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      1 //===- CFLAliasAnalysis.cpp - CFL-Based Alias Analysis Implementation ------==//
      2 //
      3 //                     The LLVM Compiler Infrastructure
      4 //
      5 // This file is distributed under the University of Illinois Open Source
      6 // License. See LICENSE.TXT for details.
      7 //
      8 //===----------------------------------------------------------------------===//
      9 //
     10 // This file implements a CFL-based context-insensitive alias analysis
     11 // algorithm. It does not depend on types. The algorithm is a mixture of the one
     12 // described in "Demand-driven alias analysis for C" by Xin Zheng and Radu
     13 // Rugina, and "Fast algorithms for Dyck-CFL-reachability with applications to
     14 // Alias Analysis" by Zhang Q, Lyu M R, Yuan H, and Su Z. -- to summarize the
     15 // papers, we build a graph of the uses of a variable, where each node is a
     16 // memory location, and each edge is an action that happened on that memory
     17 // location.  The "actions" can be one of Dereference, Reference, Assign, or
     18 // Assign.
     19 //
     20 // Two variables are considered as aliasing iff you can reach one value's node
     21 // from the other value's node and the language formed by concatenating all of
     22 // the edge labels (actions) conforms to a context-free grammar.
     23 //
     24 // Because this algorithm requires a graph search on each query, we execute the
     25 // algorithm outlined in "Fast algorithms..." (mentioned above)
     26 // in order to transform the graph into sets of variables that may alias in
     27 // ~nlogn time (n = number of variables.), which makes queries take constant
     28 // time.
     29 //===----------------------------------------------------------------------===//
     30 
     31 #include "StratifiedSets.h"
     32 #include "llvm/ADT/BitVector.h"
     33 #include "llvm/ADT/DenseMap.h"
     34 #include "llvm/ADT/None.h"
     35 #include "llvm/ADT/Optional.h"
     36 #include "llvm/Analysis/AliasAnalysis.h"
     37 #include "llvm/Analysis/Passes.h"
     38 #include "llvm/IR/Constants.h"
     39 #include "llvm/IR/Function.h"
     40 #include "llvm/IR/InstVisitor.h"
     41 #include "llvm/IR/Instructions.h"
     42 #include "llvm/IR/ValueHandle.h"
     43 #include "llvm/Pass.h"
     44 #include "llvm/Support/Allocator.h"
     45 #include "llvm/Support/Compiler.h"
     46 #include "llvm/Support/Debug.h"
     47 #include "llvm/Support/ErrorHandling.h"
     48 #include "llvm/Support/raw_ostream.h"
     49 #include <algorithm>
     50 #include <cassert>
     51 #include <forward_list>
     52 #include <memory>
     53 #include <tuple>
     54 
     55 using namespace llvm;
     56 
     57 #define DEBUG_TYPE "cfl-aa"
     58 
     59 // Try to go from a Value* to a Function*. Never returns nullptr.
     60 static Optional<Function *> parentFunctionOfValue(Value *);
     61 
     62 // Returns possible functions called by the Inst* into the given
     63 // SmallVectorImpl. Returns true if targets found, false otherwise.
     64 // This is templated because InvokeInst/CallInst give us the same
     65 // set of functions that we care about, and I don't like repeating
     66 // myself.
     67 template <typename Inst>
     68 static bool getPossibleTargets(Inst *, SmallVectorImpl<Function *> &);
     69 
     70 // Some instructions need to have their users tracked. Instructions like
     71 // `add` require you to get the users of the Instruction* itself, other
     72 // instructions like `store` require you to get the users of the first
     73 // operand. This function gets the "proper" value to track for each
     74 // type of instruction we support.
     75 static Optional<Value *> getTargetValue(Instruction *);
     76 
     77 // There are certain instructions (i.e. FenceInst, etc.) that we ignore.
     78 // This notes that we should ignore those.
     79 static bool hasUsefulEdges(Instruction *);
     80 
     81 const StratifiedIndex StratifiedLink::SetSentinel =
     82     std::numeric_limits<StratifiedIndex>::max();
     83 
     84 namespace {
     85 // StratifiedInfo Attribute things.
     86 typedef unsigned StratifiedAttr;
     87 LLVM_CONSTEXPR unsigned MaxStratifiedAttrIndex = NumStratifiedAttrs;
     88 LLVM_CONSTEXPR unsigned AttrAllIndex = 0;
     89 LLVM_CONSTEXPR unsigned AttrGlobalIndex = 1;
     90 LLVM_CONSTEXPR unsigned AttrUnknownIndex = 2;
     91 LLVM_CONSTEXPR unsigned AttrFirstArgIndex = 3;
     92 LLVM_CONSTEXPR unsigned AttrLastArgIndex = MaxStratifiedAttrIndex;
     93 LLVM_CONSTEXPR unsigned AttrMaxNumArgs = AttrLastArgIndex - AttrFirstArgIndex;
     94 
     95 LLVM_CONSTEXPR StratifiedAttr AttrNone = 0;
     96 LLVM_CONSTEXPR StratifiedAttr AttrUnknown = 1 << AttrUnknownIndex;
     97 LLVM_CONSTEXPR StratifiedAttr AttrAll = ~AttrNone;
     98 
     99 // \brief StratifiedSets call for knowledge of "direction", so this is how we
    100 // represent that locally.
    101 enum class Level { Same, Above, Below };
    102 
    103 // \brief Edges can be one of four "weights" -- each weight must have an inverse
    104 // weight (Assign has Assign; Reference has Dereference).
    105 enum class EdgeType {
    106   // The weight assigned when assigning from or to a value. For example, in:
    107   // %b = getelementptr %a, 0
    108   // ...The relationships are %b assign %a, and %a assign %b. This used to be
    109   // two edges, but having a distinction bought us nothing.
    110   Assign,
    111 
    112   // The edge used when we have an edge going from some handle to a Value.
    113   // Examples of this include:
    114   // %b = load %a              (%b Dereference %a)
    115   // %b = extractelement %a, 0 (%a Dereference %b)
    116   Dereference,
    117 
    118   // The edge used when our edge goes from a value to a handle that may have
    119   // contained it at some point. Examples:
    120   // %b = load %a              (%a Reference %b)
    121   // %b = extractelement %a, 0 (%b Reference %a)
    122   Reference
    123 };
    124 
    125 // \brief Encodes the notion of a "use"
    126 struct Edge {
    127   // \brief Which value the edge is coming from
    128   Value *From;
    129 
    130   // \brief Which value the edge is pointing to
    131   Value *To;
    132 
    133   // \brief Edge weight
    134   EdgeType Weight;
    135 
    136   // \brief Whether we aliased any external values along the way that may be
    137   // invisible to the analysis (i.e. landingpad for exceptions, calls for
    138   // interprocedural analysis, etc.)
