LLVM API Documentation

GlobalsModRef.cpp

Go to the documentation of this file.
00001 //===- GlobalsModRef.cpp - Simple Mod/Ref Analysis for Globals ------------===//
00002 //
00003 //                     The LLVM Compiler Infrastructure
00004 //
00005 // This file was developed by the LLVM research group and is distributed under
00006 // the University of Illinois Open Source License. See LICENSE.TXT for details.
00007 //
00008 //===----------------------------------------------------------------------===//
00009 //
00010 // This simple pass provides alias and mod/ref information for global values
00011 // that do not have their address taken, and keeps track of whether functions
00012 // read or write memory (are "pure").  For this simple (but very common) case,
00013 // we can provide pretty accurate and useful information.
00014 //
00015 //===----------------------------------------------------------------------===//
00016 
00017 #include "llvm/Analysis/Passes.h"
00018 #include "llvm/Module.h"
00019 #include "llvm/Pass.h"
00020 #include "llvm/Instructions.h"
00021 #include "llvm/Constants.h"
00022 #include "llvm/Analysis/AliasAnalysis.h"
00023 #include "llvm/Analysis/CallGraph.h"
00024 #include "llvm/Support/InstIterator.h"
00025 #include "llvm/Support/CommandLine.h"
00026 #include "llvm/ADT/Statistic.h"
00027 #include "llvm/ADT/SCCIterator.h"
00028 #include <set>
00029 using namespace llvm;
00030 
00031 namespace {
00032   Statistic<>
00033   NumNonAddrTakenGlobalVars("globalsmodref-aa",
00034                             "Number of global vars without address taken");
00035   Statistic<>
00036   NumNonAddrTakenFunctions("globalsmodref-aa",
00037                            "Number of functions without address taken");
00038   Statistic<>
00039   NumNoMemFunctions("globalsmodref-aa",
00040                     "Number of functions that do not access memory");
00041   Statistic<>
00042   NumReadMemFunctions("globalsmodref-aa",
00043                       "Number of functions that only read memory");
00044 
00045   /// FunctionRecord - One instance of this structure is stored for every
00046   /// function in the program.  Later, the entries for these functions are
00047   /// removed if the function is found to call an external function (in which
00048   /// case we know nothing about it.
00049   struct FunctionRecord {
00050     /// GlobalInfo - Maintain mod/ref info for all of the globals without
00051     /// addresses taken that are read or written (transitively) by this
00052     /// function.
00053     std::map<GlobalValue*, unsigned> GlobalInfo;
00054 
00055     unsigned getInfoForGlobal(GlobalValue *GV) const {
00056       std::map<GlobalValue*, unsigned>::const_iterator I = GlobalInfo.find(GV);
00057       if (I != GlobalInfo.end())
00058         return I->second;
00059       return 0;
00060     }
00061 
00062     /// FunctionEffect - Capture whether or not this function reads or writes to
00063     /// ANY memory.  If not, we can do a lot of aggressive analysis on it.
00064     unsigned FunctionEffect;
00065 
00066     FunctionRecord() : FunctionEffect(0) {}
00067   };
00068 
00069   /// GlobalsModRef - The actual analysis pass.
00070   class GlobalsModRef : public ModulePass, public AliasAnalysis {
00071     /// NonAddressTakenGlobals - The globals that do not have their addresses
00072     /// taken.
00073     std::set<GlobalValue*> NonAddressTakenGlobals;
00074 
00075     /// FunctionInfo - For each function, keep track of what globals are
00076     /// modified or read.
