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      1 //===- GlobalsModRef.cpp - Simple Mod/Ref Analysis for Globals ------------===//
      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 simple pass provides alias and mod/ref information for global values
     11 // that do not have their address taken, and keeps track of whether functions
     12 // read or write memory (are "pure").  For this simple (but very common) case,
     13 // we can provide pretty accurate and useful information.
     14 //
     15 //===----------------------------------------------------------------------===//
     16 
     17 #include "llvm/Analysis/Passes.h"
     18 #include "llvm/ADT/SCCIterator.h"
     19 #include "llvm/ADT/Statistic.h"
     20 #include "llvm/Analysis/AliasAnalysis.h"
     21 #include "llvm/Analysis/CallGraph.h"
     22 #include "llvm/Analysis/MemoryBuiltins.h"
     23 #include "llvm/Analysis/ValueTracking.h"
     24 #include "llvm/IR/Constants.h"
     25 #include "llvm/IR/DerivedTypes.h"
     26 #include "llvm/IR/InstIterator.h"
     27 #include "llvm/IR/Instructions.h"
     28 #include "llvm/IR/IntrinsicInst.h"
     29 #include "llvm/IR/Module.h"
     30 #include "llvm/Pass.h"
     31 #include "llvm/Support/CommandLine.h"
     32 #include <set>
     33 using namespace llvm;
     34 
     35 #define DEBUG_TYPE "globalsmodref-aa"
     36 
     37 STATISTIC(NumNonAddrTakenGlobalVars,
     38           "Number of global vars without address taken");
     39 STATISTIC(NumNonAddrTakenFunctions,"Number of functions without address taken");
     40 STATISTIC(NumNoMemFunctions, "Number of functions that do not access memory");
     41 STATISTIC(NumReadMemFunctions, "Number of functions that only read memory");
     42 STATISTIC(NumIndirectGlobalVars, "Number of indirect global objects");
     43 
     44 namespace {
     45   /// FunctionRecord - One instance of this structure is stored for every
     46   /// function in the program.  Later, the entries for these functions are
     47   /// removed if the function is found to call an external function (in which
     48   /// case we know nothing about it.
     49   struct FunctionRecord {
     50     /// GlobalInfo - Maintain mod/ref info for all of the globals without
     51     /// addresses taken that are read or written (transitively) by this
     52     /// function.
     53     std::map<const GlobalValue*, unsigned> GlobalInfo;
     54 
     55     /// MayReadAnyGlobal - May read global variables, but it is not known which.
     56     bool MayReadAnyGlobal;
     57 
     58     unsigned getInfoForGlobal(const GlobalValue *GV) const {
     59       unsigned Effect = MayReadAnyGlobal ? AliasAnalysis::Ref : 0;
     60       std::map<const GlobalValue*, unsigned>::const_iterator I =
     61         GlobalInfo.find(GV);
     62       if (I != GlobalInfo.end())
     63         Effect |= I->second;
     64       return Effect;
     65     }
     66 
     67     /// FunctionEffect - Capture whether or not this function reads or writes to
     68     /// ANY memory.  If not, we can do a lot of aggressive analysis on it.
     69     unsigned FunctionEffect;
     70 
     71     FunctionRecord() : MayReadAnyGlobal (false), FunctionEffect(0) {}
     72   };
     73 
     74   /// GlobalsModRef - The actual analysis pass.
     75   class GlobalsModRef : public ModulePass, public AliasAnalysis {
     76     /// NonAddressTakenGlobals - The globals that do not have their addresses
     77     /// taken.
     78     std::set<const GlobalValue*> NonAddressTakenGlobals;
     79 
     80     /// IndirectGlobals - The memory pointed to by this global is known to be
     81     /// 'owned' by the global.
     82     std::set<const GlobalValue*> IndirectGlobals;
     83 
     84     /// AllocsForIndirectGlobals - If an instruction allocates memory for an
     85     /// indirect global, this map indicates which one.
     86     std::map<const Value*, const GlobalValue*> AllocsForIndirectGlobals;
     87 
     88     /// FunctionInfo - For each function, keep track of what globals are
     89     /// modified or read.