    139   StratifiedAttrs AdditionalAttrs;
    140 
    141   Edge(Value *From, Value *To, EdgeType W, StratifiedAttrs A)
    142       : From(From), To(To), Weight(W), AdditionalAttrs(A) {}
    143 };
    144 
    145 // \brief Information we have about a function and would like to keep around
    146 struct FunctionInfo {
    147   StratifiedSets<Value *> Sets;
    148   // Lots of functions have < 4 returns. Adjust as necessary.
    149   SmallVector<Value *, 4> ReturnedValues;
    150 
    151   FunctionInfo(StratifiedSets<Value *> &&S, SmallVector<Value *, 4> &&RV)
    152       : Sets(std::move(S)), ReturnedValues(std::move(RV)) {}
    153 };
    154 
    155 struct CFLAliasAnalysis;
    156 
    157 struct FunctionHandle : public CallbackVH {
    158   FunctionHandle(Function *Fn, CFLAliasAnalysis *CFLAA)
    159       : CallbackVH(Fn), CFLAA(CFLAA) {
    160     assert(Fn != nullptr);
    161     assert(CFLAA != nullptr);
    162   }
    163 
    164   ~FunctionHandle() override {}
    165 
    166   void deleted() override { removeSelfFromCache(); }
    167   void allUsesReplacedWith(Value *) override { removeSelfFromCache(); }
    168 
    169 private:
    170   CFLAliasAnalysis *CFLAA;
    171 
    172   void removeSelfFromCache();
    173 };
    174 
    175 struct CFLAliasAnalysis : public ImmutablePass, public AliasAnalysis {
    176 private:
    177   /// \brief Cached mapping of Functions to their StratifiedSets.
    178   /// If a function's sets are currently being built, it is marked
    179   /// in the cache as an Optional without a value. This way, if we
    180   /// have any kind of recursion, it is discernable from a function
    181   /// that simply has empty sets.
    182   DenseMap<Function *, Optional<FunctionInfo>> Cache;
    183   std::forward_list<FunctionHandle> Handles;
    184 
    185 public:
    186   static char ID;
    187 
    188   CFLAliasAnalysis() : ImmutablePass(ID) {
    189     initializeCFLAliasAnalysisPass(*PassRegistry::getPassRegistry());
    190   }
    191 
    192   ~CFLAliasAnalysis() override {}
    193 
    194   void getAnalysisUsage(AnalysisUsage &AU) const override {
    195     AliasAnalysis::getAnalysisUsage(AU);
    196   }
    197 
    198   void *getAdjustedAnalysisPointer(const void *ID) override {
    199     if (ID == &AliasAnalysis::ID)
    200       return (AliasAnalysis *)this;
    201     return this;
    202   }
    203 
    204   /// \brief Inserts the given Function into the cache.
    205   void scan(Function *Fn);
    206 
    207   void evict(Function *Fn) { Cache.erase(Fn); }
    208 
    209   /// \brief Ensures that the given function is available in the cache.
    210   /// Returns the appropriate entry from the cache.
    211   const Optional<FunctionInfo> &ensureCached(Function *Fn) {
    212     auto Iter = Cache.find(Fn);
    213     if (Iter == Cache.end()) {
    214       scan(Fn);
    215       Iter = Cache.find(Fn);
    216       assert(Iter != Cache.end());
    217       assert(Iter->second.hasValue());
    218     }
    219     return Iter->second;
    220   }
    221 
    222   AliasResult query(const Location &LocA, const Location &LocB);
    223 
    224   AliasResult alias(const Location &LocA, const Location &LocB) override {
    225     if (LocA.Ptr == LocB.Ptr) {
    226       if (LocA.Size == LocB.Size) {
    227         return MustAlias;
    228       } else {
    229         return PartialAlias;
    230       }
    231     }
    232 
    233     // Comparisons between global variables and other constants should be
    234     // handled by BasicAA.
    235     // TODO: ConstantExpr handling -- CFLAA may report NoAlias when comparing
    236     // a GlobalValue and ConstantExpr, but every query needs to have at least
    237     // one Value tied to a Function, and neither GlobalValues nor ConstantExprs
    238     // are.
    239     if (isa<Constant>(LocA.Ptr) && isa<Constant>(LocB.Ptr)) {
    240       return AliasAnalysis::alias(LocA, LocB);
    241     }
    242 
    243     AliasResult QueryResult = query(LocA, LocB);
    244     if (QueryResult == MayAlias)
    245       return AliasAnalysis::alias(LocA, LocB);
    246 
    247     return QueryResult;
    248   }
    249 
    250   bool doInitialization(Module &M) override;
    251 };
    252 
    253 void FunctionHandle::removeSelfFromCache() {
    254   assert(CFLAA != nullptr);
    255   auto *Val = getValPtr();
    256   CFLAA->evict(cast<Function>(Val));
    257   setValPtr(nullptr);
    258 }
    259 
    260 // \brief Gets the edges our graph should have, based on an Instruction*
    261 class GetEdgesVisitor : public InstVisitor<GetEdgesVisitor, void> {
    262   CFLAliasAnalysis &AA;
    263   SmallVectorImpl<Edge> &Output;
    264 
    265 public:
    266   GetEdgesVisitor(CFLAliasAnalysis &AA, SmallVectorImpl<Edge> &Output)
    267       : AA(AA), Output(Output) {}
    268 
    269   void visitInstruction(Instruction &) {
    270     llvm_unreachable("Unsupported instruction encountered");
    271   }
    272 
    273   void visitPtrToIntInst(PtrToIntInst &Inst) {
    274     auto *Ptr = Inst.getOperand(0);
    275     Output.push_back(Edge(Ptr, Ptr, EdgeType::Assign, AttrUnknown));
    276   }
    277 
    278   void visitIntToPtrInst(IntToPtrInst &Inst) {
    279     auto *Ptr = &Inst;
    280     Output.push_back(Edge(Ptr, Ptr, EdgeType::Assign, AttrUnknown));
    281   }
    282 
    283   void visitCastInst(CastInst &Inst) {
    284     Output.push_back(
    285         Edge(&Inst, Inst.getOperand(0), EdgeType::Assign, AttrNone));
    286   }
    287 
    288   void visitBinaryOperator(BinaryOperator &Inst) {
    289     auto *Op1 = Inst.getOperand(0);
    290     auto *Op2 = Inst.getOperand(1);