00077     std::map<Function*, FunctionRecord> FunctionInfo;
00078 
00079   public:
00080     bool runOnModule(Module &M) {
00081       InitializeAliasAnalysis(this);                 // set up super class
00082       AnalyzeGlobals(M);                          // find non-addr taken globals
00083       AnalyzeCallGraph(getAnalysis<CallGraph>(), M); // Propagate on CG
00084       return false;
00085     }
00086 
00087     virtual void getAnalysisUsage(AnalysisUsage &AU) const {
00088       AliasAnalysis::getAnalysisUsage(AU);
00089       AU.addRequired<CallGraph>();
00090       AU.setPreservesAll();                         // Does not transform code
00091     }
00092 
00093     //------------------------------------------------
00094     // Implement the AliasAnalysis API
00095     //
00096     AliasResult alias(const Value *V1, unsigned V1Size,
00097                       const Value *V2, unsigned V2Size);
00098     ModRefResult getModRefInfo(CallSite CS, Value *P, unsigned Size);
00099     ModRefResult getModRefInfo(CallSite CS1, CallSite CS2) {
00100       return AliasAnalysis::getModRefInfo(CS1,CS2);
00101     }
00102     bool hasNoModRefInfoForCalls() const { return false; }
00103 
00104     /// getModRefBehavior - Return the behavior of the specified function if
00105     /// called from the specified call site.  The call site may be null in which
00106     /// case the most generic behavior of this function should be returned.
00107     virtual ModRefBehavior getModRefBehavior(Function *F, CallSite CS,
00108                                          std::vector<PointerAccessInfo> *Info) {
00109       if (FunctionRecord *FR = getFunctionInfo(F))
00110         if (FR->FunctionEffect == 0)
00111           return DoesNotAccessMemory;
00112         else if ((FR->FunctionEffect & Mod) == 0)
00113           return OnlyReadsMemory;
00114       return AliasAnalysis::getModRefBehavior(F, CS, Info);
00115     }
00116 
00117     virtual void deleteValue(Value *V);
00118     virtual void copyValue(Value *From, Value *To);
00119 
00120   private:
00121     /// getFunctionInfo - Return the function info for the function, or null if
00122     /// the function calls an external function (in which case we don't have
00123     /// anything useful to say about it).
00124     FunctionRecord *getFunctionInfo(Function *F) {
00125       std::map<Function*, FunctionRecord>::iterator I = FunctionInfo.find(F);
00126       if (I != FunctionInfo.end())
00127         return &I->second;
00128       return 0;
00129     }
00130 
00131     void AnalyzeGlobals(Module &M);
00132     void AnalyzeCallGraph(CallGraph &CG, Module &M);
00133     void AnalyzeSCC(std::vector<CallGraphNode *> &SCC);
00134     bool AnalyzeUsesOfGlobal(Value *V, std::vector<Function*> &Readers,
00135                              std::vector<Function*> &Writers);
00136   };
00137 
00138   RegisterOpt<GlobalsModRef> X("globalsmodref-aa",
00139                                "Simple mod/ref analysis for globals");
00140   RegisterAnalysisGroup<AliasAnalysis, GlobalsModRef> Y;
00141 }
00142 
00143 Pass *llvm::createGlobalsModRefPass() { return new GlobalsModRef(); }
00144 
00145 
00146 /// AnalyzeGlobalUses - Scan through the users of all of the internal
00147 /// GlobalValue's in the program.  If none of them have their "Address taken"
00148 /// (really, their address passed to something nontrivial), record this fact,
00149 /// and record the functions that they are used directly in.
00150 void GlobalsModRef::AnalyzeGlobals(Module &M) {
00151   std::vector<Function*> Readers, Writers;
00152   for (Module::iterator I = M.begin(), E = M.end(); I != E; ++I)
00153     if (I->hasInternalLinkage()) {
00154       if (!AnalyzeUsesOfGlobal(I, Readers, Writers)) {
00155         // Remember that we are tracking this global.