     90     std::map<const Function*, FunctionRecord> FunctionInfo;
     91 
     92   public:
     93     static char ID;
     94     GlobalsModRef() : ModulePass(ID) {
     95       initializeGlobalsModRefPass(*PassRegistry::getPassRegistry());
     96     }
     97 
     98     bool runOnModule(Module &M) override {
     99       InitializeAliasAnalysis(this, &M.getDataLayout());
    100 
    101       // Find non-addr taken globals.
    102       AnalyzeGlobals(M);
    103 
    104       // Propagate on CG.
    105       AnalyzeCallGraph(getAnalysis<CallGraphWrapperPass>().getCallGraph(), M);
    106       return false;
    107     }
    108 
    109     void getAnalysisUsage(AnalysisUsage &AU) const override {
    110       AliasAnalysis::getAnalysisUsage(AU);
    111       AU.addRequired<CallGraphWrapperPass>();
    112       AU.setPreservesAll();                         // Does not transform code
    113     }
    114 
    115     //------------------------------------------------
    116     // Implement the AliasAnalysis API
    117     //
    118     AliasResult alias(const Location &LocA, const Location &LocB) override;
    119     ModRefResult getModRefInfo(ImmutableCallSite CS,
    120                                const Location &Loc) override;
    121     ModRefResult getModRefInfo(ImmutableCallSite CS1,
    122                                ImmutableCallSite CS2) override {
    123       return AliasAnalysis::getModRefInfo(CS1, CS2);
    124     }
    125 
    126     /// getModRefBehavior - Return the behavior of the specified function if
    127     /// called from the specified call site.  The call site may be null in which
    128     /// case the most generic behavior of this function should be returned.
    129     ModRefBehavior getModRefBehavior(const Function *F) override {
    130       ModRefBehavior Min = UnknownModRefBehavior;
    131 
    132       if (FunctionRecord *FR = getFunctionInfo(F)) {
    133         if (FR->FunctionEffect == 0)
    134           Min = DoesNotAccessMemory;
    135         else if ((FR->FunctionEffect & Mod) == 0)
    136           Min = OnlyReadsMemory;
    137       }
    138 
    139       return ModRefBehavior(AliasAnalysis::getModRefBehavior(F) & Min);
    140     }
    141 
    142     /// getModRefBehavior - Return the behavior of the specified function if
    143     /// called from the specified call site.  The call site may be null in which
    144     /// case the most generic behavior of this function should be returned.
    145     ModRefBehavior getModRefBehavior(ImmutableCallSite CS) override {
    146       ModRefBehavior Min = UnknownModRefBehavior;
    147 
    148       if (const Function* F = CS.getCalledFunction())
    149         if (FunctionRecord *FR = getFunctionInfo(F)) {
    150           if (FR->FunctionEffect == 0)
    151             Min = DoesNotAccessMemory;
    152           else if ((FR->FunctionEffect & Mod) == 0)
    153             Min = OnlyReadsMemory;
    154         }
    155 
    156       return ModRefBehavior(AliasAnalysis::getModRefBehavior(CS) & Min);
    157     }
    158 
    159     void deleteValue(Value *V) override;
    160     void copyValue(Value *From, Value *To) override;
    161     void addEscapingUse(Use &U) override;
    162 
    163     /// getAdjustedAnalysisPointer - This method is used when a pass implements
    164     /// an analysis interface through multiple inheritance.  If needed, it
    165     /// should override this to adjust the this pointer as needed for the
    166     /// specified pass info.
    167     void *getAdjustedAnalysisPointer(AnalysisID PI) override {
    168       if (PI == &AliasAnalysis::ID)
    169         return (AliasAnalysis*)this;
    170       return this;
    171     }
    172 
    173   private:
    174     /// getFunctionInfo - Return the function info for the function, or null if
    175     /// we don't have anything useful to say about it.