    291     Output.push_back(Edge(&Inst, Op1, EdgeType::Assign, AttrNone));
    292     Output.push_back(Edge(&Inst, Op2, EdgeType::Assign, AttrNone));
    293   }
    294 
    295   void visitAtomicCmpXchgInst(AtomicCmpXchgInst &Inst) {
    296     auto *Ptr = Inst.getPointerOperand();
    297     auto *Val = Inst.getNewValOperand();
    298     Output.push_back(Edge(Ptr, Val, EdgeType::Dereference, AttrNone));
    299   }
    300 
    301   void visitAtomicRMWInst(AtomicRMWInst &Inst) {
    302     auto *Ptr = Inst.getPointerOperand();
    303     auto *Val = Inst.getValOperand();
    304     Output.push_back(Edge(Ptr, Val, EdgeType::Dereference, AttrNone));
    305   }
    306 
    307   void visitPHINode(PHINode &Inst) {
    308     for (unsigned I = 0, E = Inst.getNumIncomingValues(); I != E; ++I) {
    309       Value *Val = Inst.getIncomingValue(I);
    310       Output.push_back(Edge(&Inst, Val, EdgeType::Assign, AttrNone));
    311     }
    312   }
    313 
    314   void visitGetElementPtrInst(GetElementPtrInst &Inst) {
    315     auto *Op = Inst.getPointerOperand();
    316     Output.push_back(Edge(&Inst, Op, EdgeType::Assign, AttrNone));
    317     for (auto I = Inst.idx_begin(), E = Inst.idx_end(); I != E; ++I)
    318       Output.push_back(Edge(&Inst, *I, EdgeType::Assign, AttrNone));
    319   }
    320 
    321   void visitSelectInst(SelectInst &Inst) {
    322     // Condition is not processed here (The actual statement producing
    323     // the condition result is processed elsewhere). For select, the
    324     // condition is evaluated, but not loaded, stored, or assigned
    325     // simply as a result of being the condition of a select.
    326 
    327     auto *TrueVal = Inst.getTrueValue();
    328     Output.push_back(Edge(&Inst, TrueVal, EdgeType::Assign, AttrNone));
    329     auto *FalseVal = Inst.getFalseValue();
    330     Output.push_back(Edge(&Inst, FalseVal, EdgeType::Assign, AttrNone));
    331   }
    332 
    333   void visitAllocaInst(AllocaInst &) {}
    334 
    335   void visitLoadInst(LoadInst &Inst) {
    336     auto *Ptr = Inst.getPointerOperand();
    337     auto *Val = &Inst;
    338     Output.push_back(Edge(Val, Ptr, EdgeType::Reference, AttrNone));
    339   }
    340 
    341   void visitStoreInst(StoreInst &Inst) {
    342     auto *Ptr = Inst.getPointerOperand();
    343     auto *Val = Inst.getValueOperand();
    344     Output.push_back(Edge(Ptr, Val, EdgeType::Dereference, AttrNone));
    345   }
    346 
    347   void visitVAArgInst(VAArgInst &Inst) {
    348     // We can't fully model va_arg here. For *Ptr = Inst.getOperand(0), it does
    349     // two things:
    350     //  1. Loads a value from *((T*)*Ptr).
    351     //  2. Increments (stores to) *Ptr by some target-specific amount.
    352     // For now, we'll handle this like a landingpad instruction (by placing the
    353     // result in its own group, and having that group alias externals).
    354     auto *Val = &Inst;
    355     Output.push_back(Edge(Val, Val, EdgeType::Assign, AttrAll));
    356   }
    357 
    358   static bool isFunctionExternal(Function *Fn) {
    359     return Fn->isDeclaration() || !Fn->hasLocalLinkage();
    360   }
    361 
    362   // Gets whether the sets at Index1 above, below, or equal to the sets at
    363   // Index2. Returns None if they are not in the same set chain.
    364   static Optional<Level> getIndexRelation(const StratifiedSets<Value *> &Sets,
    365                                           StratifiedIndex Index1,
    366                                           StratifiedIndex Index2) {
    367     if (Index1 == Index2)
    368       return Level::Same;
    369 
    370     const auto *Current = &Sets.getLink(Index1);
    371     while (Current->hasBelow()) {
    372       if (Current->Below == Index2)
    373         return Level::Below;
    374       Current = &Sets.getLink(Current->Below);
    375     }
    376 
    377     Current = &Sets.getLink(Index1);
    378     while (Current->hasAbove()) {
    379       if (Current->Above == Index2)
    380         return Level::Above;
    381       Current = &Sets.getLink(Current->Above);
    382     }
    383 
    384     return NoneType();
    385   }
    386 
    387   bool
    388   tryInterproceduralAnalysis(const SmallVectorImpl<Function *> &Fns,
    389                              Value *FuncValue,
    390                              const iterator_range<User::op_iterator> &Args) {
    391     const unsigned ExpectedMaxArgs = 8;
    392     const unsigned MaxSupportedArgs = 50;
    393     assert(Fns.size() > 0);
    394 
    395     // I put this here to give us an upper bound on time taken by IPA. Is it
    396     // really (realistically) needed? Keep in mind that we do have an n^2 algo.
    397     if (std::distance(Args.begin(), Args.end()) > (int)MaxSupportedArgs)
    398       return false;
    399 
    400     // Exit early if we'll fail anyway
    401     for (auto *Fn : Fns) {
    402       if (isFunctionExternal(Fn) || Fn->isVarArg())
    403         return false;
    404       auto &MaybeInfo = AA.ensureCached(Fn);
    405       if (!MaybeInfo.hasValue())
    406         return false;
    407     }
    408 
    409     SmallVector<Value *, ExpectedMaxArgs> Arguments(Args.begin(), Args.end());
    410     SmallVector<StratifiedInfo, ExpectedMaxArgs> Parameters;
    411     for (auto *Fn : Fns) {
    412       auto &Info = *AA.ensureCached(Fn);
    413       auto &Sets = Info.Sets;
    414       auto &RetVals = Info.ReturnedValues;
    415 
    416       Parameters.clear();
    417       for (auto &Param : Fn->args()) {
    418         auto MaybeInfo = Sets.find(&Param);
    419         // Did a new parameter somehow get added to the function/slip by?