00156         NonAddressTakenGlobals.insert(I);
00157         ++NumNonAddrTakenFunctions;
00158       }
00159       Readers.clear(); Writers.clear();
00160     }
00161 
00162   for (Module::global_iterator I = M.global_begin(), E = M.global_end();
00163        I != E; ++I)
00164     if (I->hasInternalLinkage()) {
00165       if (!AnalyzeUsesOfGlobal(I, Readers, Writers)) {
00166         // Remember that we are tracking this global, and the mod/ref fns
00167         NonAddressTakenGlobals.insert(I);
00168         for (unsigned i = 0, e = Readers.size(); i != e; ++i)
00169           FunctionInfo[Readers[i]].GlobalInfo[I] |= Ref;
00170 
00171         if (!I->isConstant())  // No need to keep track of writers to constants
00172           for (unsigned i = 0, e = Writers.size(); i != e; ++i)
00173             FunctionInfo[Writers[i]].GlobalInfo[I] |= Mod;
00174         ++NumNonAddrTakenGlobalVars;
00175       }
00176       Readers.clear(); Writers.clear();
00177     }
00178 }
00179 
00180 /// AnalyzeUsesOfGlobal - Look at all of the users of the specified global value
00181 /// derived pointer.  If this is used by anything complex (i.e., the address
00182 /// escapes), return true.  Also, while we are at it, keep track of those
00183 /// functions that read and write to the value.
00184 bool GlobalsModRef::AnalyzeUsesOfGlobal(Value *V,
00185                                         std::vector<Function*> &Readers,
00186                                         std::vector<Function*> &Writers) {
00187   if (!isa<PointerType>(V->getType())) return true;
00188 
00189   for (Value::use_iterator UI = V->use_begin(), E = V->use_end(); UI != E; ++UI)
00190     if (LoadInst *LI = dyn_cast<LoadInst>(*UI)) {
00191       Readers.push_back(LI->getParent()->getParent());
00192     } else if (StoreInst *SI = dyn_cast<StoreInst>(*UI)) {
00193       if (V == SI->getOperand(0)) return true;  // Storing the pointer
00194       Writers.push_back(SI->getParent()->getParent());
00195     } else if (GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(*UI)) {
00196       if (AnalyzeUsesOfGlobal(GEP, Readers, Writers)) return true;
00197     } else if (CallInst *CI = dyn_cast<CallInst>(*UI)) {
00198       // Make sure that this is just the function being called, not that it is
00199       // passing into the function.
00200       for (unsigned i = 1, e = CI->getNumOperands(); i != e; ++i)
00201         if (CI->getOperand(i) == V) return true;
00202     } else if (InvokeInst *II = dyn_cast<InvokeInst>(*UI)) {
00203       // Make sure that this is just the function being called, not that it is
00204       // passing into the function.
00205       for (unsigned i = 3, e = II->getNumOperands(); i != e; ++i)
00206         if (II->getOperand(i) == V) return true;
00207     } else if (ConstantExpr *CE = dyn_cast<ConstantExpr>(*UI)) {
00208       if (CE->getOpcode() == Instruction::GetElementPtr ||
00209           CE->getOpcode() == Instruction::Cast) {
00210         if (AnalyzeUsesOfGlobal(CE, Readers, Writers))
00211           return true;
00212       } else {
00213         return true;
00214       }
00215     } else if (GlobalValue *GV = dyn_cast<GlobalValue>(*UI)) {
00216       if (AnalyzeUsesOfGlobal(GV, Readers, Writers)) return true;
00217     } else {
00218       return true;
00219     }
00220   return false;
00221 }
00222 
00223 /// AnalyzeCallGraph - At this point, we know the functions where globals are
00224 /// immediately stored to and read from.  Propagate this information up the call
00225 /// graph to all callers and compute the mod/ref info for all memory for each
00226 /// function.
00227 void GlobalsModRef::AnalyzeCallGraph(CallGraph &CG, Module &M) {
00228   // We do a bottom-up SCC traversal of the call graph.  In other words, we
00229   // visit all callees before callers (leaf-first).
00230   for (scc_iterator<CallGraph*> I = scc_begin(&CG), E = scc_end(&CG); I!=E; ++I)
00231     if ((*I).size() != 1) {
00232       AnalyzeSCC(*I);
00233     } else if (Function *F = (*I)[0]->getFunction()) {
00234       if (!F->isExternal()) {
00235         // Nonexternal function.
