: {
uint64_t Size =
TD ? TD->getTypeStoreSize(II->getArgOperand(1)->getType()) : UnknownSize;
if (isNoAlias(Location(II->getArgOperand(0), Size,
II->getMetadata(LLVMContext::MD_tbaa)),
Loc))
return NoModRef;
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 (TLI.has(LibFunc::memset_pattern16) &&
CS.getCalledFunction() &&
CS.getCalledFunction()->getName() == "memset_pattern16") {
const Function *MS = CS.getCalledFunction();
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))) {
uint64_t Len = UnknownSize;
if (const ConstantInt *LenCI = dyn_cast<ConstantInt>(CS.getArgument(2)))
Len = LenCI->getZExtValue();
const Value *Dest = CS.getArgument(0);
const Value *Src = CS.getArgument(1);
// If it can't overlap the source dest, then it doesn't modref the loc.
if (isNoAlias(Location(Dest, Len), Loc)) {
// Always reads 16 bytes of the source.
if (isNoAlias(Location(Src, 16), Loc))
return NoModRef;
// If it can't overlap the dest, then worst case it reads the loc.
Min = Ref;
// Always reads 16 bytes of the source.
} else if (isNoAlias(Location(Src, 16), Loc)) {
// If it can't overlap the source, then worst case it mutates the loc.
Min = Mod;
}
}
}
// The AliasAnalysis base class has some smarts, lets use them.
return ModRefResult(AliasAnalysis::getModRefInfo(CS, Loc) & Min);
}
/// 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, TD),
/// UnderlyingV2 is the same for V2.
///
AliasAnalysis::AliasResult
BasicAliasAnalysis::aliasGEP(const GEPOperator *GEP1, uint64_t V1Size,
const Value *V2, uint64_t V2Size,
const MDNode *V2TBAAInfo,
const Value *UnderlyingV1,
const Value *UnderlyingV2) {
int64_t GEP1BaseOffset;
SmallVector<VariableGEPIndex, 4> GEP1VariableIndices;
// If we have two gep instructions with must-alias'ing base pointers, figure
// out if the indexes to the GEP tell us anything about the derived pointer.
if (const GEPOperator *GEP2 = dyn_cast<GEPOperator>(V2)) {
// Do the base pointers alias?
AliasResult BaseAlias = aliasCheck(UnderlyingV1, UnknownSize, 0,
UnderlyingV2, UnknownSize, 0);
// If we get a No or May, then return it immediately, no amount of analysis
// will improve this situation.
if (BaseAlias != MustAlias) return BaseAlias;
// Otherwise, we have a MustAlias. Since the base pointers alias each other
// exactly, see if the computed offset from the common pointer tells us
// about the relation of the resulting pointer.
const Value *GEP1BasePtr =
DecomposeGEPExpression(GEP1, GEP1BaseOffset, GEP1VariableIndices, TD);
int64_t GEP2BaseOffset;
SmallVector<VariableGEPIndex, 4> GEP2VariableIndices;
const Value *GEP2BasePtr =
DecomposeGEPExpression(GEP2, GEP2BaseOffset, GEP2VariableIndices, TD);
// If DecomposeGEPExpression isn't able to look all the way through the
// addressing operation, we must not have TD and this is too complex for us
// to handle without it.
if (GEP1BasePtr != UnderlyingV1 || GEP2BasePtr != UnderlyingV2) {
assert(TD == 0 &&
"DecomposeGEPExpression and GetUnderlyingObject disagree!");
return MayAlias;
}
// Subtract the GEP2 pointer from the GEP1 pointer to find out their
// symbolic difference.
GEP1BaseOffset -= GEP2BaseOffset;
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, 0,
V2, V2Size, V2TBAAInfo);
if (R != MustAlias)
// If V2 may alias GEP base pointer, conservatively returns MayAlias.