    420         if (!MaybeInfo.hasValue())
    421           return false;
    422         Parameters.push_back(*MaybeInfo);
    423       }
    424 
    425       // Adding an edge from argument -> return value for each parameter that
    426       // may alias the return value
    427       for (unsigned I = 0, E = Parameters.size(); I != E; ++I) {
    428         auto &ParamInfo = Parameters[I];
    429         auto &ArgVal = Arguments[I];
    430         bool AddEdge = false;
    431         StratifiedAttrs Externals;
    432         for (unsigned X = 0, XE = RetVals.size(); X != XE; ++X) {
    433           auto MaybeInfo = Sets.find(RetVals[X]);
    434           if (!MaybeInfo.hasValue())
    435             return false;
    436 
    437           auto &RetInfo = *MaybeInfo;
    438           auto RetAttrs = Sets.getLink(RetInfo.Index).Attrs;
    439           auto ParamAttrs = Sets.getLink(ParamInfo.Index).Attrs;
    440           auto MaybeRelation =
    441               getIndexRelation(Sets, ParamInfo.Index, RetInfo.Index);
    442           if (MaybeRelation.hasValue()) {
    443             AddEdge = true;
    444             Externals |= RetAttrs | ParamAttrs;
    445           }
    446         }
    447         if (AddEdge)
    448           Output.push_back(Edge(FuncValue, ArgVal, EdgeType::Assign,
    449                                 StratifiedAttrs().flip()));
    450       }
    451 
    452       if (Parameters.size() != Arguments.size())
    453         return false;
    454 
    455       // Adding edges between arguments for arguments that may end up aliasing
    456       // each other. This is necessary for functions such as
    457       // void foo(int** a, int** b) { *a = *b; }
    458       // (Technically, the proper sets for this would be those below
    459       // Arguments[I] and Arguments[X], but our algorithm will produce
    460       // extremely similar, and equally correct, results either way)
    461       for (unsigned I = 0, E = Arguments.size(); I != E; ++I) {
    462         auto &MainVal = Arguments[I];
    463         auto &MainInfo = Parameters[I];
    464         auto &MainAttrs = Sets.getLink(MainInfo.Index).Attrs;
    465         for (unsigned X = I + 1; X != E; ++X) {
    466           auto &SubInfo = Parameters[X];
    467           auto &SubVal = Arguments[X];
    468           auto &SubAttrs = Sets.getLink(SubInfo.Index).Attrs;
    469           auto MaybeRelation =
    470               getIndexRelation(Sets, MainInfo.Index, SubInfo.Index);
    471 
    472           if (!MaybeRelation.hasValue())
    473             continue;
    474 
    475           auto NewAttrs = SubAttrs | MainAttrs;
    476           Output.push_back(Edge(MainVal, SubVal, EdgeType::Assign, NewAttrs));
    477         }
    478       }
    479     }
    480     return true;
    481   }
    482 
    483   template <typename InstT> void visitCallLikeInst(InstT &Inst) {
    484     SmallVector<Function *, 4> Targets;
    485     if (getPossibleTargets(&Inst, Targets)) {
    486       if (tryInterproceduralAnalysis(Targets, &Inst, Inst.arg_operands()))
    487         return;
    488       // Cleanup from interprocedural analysis
    489       Output.clear();
    490     }
    491 
    492     for (Value *V : Inst.arg_operands())
    493       Output.push_back(Edge(&Inst, V, EdgeType::Assign, AttrAll));
    494   }
    495 
    496   void visitCallInst(CallInst &Inst) { visitCallLikeInst(Inst); }
    497 
    498   void visitInvokeInst(InvokeInst &Inst) { visitCallLikeInst(Inst); }
    499 
    500   // Because vectors/aggregates are immutable and unaddressable,
    501   // there's nothing we can do to coax a value out of them, other
    502   // than calling Extract{Element,Value}. We can effectively treat
    503   // them as pointers to arbitrary memory locations we can store in
    504   // and load from.
    505   void visitExtractElementInst(ExtractElementInst &Inst) {
    506     auto *Ptr = Inst.getVectorOperand();
    507     auto *Val = &Inst;
    508     Output.push_back(Edge(Val, Ptr, EdgeType::Reference, AttrNone));
    509   }
    510 
    511   void visitInsertElementInst(InsertElementInst &Inst) {
    512     auto *Vec = Inst.getOperand(0);
    513     auto *Val = Inst.getOperand(1);
    514     Output.push_back(Edge(&Inst, Vec, EdgeType::Assign, AttrNone));
    515     Output.push_back(Edge(&Inst, Val, EdgeType::Dereference, AttrNone));
    516   }
    517 
    518   void visitLandingPadInst(LandingPadInst &Inst) {
    519     // Exceptions come from "nowhere", from our analysis' perspective.
    520     // So we place the instruction its own group, noting that said group may
    521     // alias externals
    522     Output.push_back(Edge(&Inst, &Inst, EdgeType::Assign, AttrAll));
    523   }
    524 
    525   void visitInsertValueInst(InsertValueInst &Inst) {
    526     auto *Agg = Inst.getOperand(0);
    527     auto *Val = Inst.getOperand(1);
    528     Output.push_back(Edge(&Inst, Agg, EdgeType::Assign, AttrNone));
    529     Output.push_back(Edge(&Inst, Val, EdgeType::Dereference, AttrNone));
    530   }
    531 
    532   void visitExtractValueInst(ExtractValueInst &Inst) {
    533     auto *Ptr = Inst.getAggregateOperand();
    534     Output.push_back(Edge(&Inst, Ptr, EdgeType::Reference, AttrNone));
    535   }
    536 
    537   void visitShuffleVectorInst(ShuffleVectorInst &Inst) {
    538     auto *From1 = Inst.getOperand(0);
    539     auto *From2 = Inst.getOperand(1);
    540     Output.push_back(Edge(&Inst, From1, EdgeType::Assign, AttrNone));
    541     Output.push_back(Edge(&Inst, From2, EdgeType::Assign, AttrNone));
    542   }
    543 };
    544 
    545 // For a given instruction, we need to know which Value* to get the
    546 // users of in order to build our graph. In some cases (i.e. add),
    547 // we simply need the Instruction*. In other cases (i.e. store),
    548 // finding the users of the Instruction* is useless; we need to find
    549 // the users of the first operand. This handles determining which
    550 // value to follow for us.
    551 //
    552 // Note: we *need* to keep this in sync with GetEdgesVisitor. Add
    553 // something to GetEdgesVisitor, add it here -- remove something from
    554 // GetEdgesVisitor, remove it here.
