//===- GlobalsModRef.cpp - Simple Mod/Ref Analysis for Globals ------------===// // // The LLVM Compiler Infrastructure // // This file was developed by the LLVM research group and 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. // //===----------------------------------------------------------------------===// #include "llvm/Analysis/Passes.h" #include "llvm/Module.h" #include "llvm/Pass.h" #include "llvm/Instructions.h" #include "llvm/Constants.h" #include "llvm/Analysis/AliasAnalysis.h" #include "llvm/Analysis/CallGraph.h" #include "llvm/Support/InstIterator.h" #include "llvm/Support/CommandLine.h" #include "llvm/ADT/Statistic.h" #include "llvm/ADT/SCCIterator.h" #include using namespace llvm; namespace { Statistic<> NumNonAddrTakenGlobalVars("globalsmodref-aa", "Number of global vars without address taken"); Statistic<> NumNonAddrTakenFunctions("globalsmodref-aa", "Number of functions without address taken"); Statistic<> NumNoMemFunctions("globalsmodref-aa", "Number of functions that do not access memory"); Statistic<> NumReadMemFunctions("globalsmodref-aa", "Number of functions that only read memory"); /// 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 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 GlobalsModRef : public ModulePass, public AliasAnalysis { /// NonAddressTakenGlobals - The globals that do not have their addresses /// taken. std::set NonAddressTakenGlobals; /// FunctionInfo - For each function, keep track of what globals are /// modified or read. std::map FunctionInfo; public: 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 /// the function calls an external function (in which case 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); void AnalyzeSCC(std::vector &SCC); bool AnalyzeUsesOfGlobal(Value *V, std::vector &Readers, std::vector &Writers); }; RegisterOpt X("globalsmodref-aa", "Simple mod/ref analysis for globals"); RegisterAnalysisGroup Y; } Pass *llvm::createGlobalsModRefPass() { return new GlobalsModRef(); } /// AnalyzeGlobalUses - 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 (!AnalyzeUsesOfGlobal(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 (!AnalyzeUsesOfGlobal(I, Readers, Writers)) { // Remember that we are tracking this global, and the mod/ref fns NonAddressTakenGlobals.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; } Readers.clear(); Writers.clear(); } } /// AnalyzeUsesOfGlobal - Look at all of the users of the specified global value /// derived 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. bool GlobalsModRef::AnalyzeUsesOfGlobal(Value *V, std::vector &Readers, std::vector &Writers) { 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(0)) return true; // Storing the pointer Writers.push_back(SI->getParent()->getParent()); } else if (GetElementPtrInst *GEP = dyn_cast(*UI)) { if (AnalyzeUsesOfGlobal(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::Cast) { if (AnalyzeUsesOfGlobal(CE, Readers, Writers)) return true; } else { return true; } } else if (GlobalValue *GV = dyn_cast(*UI)) { if (AnalyzeUsesOfGlobal(GV, Readers, Writers)) return true; } else { return true; } return false; } /// 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) if ((*I).size() != 1) { AnalyzeSCC(*I); } else if (Function *F = (*I)[0]->getFunction()) { if (!F->isExternal()) { // Nonexternal function. AnalyzeSCC(*I); } else { // Otherwise external function. Handle intrinsics and other special // cases here. if (getAnalysis().doesNotAccessMemory(F)) // If it does not access memory, process the function, causing us to // realize it doesn't do anything (the body is empty). AnalyzeSCC(*I); else { // Otherwise, don't process it. This will cause us to conservatively // assume the worst. } } } else { // Do not process the external node, assume the worst. } } void GlobalsModRef::AnalyzeSCC(std::vector &SCC) { assert(!SCC.empty() && "SCC with no functions?"); FunctionRecord &FR = FunctionInfo[SCC[0]->getFunction()]; bool CallsExternal = 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 && !CallsExternal; ++i) for (CallGraphNode::iterator CI = SCC[i]->begin(), E = SCC[i]->end(); CI != E; ++CI) if (Function *Callee = (*CI)->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 { // Okay, if we can't say anything about it, maybe some other alias // analysis can. ModRefBehavior MRB = AliasAnalysis::getModRefBehavior(Callee, CallSite()); if (MRB != DoesNotAccessMemory) { // FIXME: could make this more aggressive for functions that just // read memory. We should just say they read all globals. CallsExternal = true; break; } } } else { CallsExternal = true; break; } // If this SCC calls an external function, we can't say anything about it, so // remove all SCC functions from the FunctionInfo map. if (CallsExternal) { for (unsigned i = 0, e = SCC.size(); i != e; ++i) FunctionInfo.erase(SCC[i]->getFunction()); return; } // Otherwise, unless we already know that this function mod/refs memory, scan // the function bodies to see if there are any explicit loads or stores. if (FunctionEffect != ModRef) { 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; } /// 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 const GlobalValue *getUnderlyingObject(const Value *V) { if (!isa(V->getType())) return 0; // If we are at some type of object... return it. if (const GlobalValue *GV = dyn_cast(V)) return GV; // Traverse through different addressing mechanisms... if (const Instruction *I = dyn_cast(V)) { if (isa(I) || isa(I)) return getUnderlyingObject(I->getOperand(0)); } else if (const ConstantExpr *CE = dyn_cast(V)) { if (CE->getOpcode() == Instruction::Cast || CE->getOpcode() == Instruction::GetElementPtr) return getUnderlyingObject(CE->getOperand(0)); } return 0; } /// 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) { GlobalValue *GV1 = const_cast(getUnderlyingObject(V1)); GlobalValue *GV2 = const_cast(getUnderlyingObject(V2)); // 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 ((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 = const_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)) NonAddressTakenGlobals.erase(GV); } void GlobalsModRef::copyValue(Value *From, Value *To) { }