00236         AnalyzeSCC(*I);
00237       } else {
00238         // Otherwise external function.  Handle intrinsics and other special
00239         // cases here.
00240         if (getAnalysis<AliasAnalysis>().doesNotAccessMemory(F))
00241           // If it does not access memory, process the function, causing us to
00242           // realize it doesn't do anything (the body is empty).
00243           AnalyzeSCC(*I);
00244         else {
00245           // Otherwise, don't process it.  This will cause us to conservatively
00246           // assume the worst.
00247         }
00248       }
00249     } else {
00250       // Do not process the external node, assume the worst.
00251     }
00252 }
00253 
00254 void GlobalsModRef::AnalyzeSCC(std::vector<CallGraphNode *> &SCC) {
00255   assert(!SCC.empty() && "SCC with no functions?");
00256   FunctionRecord &FR = FunctionInfo[SCC[0]->getFunction()];
00257 
00258   bool CallsExternal = false;
00259   unsigned FunctionEffect = 0;
00260 
00261   // Collect the mod/ref properties due to called functions.  We only compute
00262   // one mod-ref set
00263   for (unsigned i = 0, e = SCC.size(); i != e && !CallsExternal; ++i)
00264     for (CallGraphNode::iterator CI = SCC[i]->begin(), E = SCC[i]->end();
00265          CI != E; ++CI)
00266       if (Function *Callee = (*CI)->getFunction()) {
00267         if (FunctionRecord *CalleeFR = getFunctionInfo(Callee)) {
00268           // Propagate function effect up.
00269           FunctionEffect |= CalleeFR->FunctionEffect;
00270 
00271           // Incorporate callee's effects on globals into our info.
00272           for (std::map<GlobalValue*, unsigned>::iterator GI =
00273                  CalleeFR->GlobalInfo.begin(), E = CalleeFR->GlobalInfo.end();
00274                GI != E; ++GI)
00275             FR.GlobalInfo[GI->first] |= GI->second;
00276 
00277         } else {
00278           // Okay, if we can't say anything about it, maybe some other alias
00279           // analysis can.
00280           ModRefBehavior MRB =
00281             AliasAnalysis::getModRefBehavior(Callee, CallSite());
00282           if (MRB != DoesNotAccessMemory) {
00283             // FIXME: could make this more aggressive for functions that just
00284             // read memory.  We should just say they read all globals.
00285             CallsExternal = true;
00286             break;
00287           }
00288         }
00289       } else {
00290         CallsExternal = true;
00291         break;
00292       }
00293 
00294   // If this SCC calls an external function, we can't say anything about it, so
00295   // remove all SCC functions from the FunctionInfo map.
00296   if (CallsExternal) {
00297     for (unsigned i = 0, e = SCC.size(); i != e; ++i)
00298       FunctionInfo.erase(SCC[i]->getFunction());
00299     return;
00300   }
00301 
00302   // Otherwise, unless we already know that this function mod/refs memory, scan
00303   // the function bodies to see if there are any explicit loads or stores.
00304   if (FunctionEffect != ModRef) {
00305     for (unsigned i = 0, e = SCC.size(); i != e && FunctionEffect != ModRef;++i)
00306       for (inst_iterator II = inst_begin(SCC[i]->getFunction()),
00307              E = inst_end(SCC[i]->getFunction());
00308            II != E && FunctionEffect != ModRef; ++II)
00309         if (isa<LoadInst>(*II))
00310           FunctionEffect |= Ref;
00311         else if (isa<StoreInst>(*II))
00312           FunctionEffect |= Mod;
00313         else if (isa<MallocInst>(*II) || isa<FreeInst>(*II))
00314           FunctionEffect |= ModRef;
00315   }
00316 
00317   if ((FunctionEffect & Mod) == 0)
00318     ++NumReadMemFunctions;
00319   if (FunctionEffect == 0)
00320     ++NumNoMemFunctions;
00321   FR.FunctionEffect = FunctionEffect;
00322 
00323   // Finally, now that we know the full effect on this SCC, clone the
00324   // information to each function in the SCC.