    555 class GetTargetValueVisitor
    556     : public InstVisitor<GetTargetValueVisitor, Value *> {
    557 public:
    558   Value *visitInstruction(Instruction &Inst) { return &Inst; }
    559 
    560   Value *visitStoreInst(StoreInst &Inst) { return Inst.getPointerOperand(); }
    561 
    562   Value *visitAtomicCmpXchgInst(AtomicCmpXchgInst &Inst) {
    563     return Inst.getPointerOperand();
    564   }
    565 
    566   Value *visitAtomicRMWInst(AtomicRMWInst &Inst) {
    567     return Inst.getPointerOperand();
    568   }
    569 
    570   Value *visitInsertElementInst(InsertElementInst &Inst) {
    571     return Inst.getOperand(0);
    572   }
    573 
    574   Value *visitInsertValueInst(InsertValueInst &Inst) {
    575     return Inst.getAggregateOperand();
    576   }
    577 };
    578 
    579 // Set building requires a weighted bidirectional graph.
    580 template <typename EdgeTypeT> class WeightedBidirectionalGraph {
    581 public:
    582   typedef std::size_t Node;
    583 
    584 private:
    585   const static Node StartNode = Node(0);
    586 
    587   struct Edge {
    588     EdgeTypeT Weight;
    589     Node Other;
    590 
    591     Edge(const EdgeTypeT &W, const Node &N) : Weight(W), Other(N) {}
    592 
    593     bool operator==(const Edge &E) const {
    594       return Weight == E.Weight && Other == E.Other;
    595     }
    596 
    597     bool operator!=(const Edge &E) const { return !operator==(E); }
    598   };
    599 
    600   struct NodeImpl {
    601     std::vector<Edge> Edges;
    602   };
    603 
    604   std::vector<NodeImpl> NodeImpls;
    605 
    606   bool inbounds(Node NodeIndex) const { return NodeIndex < NodeImpls.size(); }
    607 
    608   const NodeImpl &getNode(Node N) const { return NodeImpls[N]; }
    609   NodeImpl &getNode(Node N) { return NodeImpls[N]; }
    610 
    611 public:
    612   // ----- Various Edge iterators for the graph ----- //
    613 
    614   // \brief Iterator for edges. Because this graph is bidirected, we don't
    615   // allow modificaiton of the edges using this iterator. Additionally, the
    616   // iterator becomes invalid if you add edges to or from the node you're
    617   // getting the edges of.
    618   struct EdgeIterator : public std::iterator<std::forward_iterator_tag,
    619                                              std::tuple<EdgeTypeT, Node *>> {
    620     EdgeIterator(const typename std::vector<Edge>::const_iterator &Iter)
    621         : Current(Iter) {}
    622 
    623     EdgeIterator(NodeImpl &Impl) : Current(Impl.begin()) {}
    624 
    625     EdgeIterator &operator++() {
    626       ++Current;
    627       return *this;
    628     }
    629 
    630     EdgeIterator operator++(int) {
    631       EdgeIterator Copy(Current);
    632       operator++();
    633       return Copy;
    634     }
    635 
    636     std::tuple<EdgeTypeT, Node> &operator*() {
    637       Store = std::make_tuple(Current->Weight, Current->Other);
    638       return Store;
    639     }
    640 
    641     bool operator==(const EdgeIterator &Other) const {
    642       return Current == Other.Current;
    643     }
    644 
    645     bool operator!=(const EdgeIterator &Other) const {
    646       return !operator==(Other);
    647     }
    648 
    649   private:
    650     typename std::vector<Edge>::const_iterator Current;
    651     std::tuple<EdgeTypeT, Node> Store;
    652   };
    653 
    654   // Wrapper for EdgeIterator with begin()/end() calls.
    655   struct EdgeIterable {
    656     EdgeIterable(const std::vector<Edge> &Edges)
    657         : BeginIter(Edges.begin()), EndIter(Edges.end()) {}
    658 
    659     EdgeIterator begin() { return EdgeIterator(BeginIter); }
    660 
    661     EdgeIterator end() { return EdgeIterator(EndIter); }
    662 
    663   private:
    664     typename std::vector<Edge>::const_iterator BeginIter;
    665     typename std::vector<Edge>::const_iterator EndIter;
    666   };
    667 
    668   // ----- Actual graph-related things ----- //
    669 
    670   WeightedBidirectionalGraph() {}
    671 
    672   WeightedBidirectionalGraph(WeightedBidirectionalGraph<EdgeTypeT> &&Other)
    673       : NodeImpls(std::move(Other.NodeImpls)) {}
    674 
    675   WeightedBidirectionalGraph<EdgeTypeT> &
    676   operator=(WeightedBidirectionalGraph<EdgeTypeT> &&Other) {
    677     NodeImpls = std::move(Other.NodeImpls);
    678     return *this;
    679   }
    680 
    681   Node addNode() {
    682     auto Index = NodeImpls.size();
    683     auto NewNode = Node(Index);
    684     NodeImpls.push_back(NodeImpl());
    685     return NewNode;
    686   }
    687 
    688   void addEdge(Node From, Node To, const EdgeTypeT &Weight,
    689                const EdgeTypeT &ReverseWeight) {
    690     assert(inbounds(From));
    691     assert(inbounds(To));
    692     auto &FromNode = getNode(From);
    693     auto &ToNode = getNode(To);
    694     FromNode.Edges.push_back(Edge(Weight, To));
    695     ToNode.Edges.push_back(Edge(ReverseWeight, From));
    696   }
    697 
    698   EdgeIterable edgesFor(const Node &N) const {
    699     const auto &Node = getNode(N);
    700     return EdgeIterable(Node.Edges);
    701   }
    702 
    703   bool empty() const { return NodeImpls.empty(); }
    704   std::size_t size() const { return NodeImpls.size(); }
    705 
    706   // \brief Gets an arbitrary node in the graph as a starting point for
    707   // traversal.
    708   Node getEntryNode() {
    709     assert(inbounds(StartNode));
    710     return StartNode;
    711   }
    712 };
    713 
    714 typedef WeightedBidirectionalGraph<std::pair<EdgeType, StratifiedAttrs>> GraphT;
    715 typedef DenseMap<Value *, GraphT::Node> NodeMapT;
    716 }
    717 
    718 // -- Setting up/registering CFLAA pass -- //
    719 char CFLAliasAnalysis::ID = 0;
    720 
    721 INITIALIZE_AG_PASS(CFLAliasAnalysis, AliasAnalysis, "cfl-aa",
    722                    "CFL-Based AA implementation", false, true, false)
    723 
    724 ImmutablePass *llvm::createCFLAliasAnalysisPass() {
    725   return new CFLAliasAnalysis();
    726 }
    727 
    728 //===----------------------------------------------------------------------===//
    729 // Function declarations that require types defined in the namespace above
    730 //===----------------------------------------------------------------------===//
    731 
    732 // Given an argument number, returns the appropriate Attr index to set.