    176     FunctionRecord *getFunctionInfo(const Function *F) {
    177       std::map<const Function*, FunctionRecord>::iterator I =
    178         FunctionInfo.find(F);
    179       if (I != FunctionInfo.end())
    180         return &I->second;
    181       return nullptr;
    182     }
    183 
    184     void AnalyzeGlobals(Module &M);
    185     void AnalyzeCallGraph(CallGraph &CG, Module &M);
    186     bool AnalyzeUsesOfPointer(Value *V, std::vector<Function*> &Readers,
    187                               std::vector<Function*> &Writers,
    188                               GlobalValue *OkayStoreDest = nullptr);
    189     bool AnalyzeIndirectGlobalMemory(GlobalValue *GV);
    190   };
    191 }
    192 
    193 char GlobalsModRef::ID = 0;
    194 INITIALIZE_AG_PASS_BEGIN(GlobalsModRef, AliasAnalysis,
    195                 "globalsmodref-aa", "Simple mod/ref analysis for globals",
    196                 false, true, false)
    197 INITIALIZE_PASS_DEPENDENCY(CallGraphWrapperPass)
    198 INITIALIZE_AG_PASS_END(GlobalsModRef, AliasAnalysis,
    199                 "globalsmodref-aa", "Simple mod/ref analysis for globals",
    200                 false, true, false)
    201 
    202 Pass *llvm::createGlobalsModRefPass() { return new GlobalsModRef(); }
    203 
    204 /// AnalyzeGlobals - Scan through the users of all of the internal
    205 /// GlobalValue's in the program.  If none of them have their "address taken"
    206 /// (really, their address passed to something nontrivial), record this fact,
    207 /// and record the functions that they are used directly in.
    208 void GlobalsModRef::AnalyzeGlobals(Module &M) {
    209   std::vector<Function*> Readers, Writers;
    210   for (Module::iterator I = M.begin(), E = M.end(); I != E; ++I)
    211     if (I->hasLocalLinkage()) {
    212       if (!AnalyzeUsesOfPointer(I, Readers, Writers)) {
    213         // Remember that we are tracking this global.
    214         NonAddressTakenGlobals.insert(I);
    215         ++NumNonAddrTakenFunctions;
    216       }
    217       Readers.clear(); Writers.clear();
    218     }
    219 
    220   for (Module::global_iterator I = M.global_begin(), E = M.global_end();
    221        I != E; ++I)
    222     if (I->hasLocalLinkage()) {
    223       if (!AnalyzeUsesOfPointer(I, Readers, Writers)) {
    224         // Remember that we are tracking this global, and the mod/ref fns
    225         NonAddressTakenGlobals.insert(I);
    226 
    227         for (unsigned i = 0, e = Readers.size(); i != e; ++i)
    228           FunctionInfo[Readers[i]].GlobalInfo[I] |= Ref;
    229 
    230         if (!I->isConstant())  // No need to keep track of writers to constants
    231           for (unsigned i = 0, e = Writers.size(); i != e; ++i)
    232             FunctionInfo[Writers[i]].GlobalInfo[I] |= Mod;
    233         ++NumNonAddrTakenGlobalVars;
    234 
    235         // If this global holds a pointer type, see if it is an indirect global.
    236         if (I->getType()->getElementType()->isPointerTy() &&
    237             AnalyzeIndirectGlobalMemory(I))
    238           ++NumIndirectGlobalVars;
    239       }
    240       Readers.clear(); Writers.clear();
    241     }
    242 }
    243 
    244 /// AnalyzeUsesOfPointer - Look at all of the users of the specified pointer.
    245 /// If this is used by anything complex (i.e., the address escapes), return
    246 /// true.  Also, while we are at it, keep track of those functions that read and
    247 /// write to the value.
    248 ///
    249 /// If OkayStoreDest is non-null, stores into this global are allowed.
