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//===- ProvenanceAnalysis.cpp - ObjC ARC Optimization ---------------------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
//
/// \file
///
/// This file defines a special form of Alias Analysis called ``Provenance
/// Analysis''. The word ``provenance'' refers to the history of the ownership
/// of an object. Thus ``Provenance Analysis'' is an analysis which attempts to
/// use various techniques to determine if locally
///
/// WARNING: This file knows about certain library functions. It recognizes them
/// by name, and hardwires knowledge of their semantics.
///
/// WARNING: This file knows about how certain Objective-C library functions are
/// used. Naive LLVM IR transformations which would otherwise be
/// behavior-preserving may break these assumptions.
//
//===----------------------------------------------------------------------===//
#include "ProvenanceAnalysis.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/Analysis/AliasAnalysis.h"
#include "llvm/Analysis/ObjCARCAnalysisUtils.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/Module.h"
#include "llvm/IR/Use.h"
#include "llvm/IR/User.h"
#include "llvm/IR/Value.h"
#include "llvm/Support/Casting.h"
#include <utility>
using namespace llvm;
using namespace llvm::objcarc;
bool ProvenanceAnalysis::relatedSelect(const SelectInst *A,
const Value *B) {
// If the values are Selects with the same condition, we can do a more precise
// check: just check for relations between the values on corresponding arms.
if (const SelectInst *SB = dyn_cast<SelectInst>(B)) {
if (A->getCondition() == SB->getCondition())
return related(A->getTrueValue(), SB->getTrueValue()) ||
related(A->getFalseValue(), SB->getFalseValue());
// Check both arms of B individually. Return false if neither arm is related
// to A.
if (!(related(SB->getTrueValue(), A) || related(SB->getFalseValue(), A)))
return false;
}
// Check both arms of the Select node individually.
return related(A->getTrueValue(), B) || related(A->getFalseValue(), B);
}
bool ProvenanceAnalysis::relatedPHI(const PHINode *A,
const Value *B) {
auto comparePHISources = [this](const PHINode *PNA, const Value *B) -> bool {
// Check each unique source of the PHI node against B.
SmallPtrSet<const Value *, 4> UniqueSrc;
for (Value *PV1 : PNA->incoming_values()) {
if (UniqueSrc.insert(PV1).second && related(PV1, B))
return true;
}
// All of the arms checked out.
return false;
};
if (const PHINode *PNB = dyn_cast<PHINode>(B)) {
// If the values are PHIs in the same block, we can do a more precise as
// well as efficient check: just check for relations between the values on
// corresponding edges.
if (PNB->getParent() == A->getParent()) {
for (unsigned i = 0, e = A->getNumIncomingValues(); i != e; ++i)
if (related(A->getIncomingValue(i),
PNB->getIncomingValueForBlock(A->getIncomingBlock(i))))
return true;
return false;
}
if (!comparePHISources(PNB, A))
return false;
}
return comparePHISources(A, B);
}
/// Test if the value of P, or any value covered by its provenance, is ever
/// stored within the function (not counting callees).
static bool IsStoredObjCPointer(const Value *P) {
SmallPtrSet<const Value *, 8> Visited;
SmallVector<const Value *, 8> Worklist;
Worklist.push_back(P);
Visited.insert(P);
do {
P = Worklist.pop_back_val();
for (const Use &U : P->uses()) {
const User *Ur = U.getUser();
if (isa<StoreInst>(Ur)) {
if (U.getOperandNo() == 0)
// The pointer is stored.
return true;
// The pointed is stored through.
continue;
}
if (isa<CallInst>(Ur))
// The pointer is passed as an argument, ignore this.
continue;
if (isa<PtrToIntInst>(P))
// Assume the worst.
return true;
if (Visited.insert(Ur).second)
Worklist.push_back(Ur);
}
} while (!Worklist.empty());
// Everything checked out.
return false;
}
bool ProvenanceAnalysis::relatedCheck(const Value *A, const Value *B) {
// Ask regular AliasAnalysis, for a first approximation.
switch (AA->alias(A, B)) {
case AliasResult::NoAlias:
return false;
case AliasResult::MustAlias:
case AliasResult::PartialAlias:
return true;
case AliasResult::MayAlias:
break;
}
bool AIsIdentified = IsObjCIdentifiedObject(A);
bool BIsIdentified = IsObjCIdentifiedObject(B);
// An ObjC-Identified object can't alias a load if it is never locally stored.
// Check for an obvious escape.
if ((AIsIdentified && isa<LoadInst>(B) && !IsStoredObjCPointer(A)) ||
(BIsIdentified && isa<LoadInst>(A) && !IsStoredObjCPointer(B)))
return false;
if ((AIsIdentified && isa<LoadInst>(B)) ||
(BIsIdentified && isa<LoadInst>(A)))
return true;
// Both pointers are identified and escapes aren't an evident problem.
if (AIsIdentified && BIsIdentified && !isa<LoadInst>(A) && !isa<LoadInst>(B))
return false;
// Special handling for PHI and Select.
if (const PHINode *PN = dyn_cast<PHINode>(A))
return relatedPHI(PN, B);
if (const PHINode *PN = dyn_cast<PHINode>(B))
return relatedPHI(PN, A);
if (const SelectInst *S = dyn_cast<SelectInst>(A))
return relatedSelect(S, B);
if (const SelectInst *S = dyn_cast<SelectInst>(B))
return relatedSelect(S, A);
// Conservative.
return true;
}
bool ProvenanceAnalysis::related(const Value *A, const Value *B) {
A = GetUnderlyingObjCPtrCached(A, UnderlyingObjCPtrCache);
B = GetUnderlyingObjCPtrCached(B, UnderlyingObjCPtrCache);
// Quick check.
if (A == B)
return true;
// Begin by inserting a conservative value into the map. If the insertion
// fails, we have the answer already. If it succeeds, leave it there until we
// compute the real answer to guard against recursive queries.
if (A > B) std::swap(A, B);
std::pair<CachedResultsTy::iterator, bool> Pair =
CachedResults.insert(std::make_pair(ValuePairTy(A, B), true));
if (!Pair.second)
return Pair.first->second;
bool Result = relatedCheck(A, B);
assert(relatedCheck(B, A) == Result &&
"relatedCheck result depending on order of parameters!");
CachedResults[ValuePairTy(A, B)] = Result;
return Result;
}
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