    733 static StratifiedAttr argNumberToAttrIndex(StratifiedAttr);
    734 
    735 // Given a Value, potentially return which AttrIndex it maps to.
    736 static Optional<StratifiedAttr> valueToAttrIndex(Value *Val);
    737 
    738 // Gets the inverse of a given EdgeType.
    739 static EdgeType flipWeight(EdgeType);
    740 
    741 // Gets edges of the given Instruction*, writing them to the SmallVector*.
    742 static void argsToEdges(CFLAliasAnalysis &, Instruction *,
    743                         SmallVectorImpl<Edge> &);
    744 
    745 // Gets the "Level" that one should travel in StratifiedSets
    746 // given an EdgeType.
    747 static Level directionOfEdgeType(EdgeType);
    748 
    749 // Builds the graph needed for constructing the StratifiedSets for the
    750 // given function
    751 static void buildGraphFrom(CFLAliasAnalysis &, Function *,
    752                            SmallVectorImpl<Value *> &, NodeMapT &, GraphT &);
    753 
    754 // Gets the edges of a ConstantExpr as if it was an Instruction. This
    755 // function also acts on any nested ConstantExprs, adding the edges
    756 // of those to the given SmallVector as well.
    757 static void constexprToEdges(CFLAliasAnalysis &, ConstantExpr &,
    758                              SmallVectorImpl<Edge> &);
    759 
    760 // Given an Instruction, this will add it to the graph, along with any
    761 // Instructions that are potentially only available from said Instruction
    762 // For example, given the following line:
    763 //   %0 = load i16* getelementptr ([1 x i16]* @a, 0, 0), align 2
    764 // addInstructionToGraph would add both the `load` and `getelementptr`
    765 // instructions to the graph appropriately.
    766 static void addInstructionToGraph(CFLAliasAnalysis &, Instruction &,
    767                                   SmallVectorImpl<Value *> &, NodeMapT &,
    768                                   GraphT &);
    769 
    770 // Notes whether it would be pointless to add the given Value to our sets.
    771 static bool canSkipAddingToSets(Value *Val);
    772 
    773 // Builds the graph + StratifiedSets for a function.
    774 static FunctionInfo buildSetsFrom(CFLAliasAnalysis &, Function *);
    775 
    776 static Optional<Function *> parentFunctionOfValue(Value *Val) {
    777   if (auto *Inst = dyn_cast<Instruction>(Val)) {
    778     auto *Bb = Inst->getParent();
    779     return Bb->getParent();
    780   }
    781 
    782   if (auto *Arg = dyn_cast<Argument>(Val))
    783     return Arg->getParent();
    784   return NoneType();
    785 }
    786 
    787 template <typename Inst>
    788 static bool getPossibleTargets(Inst *Call,
    789                                SmallVectorImpl<Function *> &Output) {
    790   if (auto *Fn = Call->getCalledFunction()) {
    791     Output.push_back(Fn);
    792     return true;
    793   }
    794 
    795   // TODO: If the call is indirect, we might be able to enumerate all potential
    796   // targets of the call and return them, rather than just failing.
    797   return false;
    798 }
    799 
    800 static Optional<Value *> getTargetValue(Instruction *Inst) {
    801   GetTargetValueVisitor V;
    802   return V.visit(Inst);
    803 }
    804 
    805 static bool hasUsefulEdges(Instruction *Inst) {
    806   bool IsNonInvokeTerminator =
    807       isa<TerminatorInst>(Inst) && !isa<InvokeInst>(Inst);
    808   return !isa<CmpInst>(Inst) && !isa<FenceInst>(Inst) && !IsNonInvokeTerminator;
    809 }
    810 
    811 static Optional<StratifiedAttr> valueToAttrIndex(Value *Val) {
    812   if (isa<GlobalValue>(Val))
    813     return AttrGlobalIndex;
    814 
    815   if (auto *Arg = dyn_cast<Argument>(Val))
    816     // Only pointer arguments should have the argument attribute,
    817     // because things can't escape through scalars without us seeing a
    818     // cast, and thus, interaction with them doesn't matter.
    819     if (!Arg->hasNoAliasAttr() && Arg->getType()->isPointerTy())
    820       return argNumberToAttrIndex(Arg->getArgNo());
    821   return NoneType();
    822 }
    823 
    824 static StratifiedAttr argNumberToAttrIndex(unsigned ArgNum) {
    825   if (ArgNum >= AttrMaxNumArgs)
    826     return AttrAllIndex;
    827   return ArgNum + AttrFirstArgIndex;
    828 }
    829 
    830 static EdgeType flipWeight(EdgeType Initial) {
    831   switch (Initial) {
    832   case EdgeType::Assign:
    833     return EdgeType::Assign;
    834   case EdgeType::Dereference:
    835     return EdgeType::Reference;
    836   case EdgeType::Reference:
    837     return EdgeType::Dereference;
    838   }
    839   llvm_unreachable("Incomplete coverage of EdgeType enum");
    840 }
    841 
    842 static void argsToEdges(CFLAliasAnalysis &Analysis, Instruction *Inst,
    843                         SmallVectorImpl<Edge> &Output) {
    844   assert(hasUsefulEdges(Inst) &&
    845          "Expected instructions to have 'useful' edges");
    846   GetEdgesVisitor v(Analysis, Output);
    847   v.visit(Inst);
    848 }
    849 
    850 static Level directionOfEdgeType(EdgeType Weight) {
    851   switch (Weight) {
    852   case EdgeType::Reference:
    853     return Level::Above;
    854   case EdgeType::Dereference:
    855     return Level::Below;
    856   case EdgeType::Assign:
    857     return Level::Same;
    858   }
    859   llvm_unreachable("Incomplete switch coverage");
    860 }
    861 
    862 static void constexprToEdges(CFLAliasAnalysis &Analysis,
    863                              ConstantExpr &CExprToCollapse,
    864                              SmallVectorImpl<Edge> &Results) {
    865   SmallVector<ConstantExpr *, 4> Worklist;
    866   Worklist.push_back(&CExprToCollapse);
    867 
    868   SmallVector<Edge, 8> ConstexprEdges;
    869   while (!Worklist.empty()) {
    870     auto *CExpr = Worklist.pop_back_val();
    871     std::unique_ptr<Instruction> Inst(CExpr->getAsInstruction());
    872 
    873     if (!hasUsefulEdges(Inst.get()))
    874       continue;
    875 
    876     ConstexprEdges.clear();
    877     argsToEdges(Analysis, Inst.get(), ConstexprEdges);
    878     for (auto &Edge : ConstexprEdges) {
    879       if (Edge.From == Inst.get())
    880         Edge.From = CExpr;
    881       else if (auto *Nested = dyn_cast<ConstantExpr>(Edge.From))
    882         Worklist.push_back(Nested);
    883 
    884       if (Edge.To == Inst.get())
    885         Edge.To = CExpr;
    886       else if (auto *Nested = dyn_cast<ConstantExpr>(Edge.To))
    887         Worklist.push_back(Nested);
    888     }
    889 
    890     Results.append(ConstexprEdges.begin(), ConstexprEdges.end());
    891   }
    892 }
    893 
    894 static void addInstructionToGraph(CFLAliasAnalysis &Analysis, Instruction &Inst,
    895                                   SmallVectorImpl<Value *> &ReturnedValues,
    896                                   NodeMapT &Map, GraphT &Graph) {
    897   const auto findOrInsertNode = [&Map, &Graph](Value *Val) {
    898     auto Pair = Map.insert(std::make_pair(Val, GraphT::Node()));
    899     auto &Iter = Pair.first;
    900     if (Pair.second) {
    901       auto NewNode = Graph.addNode();
    902       Iter->second = NewNode;
    903     }
    904     return Iter->second;
    905   };
    906 
    907   // We don't want the edges of most "return" instructions, but we *do* want
    908   // to know what can be returned.