// If V2 is known not to alias GEP base pointer, then the two values
// cannot alias per GEP semantics: "A pointer value formed from a
// getelementptr instruction is associated with the addresses associated
// with the first operand of the getelementptr".
return R;
const Value *GEP1BasePtr =
DecomposeGEPExpression(GEP1, GEP1BaseOffset, GEP1VariableIndices, TD);
// If DecomposeGEPExpression isn't able to look all the way through the
// addressing operation, we must not have TD and this is too complex for us
// to handle without it.
if (GEP1BasePtr != UnderlyingV1) {
assert(TD == 0 &&
"DecomposeGEPExpression and GetUnderlyingObject disagree!");
return MayAlias;
}
}
// In the two GEP Case, if there is no difference in the offsets of the
// computed pointers, the resultant pointers are a must alias. This
// hapens when we have two lexically identical GEP's (for example).
//
// In the other case, if we have getelementptr <ptr>, 0, 0, 0, 0, ... and V2
// must aliases the GEP, the end result is a must alias also.
if (GEP1BaseOffset == 0 && GEP1VariableIndices.empty())
return MustAlias;
// If 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 {
if (V1Size != UnknownSize) {
if (-(uint64_t)GEP1BaseOffset < V1Size)
return PartialAlias;
return NoAlias;
}
}
}
// Try to distinguish something like &A[i][1] against &A[42][0].
// Grab the least significant bit set in any of the scales.
if (!GEP1VariableIndices.empty()) {
uint64_t Modulo = 0;
for (unsigned i = 0, e = GEP1VariableIndices.size(); i != e; ++i)
Modulo |= (uint64_t)GEP1VariableIndices[i].Scale;
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;
}
// 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 MDNode *SITBAAInfo,
const Value *V2, uint64_t V2Size,
const MDNode *V2TBAAInfo) {
// 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, SITBAAInfo,
SI2->getTrueValue(), V2Size, V2TBAAInfo);
if (Alias == MayAlias)
return MayAlias;
AliasResult ThisAlias =
aliasCheck(SI->getFalseValue(), SISize, SITBAAInfo,
SI2->getFalseValue(), V2Size, V2TBAAInfo);
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, V2TBAAInfo, SI->getTrueValue(), SISize, SITBAAInfo);
if (Alias == MayAlias)
return MayAlias;
AliasResult ThisAlias =
aliasCheck(V2, V2Size, V2TBAAInfo, SI->getFalseValue(), SISize, SITBAAInfo);
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 MDNode *PNTBAAInfo,
const Value *V2, uint64_t V2Size,
const MDNode *V2TBAAInfo) {
// If the values are PHIs in the same block, we can do a more precise
// as well as efficient check: just check for aliases between the values
// on corresponding edges.
if (const PHINode *PN2 = dyn_cast<PHINode>(V2))
if (PN2->getParent() == PN->getParent()) {
AliasResult Alias =
aliasCheck(PN->getIncomingValue(0), PNSize, PNTBAAInfo,
PN2->getIncomingValueForBlock(PN->getIncomingBlock(0)),
V2Size, V2TBAAInfo);
if (Alias == MayAlias)
return MayAlias;
for (unsigned i = 1, e = PN->getNumIncomingValues(); i != e; ++i) {
AliasResult ThisAlias =
aliasCheck(PN->getIncomingValue(i), PNSize, PNTBAAInfo,
PN2->getIncomingValueForBlock(PN->getIncomingBlock(i)),
V2Size, V2TBAAInfo);
Alias = MergeAliasResults(ThisAlias, Alias);
if (Alias == MayAlias)
break;
}
return Alias;
}
SmallPtrSet<Value*, 4> UniqueSrc;
SmallVector<Value*, 4> V1Srcs;
for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
Value *PV1 = PN->getIncomingValue(i);
if (isa<PHINode>(PV1))
// If any of the source itself is a PHI, return MayAlias conservatively
// to avoid compile time explosion. The worst possible case is if both
// sides are PHI nodes. In which case, this is O(m x n) time where 'm'
// and 'n' are the number of PHI sources.
return MayAlias;
if (UniqueSrc.insert(PV1))
V1Srcs.push_back(PV1);
}
AliasResult Alias = aliasCheck(V2, V2Size, V2TBAAInfo,
V1Srcs[0], PNSize, PNTBAAInfo);
// 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, V2TBAAInfo,
V, PNSize, PNTBAAInfo);
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,
const MDNode *V1TBAAInfo,
const Value *V2, uint64_t V2Size,
const MDNode *V2TBAAInfo) {
// If either of the memory references is empty, it doesn't matter what the
// pointer values are.
if (V1Size == 0 || V2Size == 0)
return NoAlias;
// Strip off any casts if they exist.
