1 //===- GlobalsModRef.cpp - Simple Mod/Ref Analysis for Globals ------------===//
3 // The LLVM Compiler Infrastructure
5 // This file was developed by the LLVM research group and is distributed under
6 // the University of Illinois Open Source License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
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.
15 //===----------------------------------------------------------------------===//
17 #include "llvm/Analysis/Passes.h"
18 #include "llvm/Module.h"
19 #include "llvm/Pass.h"
20 #include "llvm/Instructions.h"
21 #include "llvm/Constants.h"
22 #include "llvm/Analysis/AliasAnalysis.h"
23 #include "llvm/Analysis/CallGraph.h"
24 #include "llvm/Support/InstIterator.h"
25 #include "llvm/Support/CommandLine.h"
26 #include "llvm/ADT/Statistic.h"
27 #include "llvm/ADT/SCCIterator.h"
33 NumNonAddrTakenGlobalVars("globalsmodref-aa",
34 "Number of global vars without address taken");
36 NumNonAddrTakenFunctions("globalsmodref-aa",
37 "Number of functions without address taken");
39 NumNoMemFunctions("globalsmodref-aa",
40 "Number of functions that do not access memory");
42 NumReadMemFunctions("globalsmodref-aa",
43 "Number of functions that only read memory");
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
53 std::map<GlobalValue*, unsigned> GlobalInfo;
55 unsigned getInfoForGlobal(GlobalValue *GV) const {
56 std::map<GlobalValue*, unsigned>::const_iterator I = GlobalInfo.find(GV);
57 if (I != GlobalInfo.end())
62 /// FunctionEffect - Capture whether or not this function reads or writes to
63 /// ANY memory. If not, we can do a lot of aggressive analysis on it.
64 unsigned FunctionEffect;
66 FunctionRecord() : FunctionEffect(0) {}
69 /// GlobalsModRef - The actual analysis pass.
70 class GlobalsModRef : public ModulePass, public AliasAnalysis {
71 /// NonAddressTakenGlobals - The globals that do not have their addresses
73 std::set<GlobalValue*> NonAddressTakenGlobals;
75 /// FunctionInfo - For each function, keep track of what globals are
77 std::map<Function*, FunctionRecord> FunctionInfo;
80 bool runOnModule(Module &M) {
81 InitializeAliasAnalysis(this); // set up super class
82 AnalyzeGlobals(M); // find non-addr taken globals
83 AnalyzeCallGraph(getAnalysis<CallGraph>(), M); // Propagate on CG
87 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
88 AliasAnalysis::getAnalysisUsage(AU);
89 AU.addRequired<CallGraph>();
90 AU.setPreservesAll(); // Does not transform code
93 //------------------------------------------------
94 // Implement the AliasAnalysis API
96 AliasResult alias(const Value *V1, unsigned V1Size,
97 const Value *V2, unsigned V2Size);
98 ModRefResult getModRefInfo(CallSite CS, Value *P, unsigned Size);
99 ModRefResult getModRefInfo(CallSite CS1, CallSite CS2) {
100 return AliasAnalysis::getModRefInfo(CS1,CS2);
102 bool hasNoModRefInfoForCalls() const { return false; }
104 /// getModRefBehavior - Return the behavior of the specified function if
105 /// called from the specified call site. The call site may be null in which
106 /// case the most generic behavior of this function should be returned.
107 virtual ModRefBehavior getModRefBehavior(Function *F, CallSite CS,
108 std::vector<PointerAccessInfo> *Info) {
109 if (FunctionRecord *FR = getFunctionInfo(F))
110 if (FR->FunctionEffect == 0)
111 return DoesNotAccessMemory;
112 else if ((FR->FunctionEffect & Mod) == 0)
113 return OnlyReadsMemory;
114 return AliasAnalysis::getModRefBehavior(F, CS, Info);
117 virtual void deleteValue(Value *V);
118 virtual void copyValue(Value *From, Value *To);
121 /// getFunctionInfo - Return the function info for the function, or null if
122 /// the function calls an external function (in which case we don't have
123 /// anything useful to say about it).