    909   if (isa<ReturnInst>(&Inst))
    910     ReturnedValues.push_back(&Inst);
    911 
    912   if (!hasUsefulEdges(&Inst))
    913     return;
    914 
    915   SmallVector<Edge, 8> Edges;
    916   argsToEdges(Analysis, &Inst, Edges);
    917 
    918   // In the case of an unused alloca (or similar), edges may be empty. Note
    919   // that it exists so we can potentially answer NoAlias.
    920   if (Edges.empty()) {
    921     auto MaybeVal = getTargetValue(&Inst);
    922     assert(MaybeVal.hasValue());
    923     auto *Target = *MaybeVal;
    924     findOrInsertNode(Target);
    925     return;
    926   }
    927 
    928   const auto addEdgeToGraph = [&Graph, &findOrInsertNode](const Edge &E) {
    929     auto To = findOrInsertNode(E.To);
    930     auto From = findOrInsertNode(E.From);
    931     auto FlippedWeight = flipWeight(E.Weight);
    932     auto Attrs = E.AdditionalAttrs;
    933     Graph.addEdge(From, To, std::make_pair(E.Weight, Attrs),
    934                   std::make_pair(FlippedWeight, Attrs));
    935   };
    936 
    937   SmallVector<ConstantExpr *, 4> ConstantExprs;
    938   for (const Edge &E : Edges) {
    939     addEdgeToGraph(E);
    940     if (auto *Constexpr = dyn_cast<ConstantExpr>(E.To))
    941       ConstantExprs.push_back(Constexpr);
    942     if (auto *Constexpr = dyn_cast<ConstantExpr>(E.From))
    943       ConstantExprs.push_back(Constexpr);
    944   }
    945 
    946   for (ConstantExpr *CE : ConstantExprs) {
    947     Edges.clear();
    948     constexprToEdges(Analysis, *CE, Edges);
    949     std::for_each(Edges.begin(), Edges.end(), addEdgeToGraph);
    950   }
    951 }
    952 
    953 // Aside: We may remove graph construction entirely, because it doesn't really
    954 // buy us much that we don't already have. I'd like to add interprocedural
    955 // analysis prior to this however, in case that somehow requires the graph
    956 // produced by this for efficient execution
    957 static void buildGraphFrom(CFLAliasAnalysis &Analysis, Function *Fn,
    958                            SmallVectorImpl<Value *> &ReturnedValues,
    959                            NodeMapT &Map, GraphT &Graph) {
    960   for (auto &Bb : Fn->getBasicBlockList())
    961     for (auto &Inst : Bb.getInstList())
    962       addInstructionToGraph(Analysis, Inst, ReturnedValues, Map, Graph);
    963 }
    964 
    965 static bool canSkipAddingToSets(Value *Val) {
    966   // Constants can share instances, which may falsely unify multiple
    967   // sets, e.g. in
    968   // store i32* null, i32** %ptr1
    969   // store i32* null, i32** %ptr2
    970   // clearly ptr1 and ptr2 should not be unified into the same set, so
    971   // we should filter out the (potentially shared) instance to
    972   // i32* null.
    973   if (isa<Constant>(Val)) {
    974     bool Container = isa<ConstantVector>(Val) || isa<ConstantArray>(Val) ||
    975                      isa<ConstantStruct>(Val);
    976     // TODO: Because all of these things are constant, we can determine whether
    977     // the data is *actually* mutable at graph building time. This will probably
    978     // come for free/cheap with offset awareness.
