//===- GlobalsModRef.cpp - Simple Mod/Ref Analysis for Globals ------------===// // // The LLVM Compiler Infrastructure // // This file is distributed under the University of Illinois Open Source // License. See LICENSE.TXT for details. // //===----------------------------------------------------------------------===// // // This simple pass provides alias and mod/ref information for global values // that do not have their address taken, and keeps track of whether functions // read or write memory (are "pure"). For this simple (but very common) case, // we can provide pretty accurate and useful information. // //===----------------------------------------------------------------------===// #define DEBUG_TYPE "globalsmodref-aa" #include "llvm/Analysis/Passes.h" #include "llvm/Module.h" #include "llvm/Pass.h" #include "llvm/Instructions.h" #include "llvm/Constants.h" #include "llvm/DerivedTypes.h" #include "llvm/Analysis/AliasAnalysis.h" #include "llvm/Analysis/CallGraph.h" #include "llvm/Support/Compiler.h" #include "llvm/Support/CommandLine.h" #include "llvm/Support/InstIterator.h" #include "llvm/ADT/Statistic.h" #include "llvm/ADT/SCCIterator.h" #include using namespace llvm; STATISTIC(NumNonAddrTakenGlobalVars, "Number of global vars without address taken"); STATISTIC(NumNonAddrTakenFunctions,"Number of functions without address taken"); STATISTIC(NumNoMemFunctions, "Number of functions that do not access memory"); STATISTIC(NumReadMemFunctions, "Number of functions that only read memory"); STATISTIC(NumIndirectGlobalVars, "Number of indirect global objects"); namespace { /// FunctionRecord - One instance of this structure is stored for every /// function in the program. Later, the entries for these functions are /// removed if the function is found to call an external function (in which /// case we know nothing about it. struct VISIBILITY_HIDDEN FunctionRecord { /// GlobalInfo - Maintain mod/ref info for all of the globals without /// addresses taken that are read or written (transitively) by this /// function. std::map GlobalInfo; unsigned getInfoForGlobal(GlobalValue *GV) const { std::map::const_iterator I = GlobalInfo.find(GV); if (I != GlobalInfo.end()) return I->second; return 0; } /// FunctionEffect - Capture whether or not this function reads or writes to /// ANY memory. If not, we can do a lot of aggressive analysis on it. unsigned FunctionEffect; FunctionRecord() : FunctionEffect(0) {} }; /// GlobalsModRef - The actual analysis pass. class VISIBILITY_HIDDEN GlobalsModRef : public ModulePass, public AliasAnalysis { /// NonAddressTakenGlobals - The globals that do not have their addresses /// taken. std::set NonAddressTakenGlobals; /// ReadGlobals - The globals without addresses taken that are read by /// some function. std::set ReadGlobals; /// IndirectGlobals - The memory pointed to by this global is known to be /// 'owned' by the global. std::set IndirectGlobals; /// AllocsForIndirectGlobals - If an instruction allocates memory for an /// indirect global, this map indicates which one. std::map AllocsForIndirectGlobals; /// FunctionInfo - For each function, keep track of what globals are /// modified or read. std::map FunctionInfo; public: static char ID; GlobalsModRef() : ModulePass(&ID) {} bool runOnModule(Module &M) { InitializeAliasAnalysis(this); // set up super class AnalyzeGlobals(M); // find non-addr taken globals AnalyzeCallGraph(getAnalysis(), M); // Propagate on CG return false; } virtual void getAnalysisUsage(AnalysisUsage &AU) const { AliasAnalysis::getAnalysisUsage(AU); AU.addRequired(); AU.setPreservesAll(); // Does not transform code } //------------------------------------------------ // Implement the AliasAnalysis API // AliasResult alias(const Value *V1, unsigned V1Size, const Value *V2, unsigned V2Size); ModRefResult getModRefInfo(CallSite CS, Value *P, unsigned Size); ModRefResult getModRefInfo(CallSite CS1, CallSite CS2) { return