    250 bool GlobalsModRef::AnalyzeUsesOfPointer(Value *V,
    251                                          std::vector<Function*> &Readers,
    252                                          std::vector<Function*> &Writers,
    253                                          GlobalValue *OkayStoreDest) {
    254   if (!V->getType()->isPointerTy()) return true;
    255 
    256   for (Use &U : V->uses()) {
    257     User *I = U.getUser();
    258     if (LoadInst *LI = dyn_cast<LoadInst>(I)) {
    259       Readers.push_back(LI->getParent()->getParent());
    260     } else if (StoreInst *SI = dyn_cast<StoreInst>(I)) {
    261       if (V == SI->getOperand(1)) {
    262         Writers.push_back(SI->getParent()->getParent());
    263       } else if (SI->getOperand(1) != OkayStoreDest) {
    264         return true;  // Storing the pointer
    265       }
    266     } else if (Operator::getOpcode(I) == Instruction::GetElementPtr) {
    267       if (AnalyzeUsesOfPointer(I, Readers, Writers))
    268         return true;
    269     } else if (Operator::getOpcode(I) == Instruction::BitCast) {
    270       if (AnalyzeUsesOfPointer(I, Readers, Writers, OkayStoreDest))
    271         return true;
    272     } else if (auto CS = CallSite(I)) {
    273       // Make sure that this is just the function being called, not that it is
    274       // passing into the function.
    275       if (!CS.isCallee(&U)) {
    276         // Detect calls to free.
    277         if (isFreeCall(I, TLI))
    278           Writers.push_back(CS->getParent()->getParent());
    279         else
    280           return true; // Argument of an unknown call.
    281       }
    282     } else if (ICmpInst *ICI = dyn_cast<ICmpInst>(I)) {
    283       if (!isa<ConstantPointerNull>(ICI->getOperand(1)))
    284         return true;  // Allow comparison against null.
    285     } else {
    286       return true;
    287     }
    288   }
    289 
    290   return false;
    291 }
    292 
    293 /// AnalyzeIndirectGlobalMemory - We found an non-address-taken global variable
    294 /// which holds a pointer type.  See if the global always points to non-aliased
    295 /// heap memory: that is, all initializers of the globals are allocations, and
    296 /// those allocations have no use other than initialization of the global.
    297 /// Further, all loads out of GV must directly use the memory, not store the
    298 /// pointer somewhere.  If this is true, we consider the memory pointed to by
    299 /// GV to be owned by GV and can disambiguate other pointers from it.
    300 bool GlobalsModRef::AnalyzeIndirectGlobalMemory(GlobalValue *GV) {
    301   // Keep track of values related to the allocation of the memory, f.e. the
    302   // value produced by the malloc call and any casts.
    303   std::vector<Value*> AllocRelatedValues;
    304 
    305   // Walk the user list of the global.  If we find anything other than a direct
    306   // load or store, bail out.
    307   for (User *U : GV->users()) {
    308     if (LoadInst *LI = dyn_cast<LoadInst>(U)) {
    309       // The pointer loaded from the global can only be used in simple ways:
    310       // we allow addressing of it and loading storing to it.  We do *not* allow
    311       // storing the loaded pointer somewhere else or passing to a function.
    312       std::vector<Function*> ReadersWriters;
    313       if (AnalyzeUsesOfPointer(LI, ReadersWriters, ReadersWriters))
    314         return false;  // Loaded pointer escapes.
    315       // TODO: Could try some IP mod/ref of the loaded pointer.
    316     } else if (StoreInst *SI = dyn_cast<StoreInst>(U)) {
    317       // Storing the global itself.
    318       if (SI->getOperand(0) == GV) return false;
    319 
    320       // If storing the null pointer, ignore it.
    321       if (isa<ConstantPointerNull>(SI->getOperand(0)))
    322         continue;
    323 
    324       // Check the value being stored.
    325       Value *Ptr = GetUnderlyingObject(SI->getOperand(0),
    326                                        GV->getParent()->getDataLayout());
    327 
    328       if (!isAllocLikeFn(Ptr, TLI))
    329         return false;  // Too hard to analyze.
    330 
    331       // Analyze all uses of the allocation.  If any of them are used in a
    332       // non-simple way (e.g. stored to another global) bail out.
    333       std::vector<Function*> ReadersWriters;
    334       if (AnalyzeUsesOfPointer(Ptr, ReadersWriters, ReadersWriters, GV))
    335         return false;  // Loaded pointer escapes.
    336 
    337       // Remember that this allocation is related to the indirect global.
    338       AllocRelatedValues.push_back(Ptr);
    339     } else {
    340       // Something complex, bail out.
    341       return false;
    342     }
    343   }
    344 
    345   // Okay, this is an indirect global.  Remember all of the allocations for
    346   // this global in AllocsForIndirectGlobals.