    979     bool CanStoreMutableData =
    980         isa<GlobalValue>(Val) || isa<ConstantExpr>(Val) || Container;
    981     return !CanStoreMutableData;
    982   }
    983 
    984   return false;
    985 }
    986 
    987 static FunctionInfo buildSetsFrom(CFLAliasAnalysis &Analysis, Function *Fn) {
    988   NodeMapT Map;
    989   GraphT Graph;
    990   SmallVector<Value *, 4> ReturnedValues;
    991 
    992   buildGraphFrom(Analysis, Fn, ReturnedValues, Map, Graph);
    993 
    994   DenseMap<GraphT::Node, Value *> NodeValueMap;
    995   NodeValueMap.resize(Map.size());
    996   for (const auto &Pair : Map)
    997     NodeValueMap.insert(std::make_pair(Pair.second, Pair.first));
    998 
    999   const auto findValueOrDie = [&NodeValueMap](GraphT::Node Node) {
   1000     auto ValIter = NodeValueMap.find(Node);
   1001     assert(ValIter != NodeValueMap.end());
   1002     return ValIter->second;
   1003   };
   1004 
   1005   StratifiedSetsBuilder<Value *> Builder;
   1006 
   1007   SmallVector<GraphT::Node, 16> Worklist;
   1008   for (auto &Pair : Map) {
   1009     Worklist.clear();
   1010 
   1011     auto *Value = Pair.first;
   1012     Builder.add(Value);
   1013     auto InitialNode = Pair.second;
   1014     Worklist.push_back(InitialNode);
   1015     while (!Worklist.empty()) {
   1016       auto Node = Worklist.pop_back_val();
   1017       auto *CurValue = findValueOrDie(Node);
   1018       if (canSkipAddingToSets(CurValue))
   1019         continue;
   1020 
   1021       for (const auto &EdgeTuple : Graph.edgesFor(Node)) {
   1022         auto Weight = std::get<0>(EdgeTuple);
   1023         auto Label = Weight.first;
   1024         auto &OtherNode = std::get<1>(EdgeTuple);
   1025         auto *OtherValue = findValueOrDie(OtherNode);
   1026 
   1027         if (canSkipAddingToSets(OtherValue))
   1028           continue;
   1029 
   1030         bool Added;
   1031         switch (directionOfEdgeType(Label)) {
   1032         case Level::Above:
   1033           Added = Builder.addAbove(CurValue, OtherValue);
   1034           break;
   1035         case Level::Below:
   1036           Added = Builder.addBelow(CurValue, OtherValue);
   1037           break;
   1038         case Level::Same:
   1039           Added = Builder.addWith(CurValue, OtherValue);
   1040           break;
   1041         }
   1042 
   1043         auto Aliasing = Weight.second;
   1044         if (auto MaybeCurIndex = valueToAttrIndex(CurValue))
   1045           Aliasing.set(*MaybeCurIndex);
   1046         if (auto MaybeOtherIndex = valueToAttrIndex(OtherValue))
   1047           Aliasing.set(*MaybeOtherIndex);
   1048         Builder.noteAttributes(CurValue, Aliasing);
   1049         Builder.noteAttributes(OtherValue, Aliasing);
   1050 
   1051         if (Added)
   1052           Worklist.push_back(OtherNode);
   1053       }
   1054     }
   1055   }
   1056 
   1057   // There are times when we end up with parameters not in our graph (i.e. if
   1058   // it's only used as the condition of a branch). Other bits of code depend on
   1059   // things that were present during construction being present in the graph.
   1060   // So, we add all present arguments here.
   1061   for (auto &Arg : Fn->args()) {
   1062     if (!Builder.add(&Arg))
   1063       continue;
   1064 
   1065     auto Attrs = valueToAttrIndex(&Arg);
   1066     if (Attrs.hasValue())
   1067       Builder.noteAttributes(&Arg, *Attrs);
   1068   }
   1069 
   1070   return FunctionInfo(Builder.build(), std::move(ReturnedValues));
   1071 }
   1072 
   1073 void CFLAliasAnalysis::scan(Function *Fn) {
   1074   auto InsertPair = Cache.insert(std::make_pair(Fn, Optional<FunctionInfo>()));
   1075   (void)InsertPair;
   1076   assert(InsertPair.second &&
   1077          "Trying to scan a function that has already been cached");
   1078 
   1079   FunctionInfo Info(buildSetsFrom(*this, Fn));
   1080   Cache[Fn] = std::move(Info);
   1081   Handles.push_front(FunctionHandle(Fn, this));
   1082 }
   1083 
   1084 AliasAnalysis::AliasResult
   1085 CFLAliasAnalysis::query(const AliasAnalysis::Location &LocA,
   1086                         const AliasAnalysis::Location &LocB) {
   1087   auto *ValA = const_cast<Value *>(LocA.Ptr);
   1088   auto *ValB = const_cast<Value *>(LocB.Ptr);
   1089 
   1090   Function *Fn = nullptr;
   1091   auto MaybeFnA = parentFunctionOfValue(ValA);
   1092   auto MaybeFnB = parentFunctionOfValue(ValB);
   1093   if (!MaybeFnA.hasValue() && !MaybeFnB.hasValue()) {
   1094     // The only times this is known to happen are when globals + InlineAsm
   1095     // are involved
   1096     DEBUG(dbgs() << "CFLAA: could not extract parent function information.\n");
   1097     return AliasAnalysis::MayAlias;
   1098   }
   1099 
   1100   if (MaybeFnA.hasValue()) {
   1101     Fn = *MaybeFnA;
   1102     assert((!MaybeFnB.hasValue() || *MaybeFnB == *MaybeFnA) &&
   1103            "Interprocedural queries not supported");
   1104   } else {
   1105     Fn = *MaybeFnB;
   1106   }
   1107 
   1108   assert(Fn != nullptr);
   1109   auto &MaybeInfo = ensureCached(Fn);
   1110   assert(MaybeInfo.hasValue());
   1111 
   1112   auto &Sets = MaybeInfo->Sets;
   1113   auto MaybeA = Sets.find(ValA);
   1114   if (!MaybeA.hasValue())
   1115     return AliasAnalysis::MayAlias;
   1116 
   1117   auto MaybeB = Sets.find(ValB);
   1118   if (!MaybeB.hasValue())
   1119     return AliasAnalysis::MayAlias;
   1120 
   1121   auto SetA = *MaybeA;
   1122   auto SetB = *MaybeB;
   1123   auto AttrsA = Sets.getLink(SetA.Index).Attrs;
   1124   auto AttrsB = Sets.getLink(SetB.Index).Attrs;
   1125 
   1126   // Stratified set attributes are used as markets to signify whether a member
   1127   // of a StratifiedSet (or a member of a set above the current set) has
   1128   // interacted with either arguments or globals. "Interacted with" meaning
   1129   // its value may be different depending on the value of an argument or
   1130   // global. The thought behind this is that, because arguments and globals
   1131   // may alias each other, if AttrsA and AttrsB have touched args/globals,
   1132   // we must conservatively say that they alias. However, if at least one of
   1133   // the sets has no values that could legally be altered by changing the value
   1134   // of an argument or global, then we don't have to be as conservative.
   1135   if (AttrsA.any() && AttrsB.any())
   1136     return AliasAnalysis::MayAlias;
   1137 
   1138   // We currently unify things even if the accesses to them may not be in
   1139   // bounds, so we can't return partial alias here because we don't
   1140   // know whether the pointer is really within the object or not.
   1141   // IE Given an out of bounds GEP and an alloca'd pointer, we may
   1142   // unify the two. We can't return partial alias for this case.
   1143   // Since we do not currently track enough information to
   1144   // differentiate
   1145 
   1146   if (SetA.Index == SetB.Index)
   1147     return AliasAnalysis::MayAlias;
   1148 
   1149   return AliasAnalysis::NoAlias;
   1150 }
   1151 
   1152 bool CFLAliasAnalysis::doInitialization(Module &M) {
   1153   InitializeAliasAnalysis(this, &M.getDataLayout());
   1154   return true;
   1155 }
   1156