124 FunctionRecord *getFunctionInfo(Function *F) {
125 std::map<Function*, FunctionRecord>::iterator I = FunctionInfo.find(F);
126 if (I != FunctionInfo.end())
131 void AnalyzeGlobals(Module &M);
132 void AnalyzeCallGraph(CallGraph &CG, Module &M);
133 void AnalyzeSCC(std::vector<CallGraphNode *> &SCC);
134 bool AnalyzeUsesOfGlobal(Value *V, std::vector<Function*> &Readers,
135 std::vector<Function*> &Writers);
138 RegisterOpt<GlobalsModRef> X("globalsmodref-aa",
139 "Simple mod/ref analysis for globals");
140 RegisterAnalysisGroup<AliasAnalysis, GlobalsModRef> Y;
143 Pass *llvm::createGlobalsModRefPass() { return new GlobalsModRef(); }
146 /// AnalyzeGlobalUses - Scan through the users of all of the internal
147 /// GlobalValue's in the program. If none of them have their "Address taken"
148 /// (really, their address passed to something nontrivial), record this fact,
149 /// and record the functions that they are used directly in.
150 void GlobalsModRef::AnalyzeGlobals(Module &M) {
151 std::vector<Function*> Readers, Writers;
152 for (Module::iterator I = M.begin(), E = M.end(); I != E; ++I)
153 if (I->hasInternalLinkage()) {
154 if (!AnalyzeUsesOfGlobal(I, Readers, Writers)) {
155 // Remember that we are tracking this global.
156 NonAddressTakenGlobals.insert(I);
157 ++NumNonAddrTakenFunctions;
159 Readers.clear(); Writers.clear();
162 for (Module::global_iterator I = M.global_begin(), E = M.global_end();
164 if (I->hasInternalLinkage()) {
165 if (!AnalyzeUsesOfGlobal(I, Readers, Writers)) {
166 // Remember that we are tracking this global, and the mod/ref fns
167 NonAddressTakenGlobals.insert(I);
168 for (unsigned i = 0, e = Readers.size(); i != e; ++i)
169 FunctionInfo[Readers[i]].GlobalInfo[I] |= Ref;
171 if (!I->isConstant()) // No need to keep track of writers to constants
172 for (unsigned i = 0, e = Writers.size(); i != e; ++i)
173 FunctionInfo[Writers[i]].GlobalInfo[I] |= Mod;
174 ++NumNonAddrTakenGlobalVars;
176 Readers.clear(); Writers.clear();
180 /// AnalyzeUsesOfGlobal - Look at all of the users of the specified global value
181 /// derived pointer. If this is used by anything complex (i.e., the address
182 /// escapes), return true. Also, while we are at it, keep track of those
183 /// functions that read and write to the value.
184 bool GlobalsModRef::AnalyzeUsesOfGlobal(Value *V,
185 std::vector<Function*> &Readers,
186 std::vector<Function*> &Writers) {
187 if (!isa<PointerType>(V->getType())) return true;
189 for (Value::use_iterator UI = V->use_begin(), E = V->use_end(); UI != E; ++UI)
190 if (LoadInst *LI = dyn_cast<LoadInst>(*UI)) {
191 Readers.push_back(LI->getParent()->getParent());
192 } else if (StoreInst *SI = dyn_cast<StoreInst>(*UI)) {
193 if (V == SI->getOperand(0)) return true; // Storing the pointer
194 Writers.push_back(SI->getParent()->getParent());
195 } else if (GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(*UI)) {
196 if (AnalyzeUsesOfGlobal(GEP, Readers, Writers)) return true;
197 } else if (CallInst *CI = dyn_cast<CallInst>(*UI)) {
198 // Make sure that this is just the function being called, not that it is
199 // passing into the function.