00325   for (unsigned i = 1, e = SCC.size(); i != e; ++i)
00326     FunctionInfo[SCC[i]->getFunction()] = FR;
00327 }
00328 
00329 
00330 
00331 /// getUnderlyingObject - This traverses the use chain to figure out what object
00332 /// the specified value points to.  If the value points to, or is derived from,
00333 /// a global object, return it.
00334 static const GlobalValue *getUnderlyingObject(const Value *V) {
00335   if (!isa<PointerType>(V->getType())) return 0;
00336 
00337   // If we are at some type of object... return it.
00338   if (const GlobalValue *GV = dyn_cast<GlobalValue>(V)) return GV;
00339 
00340   // Traverse through different addressing mechanisms...
00341   if (const Instruction *I = dyn_cast<Instruction>(V)) {
00342     if (isa<CastInst>(I) || isa<GetElementPtrInst>(I))
00343       return getUnderlyingObject(I->getOperand(0));
00344   } else if (const ConstantExpr *CE = dyn_cast<ConstantExpr>(V)) {
00345     if (CE->getOpcode() == Instruction::Cast ||
00346         CE->getOpcode() == Instruction::GetElementPtr)
00347       return getUnderlyingObject(CE->getOperand(0));
00348   }
00349   return 0;
00350 }
00351 
00352 /// alias - If one of the pointers is to a global that we are tracking, and the
00353 /// other is some random pointer, we know there cannot be an alias, because the
00354 /// address of the global isn't taken.
00355 AliasAnalysis::AliasResult
00356 GlobalsModRef::alias(const Value *V1, unsigned V1Size,
00357                      const Value *V2, unsigned V2Size) {
00358   GlobalValue *GV1 = const_cast<GlobalValue*>(getUnderlyingObject(V1));
00359   GlobalValue *GV2 = const_cast<GlobalValue*>(getUnderlyingObject(V2));
00360 
00361   // If the global's address is taken, pretend we don't know it's a pointer to
00362   // the global.
00363   if (GV1 && !NonAddressTakenGlobals.count(GV1)) GV1 = 0;
00364   if (GV2 && !NonAddressTakenGlobals.count(GV2)) GV2 = 0;
00365 
00366   if ((GV1 || GV2) && GV1 != GV2)
00367     return NoAlias;
00368 
00369   return AliasAnalysis::alias(V1, V1Size, V2, V2Size);
00370 }
00371 
00372 AliasAnalysis::ModRefResult
00373 GlobalsModRef::getModRefInfo(CallSite CS, Value *P, unsigned Size) {
00374   unsigned Known = ModRef;
00375 
00376   // If we are asking for mod/ref info of a direct call with a pointer to a
00377   // global we are tracking, return information if we have it.
00378   if (GlobalValue *GV = const_cast<GlobalValue*>(getUnderlyingObject(P)))
00379     if (GV->hasInternalLinkage())
00380       if (Function *F = CS.getCalledFunction())
00381         if (NonAddressTakenGlobals.count(GV))
00382           if (FunctionRecord *FR = getFunctionInfo(F))
00383             Known = FR->getInfoForGlobal(GV);
00384 
00385   if (Known == NoModRef)
00386     return NoModRef; // No need to query other mod/ref analyses
00387   return ModRefResult(Known & AliasAnalysis::getModRefInfo(CS, P, Size));
00388 }
00389 
00390 
00391 //===----------------------------------------------------------------------===//
00392 // Methods to update the analysis as a result of the client transformation.
00393 //
00394 void GlobalsModRef::deleteValue(Value *V) {
00395   if (GlobalValue *GV = dyn_cast<GlobalValue>(V))
00396     NonAddressTakenGlobals.erase(GV);
00397 }
00398 
00399 void GlobalsModRef::copyValue(Value *From, Value *To) {
00400 }