    347   while (!AllocRelatedValues.empty()) {
    348     AllocsForIndirectGlobals[AllocRelatedValues.back()] = GV;
    349     AllocRelatedValues.pop_back();
    350   }
    351   IndirectGlobals.insert(GV);
    352   return true;
    353 }
    354 
    355 /// AnalyzeCallGraph - At this point, we know the functions where globals are
    356 /// immediately stored to and read from.  Propagate this information up the call
    357 /// graph to all callers and compute the mod/ref info for all memory for each
    358 /// function.
    359 void GlobalsModRef::AnalyzeCallGraph(CallGraph &CG, Module &M) {
    360   // We do a bottom-up SCC traversal of the call graph.  In other words, we
    361   // visit all callees before callers (leaf-first).
    362   for (scc_iterator<CallGraph*> I = scc_begin(&CG); !I.isAtEnd(); ++I) {
    363     const std::vector<CallGraphNode *> &SCC = *I;
    364     assert(!SCC.empty() && "SCC with no functions?");
    365 
    366     if (!SCC[0]->getFunction()) {
    367       // Calls externally - can't say anything useful.  Remove any existing
    368       // function records (may have been created when scanning globals).
    369       for (unsigned i = 0, e = SCC.size(); i != e; ++i)
    370         FunctionInfo.erase(SCC[i]->getFunction());
    371       continue;
    372     }
    373 
    374     FunctionRecord &FR = FunctionInfo[SCC[0]->getFunction()];
    375 
    376     bool KnowNothing = false;
    377     unsigned FunctionEffect = 0;
    378 
    379     // Collect the mod/ref properties due to called functions.  We only compute
    380     // one mod-ref set.
    381     for (unsigned i = 0, e = SCC.size(); i != e && !KnowNothing; ++i) {
    382       Function *F = SCC[i]->getFunction();
    383       if (!F) {
    384         KnowNothing = true;
    385         break;
    386       }
    387 
    388       if (F->isDeclaration()) {
    389         // Try to get mod/ref behaviour from function attributes.
    390         if (F->doesNotAccessMemory()) {
    391           // Can't do better than that!
    392         } else if (F->onlyReadsMemory()) {
    393           FunctionEffect |= Ref;
    394           if (!F->isIntrinsic())
    395             // This function might call back into the module and read a global -
    396             // consider every global as possibly being read by this function.
    397             FR.MayReadAnyGlobal = true;
    398         } else {
    399           FunctionEffect |= ModRef;
    400           // Can't say anything useful unless it's an intrinsic - they don't
    401           // read or write global variables of the kind considered here.
    402           KnowNothing = !F->isIntrinsic();
    403         }
    404         continue;
    405       }
    406 
    407       for (CallGraphNode::iterator CI = SCC[i]->begin(), E = SCC[i]->end();
    408            CI != E && !KnowNothing; ++CI)
    409         if (Function *Callee = CI->second->getFunction()) {
    410           if (FunctionRecord *CalleeFR = getFunctionInfo(Callee)) {
    411             // Propagate function effect up.
    412             FunctionEffect |= CalleeFR->FunctionEffect;
    413 
    414             // Incorporate callee's effects on globals into our info.
    415             for (const auto &G : CalleeFR->GlobalInfo)
    416               FR.GlobalInfo[G.first] |= G.second;
    417             FR.MayReadAnyGlobal |= CalleeFR->MayReadAnyGlobal;
    418           } else {
    419             // Can't say anything about it.  However, if it is inside our SCC,
    420             // then nothing needs to be done.
    421             CallGraphNode *CalleeNode = CG[Callee];
    422             if (std::find(SCC.begin(), SCC.end(), CalleeNode) == SCC.end())
    423               KnowNothing = true;
    424           }
    425         } else {
    426           KnowNothing = true;
    427         }
    428     }
    429 
    430     // If we can't say anything useful about this SCC, remove all SCC functions
    431     // from the FunctionInfo map.
    432     if (KnowNothing) {
    433       for (unsigned i = 0, e = SCC.size(); i != e; ++i)
    434         FunctionInfo.erase(SCC[i]->getFunction());
    435       continue;
    436     }
    437 
    438     // Scan the function bodies for explicit loads or stores.