V1 = V1->stripPointerCasts();
V2 = V2->stripPointerCasts();
// Are we checking for alias of the same value?
if (V1 == V2) return MustAlias;
if (!V1->getType()->isPointerTy() || !V2->getType()->isPointerTy())
return NoAlias; // Scalars cannot alias each other
// Figure out what objects these things are pointing to if we can.
const Value *O1 = GetUnderlyingObject(V1, TD);
const Value *O2 = GetUnderlyingObject(V2, TD);
// Null values in the default address space don't point to any object, so they
// don't alias any other pointer.
if (const ConstantPointerNull *CPN = dyn_cast<ConstantPointerNull>(O1))
if (CPN->getType()->getAddressSpace() == 0)
return NoAlias;
if (const ConstantPointerNull *CPN = dyn_cast<ConstantPointerNull>(O2))
if (CPN->getType()->getAddressSpace() == 0)
return NoAlias;
if (O1 != O2) {
// If V1/V2 point to two different objects we know that we have no alias.
if (isIdentifiedObject(O1) && isIdentifiedObject(O2))
return NoAlias;
// Constant pointers can't alias with non-const isIdentifiedObject objects.
if ((isa<Constant>(O1) && isIdentifiedObject(O2) && !isa<Constant>(O2)) ||
(isa<Constant>(O2) && isIdentifiedObject(O1) && !isa<Constant>(O1)))
return NoAlias;
// Arguments can't alias with local allocations or noalias calls
// in the same function.
if (((isa<Argument>(O1) && (isa<AllocaInst>(O2) || isNoAliasCall(O2))) ||
(isa<Argument>(O2) && (isa<AllocaInst>(O1) || isNoAliasCall(O1)))))
return NoAlias;
// Most objects can't alias null.
if ((isa<ConstantPointerNull>(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 (TD)
if ((V1Size != UnknownSize && isObjectSmallerThan(O2, V1Size, *TD)) ||
(V2Size != UnknownSize && isObjectSmallerThan(O1, V2Size, *TD)))
return NoAlias;
// Check the cache before climbing up use-def chains. This also terminates
// otherwise infinitely recursive queries.
LocPair Locs(Location(V1, V1Size, V1TBAAInfo),
Location(V2, V2Size, V2TBAAInfo));
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);
}
if (const GEPOperator *GV1 = dyn_cast<GEPOperator>(V1)) {
AliasResult Result = aliasGEP(GV1, V1Size, V2, V2Size, V2TBAAInfo, O1, O2);
if (Result != MayAlias) return AliasCache[Locs] = Result;
}
if (isa<PHINode>(V2) && !isa<PHINode>(V1)) {
std::swap(V1, V2);
std::swap(V1Size, V2Size);
}
if (const PHINode *PN = dyn_cast<PHINode>(V1)) {
AliasResult Result = aliasPHI(PN, V1Size, V1TBAAInfo,
V2, V2Size, V2TBAAInfo);
if (Result != MayAlias) return AliasCache[Locs] = Result;
}
if (isa<SelectInst>(V2) && !isa<SelectInst>(V1)) {
std::swap(V1, V2);
std::swap(V1Size, V2Size);
}
if (const SelectInst *S1 = dyn_cast<SelectInst>(V1)) {
AliasResult Result = aliasSelect(S1, V1Size, V1TBAAInfo,
V2, V2Size, V2TBAAInfo);
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 (TD && O1 == O2)
if ((V1Size != UnknownSize && isObjectSize(O1, V1Size, *TD)) ||
(V2Size != UnknownSize && isObjectSize(O2, V2Size, *TD)))
return AliasCache[Locs] = PartialAlias;
AliasResult Result =
AliasAnalysis::alias(Location(V1, V1Size, V1TBAAInfo),
Location(V2, V2Size, V2TBAAInfo));
return AliasCache[Locs] = Result;
}