200 for (unsigned i = 1, e = CI->getNumOperands(); i != e; ++i)
201 if (CI->getOperand(i) == V) return true;
202 } else if (InvokeInst *II = dyn_cast<InvokeInst>(*UI)) {
203 // Make sure that this is just the function being called, not that it is
204 // passing into the function.
205 for (unsigned i = 3, e = II->getNumOperands(); i != e; ++i)
206 if (II->getOperand(i) == V) return true;
207 } else if (ConstantExpr *CE = dyn_cast<ConstantExpr>(*UI)) {
208 if (CE->getOpcode() == Instruction::GetElementPtr ||
209 CE->getOpcode() == Instruction::Cast) {
210 if (AnalyzeUsesOfGlobal(CE, Readers, Writers))
215 } else if (GlobalValue *GV = dyn_cast<GlobalValue>(*UI)) {
216 if (AnalyzeUsesOfGlobal(GV, Readers, Writers)) return true;
223 /// AnalyzeCallGraph - At this point, we know the functions where globals are
224 /// immediately stored to and read from. Propagate this information up the call
225 /// graph to all callers and compute the mod/ref info for all memory for each
227 void GlobalsModRef::AnalyzeCallGraph(CallGraph &CG, Module &M) {
228 // We do a bottom-up SCC traversal of the call graph. In other words, we
229 // visit all callees before callers (leaf-first).
230 for (scc_iterator<CallGraph*> I = scc_begin(&CG), E = scc_end(&CG); I!=E; ++I)
231 if ((*I).size() != 1) {
233 } else if (Function *F = (*I)[0]->getFunction()) {
234 if (!F->isExternal()) {
235 // Nonexternal function.
238 // Otherwise external function. Handle intrinsics and other special
240 if (getAnalysis<AliasAnalysis>().doesNotAccessMemory(F))
241 // If it does not access memory, process the function, causing us to
242 // realize it doesn't do anything (the body is empty).
245 // Otherwise, don't process it. This will cause us to conservatively
250 // Do not process the external node, assume the worst.
254 void GlobalsModRef::AnalyzeSCC(std::vector<CallGraphNode *> &SCC) {
255 assert(!SCC.empty() && "SCC with no functions?");
256 FunctionRecord &FR = FunctionInfo[SCC[0]->getFunction()];
258 bool CallsExternal = false;
259 unsigned FunctionEffect = 0;
261 // Collect the mod/ref properties due to called functions. We only compute
263 for (unsigned i = 0, e = SCC.size(); i != e && !CallsExternal; ++i)
264 for (CallGraphNode::iterator CI = SCC[i]->begin(), E = SCC[i]->end();
266 if (Function *Callee = CI->second->getFunction()) {
267 if (FunctionRecord *CalleeFR = getFunctionInfo(Callee)) {
268 // Propagate function effect up.
269 FunctionEffect |= CalleeFR->FunctionEffect;
271 // Incorporate callee's effects on globals into our info.
272 for (std::map<GlobalValue*, unsigned>::iterator GI =
273 CalleeFR->GlobalInfo.begin(), E = CalleeFR->GlobalInfo.end();
275 FR.GlobalInfo[GI->first] |= GI->second;
278 // Okay, if we can't say anything about it, maybe some other alias
281 AliasAnalysis::getModRefBehavior(Callee, CallSite());
282 if (MRB != DoesNotAccessMemory) {
283 // FIXME: could make this more aggressive for functions that just
284 // read memory. We should just say they read all globals.
285 CallsExternal = true;
290 CallsExternal = true;
294 // If this SCC calls an external function, we can't say anything about it, so
295 // remove all SCC functions from the FunctionInfo map.
297 for (unsigned i = 0, e = SCC.size(); i != e; ++i)
298 FunctionInfo.erase(SCC[i]->getFunction());
302 // Otherwise, unless we already know that this function mod/refs memory, scan
303 // the function bodies to see if there are any explicit loads or stores.