    439     for (unsigned i = 0, e = SCC.size(); i != e && FunctionEffect != ModRef;++i)
    440       for (inst_iterator II = inst_begin(SCC[i]->getFunction()),
    441              E = inst_end(SCC[i]->getFunction());
    442            II != E && FunctionEffect != ModRef; ++II)
    443         if (LoadInst *LI = dyn_cast<LoadInst>(&*II)) {
    444           FunctionEffect |= Ref;
    445           if (LI->isVolatile())
    446             // Volatile loads may have side-effects, so mark them as writing
    447             // memory (for example, a flag inside the processor).
    448             FunctionEffect |= Mod;
    449         } else if (StoreInst *SI = dyn_cast<StoreInst>(&*II)) {
    450           FunctionEffect |= Mod;
    451           if (SI->isVolatile())
    452             // Treat volatile stores as reading memory somewhere.
    453             FunctionEffect |= Ref;
    454         } else if (isAllocationFn(&*II, TLI) || isFreeCall(&*II, TLI)) {
    455           FunctionEffect |= ModRef;
    456         } else if (IntrinsicInst *Intrinsic = dyn_cast<IntrinsicInst>(&*II)) {
    457           // The callgraph doesn't include intrinsic calls.
    458           Function *Callee = Intrinsic->getCalledFunction();
    459           ModRefBehavior Behaviour = AliasAnalysis::getModRefBehavior(Callee);
    460           FunctionEffect |= (Behaviour & ModRef);
    461         }
    462 
    463     if ((FunctionEffect & Mod) == 0)
    464       ++NumReadMemFunctions;
    465     if (FunctionEffect == 0)
    466       ++NumNoMemFunctions;
    467     FR.FunctionEffect = FunctionEffect;
    468 
    469     // Finally, now that we know the full effect on this SCC, clone the
    470     // information to each function in the SCC.
    471     for (unsigned i = 1, e = SCC.size(); i != e; ++i)
    472       FunctionInfo[SCC[i]->getFunction()] = FR;
    473   }
    474 }
    475 
    476 
    477 
    478 /// alias - If one of the pointers is to a global that we are tracking, and the
    479 /// other is some random pointer, we know there cannot be an alias, because the
    480 /// address of the global isn't taken.
    481 AliasAnalysis::AliasResult
    482 GlobalsModRef::alias(const Location &LocA,
    483                      const Location &LocB) {
    484   // Get the base object these pointers point to.
    485   const Value *UV1 = GetUnderlyingObject(LocA.Ptr, *DL);
    486   const Value *UV2 = GetUnderlyingObject(LocB.Ptr, *DL);
    487 
    488   // If either of the underlying values is a global, they may be non-addr-taken
    489   // globals, which we can answer queries about.
    490   const GlobalValue *GV1 = dyn_cast<GlobalValue>(UV1);
    491   const GlobalValue *GV2 = dyn_cast<GlobalValue>(UV2);
    492   if (GV1 || GV2) {
    493     // If the global's address is taken, pretend we don't know it's a pointer to
    494     // the global.
    495     if (GV1 && !NonAddressTakenGlobals.count(GV1)) GV1 = nullptr;
    496     if (GV2 && !NonAddressTakenGlobals.count(GV2)) GV2 = nullptr;
    497 
    498     // If the two pointers are derived from two different non-addr-taken
    499     // globals, or if one is and the other isn't, we know these can't alias.
    500     if ((GV1 || GV2) && GV1 != GV2)
    501       return NoAlias;
    502 
    503     // Otherwise if they are both derived from the same addr-taken global, we
    504     // can't know the two accesses don't overlap.
    505   }
    506 
    507   // These pointers may be based on the memory owned by an indirect global.  If
    508   // so, we may be able to handle this.  First check to see if the base pointer
    509   // is a direct load from an indirect global.