AliasAnalysis::getModRefInfo(CS1,CS2); } bool hasNoModRefInfoForCalls() const { return false; } /// getModRefBehavior - Return the behavior of the specified function if /// called from the specified call site. The call site may be null in which /// case the most generic behavior of this function should be returned. virtual ModRefBehavior getModRefBehavior(Function *F, CallSite CS, std::vector *Info) { if (FunctionRecord *FR = getFunctionInfo(F)) { if (FR->FunctionEffect == 0) return DoesNotAccessMemory; else if ((FR->FunctionEffect & Mod) == 0) return OnlyReadsMemory; } return AliasAnalysis::getModRefBehavior(F, CS, Info); } virtual void deleteValue(Value *V); virtual void copyValue(Value *From, Value *To); private: /// getFunctionInfo - Return the function info for the function, or null if /// we don't have anything useful to say about it. FunctionRecord *getFunctionInfo(Function *F) { std::map::iterator I = FunctionInfo.find(F); if (I != FunctionInfo.end()) return &I->second; return 0; } void AnalyzeGlobals(Module &M); void AnalyzeCallGraph(CallGraph &CG, Module &M); bool AnalyzeUsesOfPointer(Value *V, std::vector &Readers, std::vector &Writers, GlobalValue *OkayStoreDest = 0); bool AnalyzeIndirectGlobalMemory(GlobalValue *GV); }; } char GlobalsModRef::ID = 0; static RegisterPass X("globalsmodref-aa", "Simple mod/ref analysis for globals", false, true); static RegisterAnalysisGroup Y(X); Pass *llvm::createGlobalsModRefPass() { return new GlobalsModRef(); } /// getUnderlyingObject - This traverses the use chain to figure out what object /// the specified value points to. If the value points to, or is derived from, /// a global object, return it. static Value *getUnderlyingObject(Value *V) { if (!isa(V->getType())) return V; // If we are at some type of object... return it. if (GlobalValue *GV = dyn_cast(V)) return GV; // Traverse through different addressing mechanisms. if (Instruction *I = dyn_cast(V)) { if (isa(I) || isa(I)) return getUnderlyingObject(I->getOperand(0)); } else if (ConstantExpr *CE = dyn_cast(V)) { if (CE->getOpcode() == Instruction::BitCast || CE->getOpcode() == Instruction::GetElementPtr) return getUnderlyingObject(CE->getOperand(0)); } // Otherwise, we don't know what this is, return it as the base pointer. return V; } /// AnalyzeGlobals - Scan through the users of all of the internal /// GlobalValue's in the program. If none of them have their "address taken" /// (really, their address passed to something nontrivial), record this fact, /// and record the functions that they are used directly in. void GlobalsModRef::AnalyzeGlobals(Module &M) { std::vector Readers, Writers; for (Module::iterator I = M.begin(), E = M.end(); I != E; ++I) if (I->hasInternalLinkage()) { if (!AnalyzeUsesOfPointer(I, Readers, Writers)) { // Remember that we are tracking this global. NonAddressTakenGlobals.insert(I); ++NumNonAddrTakenFunctions; } Readers.clear(); Writers.clear(); } for (Module::global_iterator I = M.global_begin(), E = M.global_end(); I != E; ++I) if (I->hasInternalLinkage()) { if (!AnalyzeUsesOfPointer(I, Readers, Writers)) { // Remember that we are tracking this global, and the mod/ref fns NonAddressTakenGlobals.insert(I); if (!Readers.empty()) // Some function read this global - remember that. ReadGlobals.insert(I); for (unsigned i = 0, e = Readers.size(); i != e; ++i) FunctionInfo[Readers[i]].GlobalInfo[I] |= Ref; if (!I->isConstant()) // No need to keep track of writers to constants for (unsigned i = 0, e = Writers.size(); i != e; ++i) FunctionInfo[Writers[i]].GlobalInfo[I] |= Mod; ++NumNonAddrTakenGlobalVars; // If this global holds a pointer type, see if it is an indirect global. if (isa(I->getType()->getElementType()) && AnalyzeIndirectGlobalMemory(I)) ++NumIndirectGlobalVars; } Readers.clear(); Writers.clear(); } } /// AnalyzeUsesOfPointer - Look at all of the users of the specified pointer. /// If this is used by anything complex (i.e., the address escapes), return /// true. Also, while we are at it, keep track of those functions that read and /// write to the value. /// /// If OkayStoreDest is non-null, stores into this global are allowed. bool GlobalsModRef::AnalyzeUsesOfPointer(Value *V, std::vector &Readers, std::vector &Writers, GlobalValue *OkayStoreDest) { if (!isa(V->getType())) return true; for (Value::use_iterator UI = V->use_begin(), E = V->use_end(); UI != E; ++UI) if (LoadInst *LI = dyn_cast(*UI)) { Readers.push_back(LI->getParent()->getParent()); } else if (StoreInst *SI = dyn_cast(*UI)) { if (V == SI->getOperand(1)) { Writers.push_back(SI->getParent()->getParent()); } else if (SI->getOperand(1) != OkayStoreDest) { return true; // Storing the pointer } } else if (GetElementPtrInst *GEP = dyn_cast(*UI)) { if (AnalyzeUsesOfPointer(GEP, Readers, Writers)) return true; } else if (CallInst *CI = dyn_cast(*UI)) { // Make sure that this is just the function being called, not that it is // passing into the function. for (unsigned i = 1, e = CI->getNumOperands(); i != e; ++i) if (CI->getOperand(i) == V) return true; } else if (InvokeInst *II = dyn_cast(*UI)) { // Make sure that this is just the function being called, not that it is // passing into the function. for (unsigned i = 3, e = II->getNumOperands(); i != e; ++i) if (II->getOperand(i) == V) return true; } else if (ConstantExpr *CE = dyn_cast(*UI)) { if (CE->getOpcode() == Instruction::GetElementPtr || CE->getOpcode() == Instruction::BitCast) { if (AnalyzeUsesOfPointer(CE, Readers, Writers)) return true; } else { return true; } } else if (ICmpInst *ICI = dyn_cast(*UI)) { if (!isa(ICI->getOperand(1))) return true; // Allow comparison against null. } else if (FreeInst *F = dyn_cast(*UI)) { Writers.push_back(F->getParent()->getParent()); } else { return true; } return false; } /// AnalyzeIndirectGlobalMemory - We found an non-address-taken global variable /// which holds a pointer type. See if the global always points to non-aliased /// heap memory: that is, all initializers of the globals are allocations, and /// those allocations have no use other than initialization of the global. /// Further, all loads out of GV must directly use the memory, not store the /// pointer somewhere. If this is true, we consider the memory pointed to by /// GV to be owned by GV and can disambiguate other pointers from it. bool GlobalsModRef::AnalyzeIndirectGlobalMemory(GlobalValue *GV) { // Keep track of values related to the allocation of the memory, f.e. the // value produced by the malloc call and any casts. std::vector AllocRelatedValues; // Walk the user list of the global. If we find anything other than a direct // load or store, bail out. for (Value::use_iterator I = GV->use_begin(), E = GV->use_end(); I != E; ++I){ if (LoadInst *LI = dyn_cast(*I)) { // The pointer loaded from the global can only be used in simple ways: // we allow addressing of it and loading storing to it. We do *not* allow // storing the loaded pointer somewhere else or passing to a function. std::vector ReadersWriters; if (AnalyzeUsesOfPointer(LI, ReadersWriters, ReadersWriters)) return false; // Loaded pointer escapes. // TODO: Could try some IP mod/ref of the loaded pointer. } else if (StoreInst *SI = dyn_cast(*I)) { // Storing the global itself. if (SI->getOperand(0) == GV) return false; // If storing the null pointer, ignore it. if (isa(SI->getOperand(0))) continue; // Check the value being stored. Value *Ptr = getUnderlyingObject(SI->getOperand(0)); if (isa(Ptr)) { // Okay, easy case. } else if (CallInst *CI = dyn_cast(Ptr)) { Function *F = CI->getCalledFunction(); if (!F || !F->isDeclaration()) return false; // Too hard to analyze. if (F->getName() != "calloc") return false; // Not calloc. } else { return false; // Too hard to analyze. } // Analyze all uses of the allocation. If any of them are used in a // non-simple way (e.g. stored to another global) bail out. std::vector ReadersWriters; if (AnalyzeUsesOfPointer(Ptr, ReadersWriters, ReadersWriters, GV)) return false; // Loaded pointer escapes. // Remember that this allocation is related to the indirect global. AllocRelatedValues.push_back(Ptr); } else { // Something complex, bail out. return false; } } // Okay, this is an indirect global. Remember all of the allocations for // this global in AllocsForIndirectGlobals. while (!AllocRelatedValues.empty()) { AllocsForIndirectGlobals[AllocRelatedValues.back()] = GV; AllocRelatedValues.pop_back(); } IndirectGlobals.insert(GV); return true; } /// AnalyzeCallGraph - At this point, we know the functions where globals are /// immediately stored to and read from. Propagate this information up the call /// graph to all callers and compute the mod/ref info for all memory for each /// function. void GlobalsModRef::AnalyzeCallGraph(CallGraph &CG, Module &M) { // We do a bottom-up SCC traversal of the call graph. In other words, we // visit all callees before callers (leaf-first). for (scc_iterator I = scc_begin(&CG), E = scc_end(&CG); I != E; ++I) { std::vector &SCC = *I; assert(!SCC.empty() && "SCC with no functions?"); if (!SCC[0]->getFunction()) { // Calls externally - can't say anything useful. Remove any existing // function records (may have been created when scanning globals). for (unsigned i = 0, e = SCC.size(); i != e; ++i) FunctionInfo.erase(SCC[i]->getFunction()); continue; } FunctionRecord &FR = FunctionInfo[SCC[0]->getFunction()]; bool KnowNothing = false; unsigned FunctionEffect = 0; // Collect the mod/ref properties due to called functions. We only compute // one mod-ref set. for (unsigned i = 0, e = SCC.size(); i != e && !KnowNothing; ++i) { Function *F = SCC[i]->getFunction(); if (!F) { KnowNothing = true; break; } if (F->isDeclaration()) { // Try to get mod/ref behaviour from function attributes. if (F->doesNotAccessMemory()) { // Can't do better than that! } else if (F->onlyReadsMemory()) { FunctionEffect |= Ref; // This function might call back into the module and read a global, so // mark all globals read somewhere as being read by this function. for (std::set::iterator GI = ReadGlobals.begin(), E = ReadGlobals.end(); GI != E; ++GI) FR.GlobalInfo[*GI] |= Ref; } else { // Can't say anything useful. KnowNothing = true; } continue; } for (CallGraphNode::iterator CI = SCC[i]->begin(), E = SCC[i]->end(); CI != E && !KnowNothing; ++CI) if (Function *Callee = CI->second->getFunction()) { if (FunctionRecord *CalleeFR = getFunctionInfo(Callee)) { // Propagate function effect up. FunctionEffect |= CalleeFR->FunctionEffect; // Incorporate callee's effects on globals into our info. for (std::map::iterator GI = CalleeFR->GlobalInfo.begin(), E = CalleeFR->GlobalInfo.end(); GI != E; ++GI) FR.GlobalInfo[GI->first] |= GI->second; } else { // Can't say anything about it. However, if it is inside our SCC, // then nothing needs to be done. CallGraphNode *CalleeNode = CG[Callee]; if (std::find(SCC.begin(), SCC.end(), CalleeNode) == SCC.end()) KnowNothing = true; } } else { KnowNothing = true; } } // If we can't say anything useful about this SCC, remove all SCC functions // from the FunctionInfo map. if (KnowNothing) { for (unsigned i = 0, e = SCC.size(); i != e; ++i) FunctionInfo.erase(SCC[i]->getFunction()); continue; } // Scan the function bodies for explicit loads or stores. for (unsigned i = 0, e = SCC.size(); i != e && FunctionEffect != ModRef;++i) for (inst_iterator II = inst_begin(SCC[i]->getFunction()), E = inst_end(SCC[i]->getFunction()); II != E && FunctionEffect != ModRef; ++II) if (isa(*II)) FunctionEffect |= Ref; else if (isa(*II)) FunctionEffect |= Mod; else if (isa(*II) || isa(*II)) FunctionEffect |= ModRef; if ((FunctionEffect & Mod) == 0) ++NumReadMemFunctions; if (FunctionEffect == 0) ++NumNoMemFunctions; FR.FunctionEffect = FunctionEffect; // Finally, now that we know the full effect on this SCC, clone the // information to each function in the SCC. for (unsigned i = 1, e = SCC.size(); i != e; ++i) FunctionInfo[SCC[i]->getFunction()] = FR; } } /// alias - If one of the pointers is to a global that we are tracking, and the /// other is some random pointer, we know there cannot be an alias, because the /// address of the global isn't taken. AliasAnalysis::AliasResult GlobalsModRef::alias(const Value *V1, unsigned V1Size, const Value *V2, unsigned V2Size) { // Get the base object these pointers point to. Value *UV1 = getUnderlyingObject(const_cast(V1)); Value *UV2 = getUnderlyingObject(const_cast(V2)); // If either of the underlying values is a global, they may be non-addr-taken // globals, which we can answer queries about. GlobalValue *GV1 = dyn_cast(UV1); GlobalValue *GV2 = dyn_cast(UV2); if (GV1 || GV2) { // If the global's address is taken, pretend we don't know it's a pointer to // the global. if (GV1 && !NonAddressTakenGlobals.count(GV1)) GV1 = 0; if (GV2 && !NonAddressTakenGlobals.count(GV2)) GV2 = 0; // If the the two pointers are derived from two different non-addr-taken // globals, or if one is and the other isn't, we know these can't alias. if ((GV1 || GV2) && GV1 != GV2) return NoAlias; // Otherwise if they are both derived from the same addr-taken global, we // can't know the two accesses don't overlap. } // These pointers may be based on the memory owned by an indirect global. If // so, we may be able to handle this. First check to see if the base pointer // is a direct load from an indirect global. GV1 = GV2 = 0; if (LoadInst *LI = dyn_cast(UV1)) if (GlobalVariable *GV = dyn_cast(LI->getOperand(0))) if (IndirectGlobals.count(GV)) GV1 = GV; if (LoadInst *LI = dyn_cast(UV2)) if (GlobalVariable *GV = dyn_cast(LI->getOperand(0))) if (IndirectGlobals.count(GV)) GV2 = GV; // These pointers may also be from an allocation for the indirect global. If // so, also handle them. if (AllocsForIndirectGlobals.count(UV1)) GV1 = AllocsForIndirectGlobals[UV1]; if (AllocsForIndirectGlobals.count(UV2)) GV2 = AllocsForIndirectGlobals[UV2]; // Now that we know whether the two pointers are related to indirect globals, // use this to disambiguate the pointers. If either pointer is based on an // indirect global and if they are not both based on the same indirect global, // they cannot alias. if ((GV1 || GV2) && GV1 != GV2) return NoAlias; return AliasAnalysis::alias(V1, V1Size, V2, V2Size); } AliasAnalysis::ModRefResult GlobalsModRef::getModRefInfo(CallSite CS, Value *P, unsigned Size) { unsigned Known = ModRef; // If we are asking for mod/ref info of a direct call with a pointer to a // global we are tracking, return information if we have it. if (GlobalValue *GV = dyn_cast(getUnderlyingObject(P))) if (GV->hasInternalLinkage()) if (Function *F = CS.getCalledFunction()) if (NonAddressTakenGlobals.count(GV)) if (FunctionRecord *FR = getFunctionInfo(F)) Known = FR->getInfoForGlobal(GV); if (Known == NoModRef) return NoModRef; // No need to query other mod/ref analyses return ModRefResult(Known & AliasAnalysis::getModRefInfo(CS, P, Size)); } //===----------------------------------------------------------------------===// // Methods to update the analysis as a result of the client transformation. // void GlobalsModRef::deleteValue(Value *V) { if (GlobalValue *GV = dyn_cast(V)) { if (NonAddressTakenGlobals.erase(GV)) { // This global might be an indirect global. If so, remove it and remove // any AllocRelatedValues for it. if (IndirectGlobals.erase(GV)) { // Remove any entries in AllocsForIndirectGlobals for this global. for (std::map::iterator I = AllocsForIndirectGlobals.begin(), E = AllocsForIndirectGlobals.end(); I != E; ) { if (I->second == GV) { AllocsForIndirectGlobals.erase(I++); } else { ++I; } } } } } // Otherwise, if this is an allocation related to an indirect global, remove // it. AllocsForIndirectGlobals.erase(V); AliasAnalysis::deleteValue(V); } void GlobalsModRef::copyValue(Value *From, Value *To) { AliasAnalysis::copyValue(From, To); }