    510   GV1 = GV2 = nullptr;
    511   if (const LoadInst *LI = dyn_cast<LoadInst>(UV1))
    512     if (GlobalVariable *GV = dyn_cast<GlobalVariable>(LI->getOperand(0)))
    513       if (IndirectGlobals.count(GV))
    514         GV1 = GV;
    515   if (const LoadInst *LI = dyn_cast<LoadInst>(UV2))
    516     if (const GlobalVariable *GV = dyn_cast<GlobalVariable>(LI->getOperand(0)))
    517       if (IndirectGlobals.count(GV))
    518         GV2 = GV;
    519 
    520   // These pointers may also be from an allocation for the indirect global.  If
    521   // so, also handle them.
    522   if (AllocsForIndirectGlobals.count(UV1))
    523     GV1 = AllocsForIndirectGlobals[UV1];
    524   if (AllocsForIndirectGlobals.count(UV2))
    525     GV2 = AllocsForIndirectGlobals[UV2];
    526 
    527   // Now that we know whether the two pointers are related to indirect globals,
    528   // use this to disambiguate the pointers.  If either pointer is based on an
    529   // indirect global and if they are not both based on the same indirect global,
    530   // they cannot alias.
    531   if ((GV1 || GV2) && GV1 != GV2)
    532     return NoAlias;
    533 
    534   return AliasAnalysis::alias(LocA, LocB);
    535 }
    536 
    537 AliasAnalysis::ModRefResult
    538 GlobalsModRef::getModRefInfo(ImmutableCallSite CS,
    539                              const Location &Loc) {
    540   unsigned Known = ModRef;
    541 
    542   // If we are asking for mod/ref info of a direct call with a pointer to a
    543   // global we are tracking, return information if we have it.
    544   const DataLayout &DL = CS.getCaller()->getParent()->getDataLayout();
    545   if (const GlobalValue *GV =
    546           dyn_cast<GlobalValue>(GetUnderlyingObject(Loc.Ptr, DL)))
    547     if (GV->hasLocalLinkage())
    548       if (const Function *F = CS.getCalledFunction())
    549         if (NonAddressTakenGlobals.count(GV))
    550           if (const FunctionRecord *FR = getFunctionInfo(F))
    551             Known = FR->getInfoForGlobal(GV);
    552 
    553   if (Known == NoModRef)
    554     return NoModRef; // No need to query other mod/ref analyses
    555   return ModRefResult(Known & AliasAnalysis::getModRefInfo(CS, Loc));
    556 }
    557 
    558 
    559 //===----------------------------------------------------------------------===//
    560 // Methods to update the analysis as a result of the client transformation.
    561 //
    562 void GlobalsModRef::deleteValue(Value *V) {
    563   if (GlobalValue *GV = dyn_cast<GlobalValue>(V)) {
    564     if (NonAddressTakenGlobals.erase(GV)) {
    565       // This global might be an indirect global.  If so, remove it and remove
    566       // any AllocRelatedValues for it.
    567       if (IndirectGlobals.erase(GV)) {
    568         // Remove any entries in AllocsForIndirectGlobals for this global.
    569         for (std::map<const Value*, const GlobalValue*>::iterator
    570              I = AllocsForIndirectGlobals.begin(),
    571              E = AllocsForIndirectGlobals.end(); I != E; ) {
    572           if (I->second == GV) {
    573             AllocsForIndirectGlobals.erase(I++);
    574           } else {
    575             ++I;
    576           }
    577         }
    578       }
    579     }
    580   }
    581 
    582   // Otherwise, if this is an allocation related to an indirect global, remove
    583   // it.
    584   AllocsForIndirectGlobals.erase(V);
    585 
    586   AliasAnalysis::deleteValue(V);
    587 }
    588 
    589 void GlobalsModRef::copyValue(Value *From, Value *To) {
    590   AliasAnalysis::copyValue(From, To);
    591 }
    592 
    593 void GlobalsModRef::addEscapingUse(Use &U) {
    594   // For the purposes of this analysis, it is conservatively correct to treat
    595   // a newly escaping value equivalently to a deleted one.  We could perhaps
    596   // be more precise by processing the new use and attempting to update our
    597   // saved analysis results to accommodate it.
    598   deleteValue(U);
    599 
    600   AliasAnalysis::addEscapingUse(U);
    601 }
    602