304 if (FunctionEffect != ModRef) {
305 for (unsigned i = 0, e = SCC.size(); i != e && FunctionEffect != ModRef;++i)
306 for (inst_iterator II = inst_begin(SCC[i]->getFunction()),
307 E = inst_end(SCC[i]->getFunction());
308 II != E && FunctionEffect != ModRef; ++II)
309 if (isa<LoadInst>(*II))
310 FunctionEffect |= Ref;
311 else if (isa<StoreInst>(*II))
312 FunctionEffect |= Mod;
313 else if (isa<MallocInst>(*II) || isa<FreeInst>(*II))
314 FunctionEffect |= ModRef;
317 if ((FunctionEffect & Mod) == 0)
318 ++NumReadMemFunctions;
319 if (FunctionEffect == 0)
321 FR.FunctionEffect = FunctionEffect;
323 // Finally, now that we know the full effect on this SCC, clone the
324 // information to each function in the SCC.
325 for (unsigned i = 1, e = SCC.size(); i != e; ++i)
326 FunctionInfo[SCC[i]->getFunction()] = FR;
331 /// getUnderlyingObject - This traverses the use chain to figure out what object
332 /// the specified value points to. If the value points to, or is derived from,
333 /// a global object, return it.
334 static const GlobalValue *getUnderlyingObject(const Value *V) {
335 if (!isa<PointerType>(V->getType())) return 0;
337 // If we are at some type of object... return it.
338 if (const GlobalValue *GV = dyn_cast<GlobalValue>(V)) return GV;
340 // Traverse through different addressing mechanisms...
341 if (const Instruction *I = dyn_cast<Instruction>(V)) {
342 if (isa<CastInst>(I) || isa<GetElementPtrInst>(I))
343 return getUnderlyingObject(I->getOperand(0));
344 } else if (const ConstantExpr *CE = dyn_cast<ConstantExpr>(V)) {
345 if (CE->getOpcode() == Instruction::Cast ||
346 CE->getOpcode() == Instruction::GetElementPtr)
347 return getUnderlyingObject(CE->getOperand(0));
352 /// alias - If one of the pointers is to a global that we are tracking, and the
353 /// other is some random pointer, we know there cannot be an alias, because the
354 /// address of the global isn't taken.
355 AliasAnalysis::AliasResult
356 GlobalsModRef::alias(const Value *V1, unsigned V1Size,
357 const Value *V2, unsigned V2Size) {
358 GlobalValue *GV1 = const_cast<GlobalValue*>(getUnderlyingObject(V1));
359 GlobalValue *GV2 = const_cast<GlobalValue*>(getUnderlyingObject(V2));
361 // If the global's address is taken, pretend we don't know it's a pointer to
363 if (GV1 && !NonAddressTakenGlobals.count(GV1)) GV1 = 0;
364 if (GV2 && !NonAddressTakenGlobals.count(GV2)) GV2 = 0;
366 if ((GV1 || GV2) && GV1 != GV2)
369 return AliasAnalysis::alias(V1, V1Size, V2, V2Size);
372 AliasAnalysis::ModRefResult
373 GlobalsModRef::getModRefInfo(CallSite CS, Value *P, unsigned Size) {
374 unsigned Known = ModRef;
376 // If we are asking for mod/ref info of a direct call with a pointer to a
377 // global we are tracking, return information if we have it.
378 if (GlobalValue *GV = const_cast<GlobalValue*>(getUnderlyingObject(P)))
379 if (GV->hasInternalLinkage())
380 if (Function *F = CS.getCalledFunction())
381 if (NonAddressTakenGlobals.count(GV))
382 if (FunctionRecord *FR = getFunctionInfo(F))
383 Known = FR->getInfoForGlobal(GV);
385 if (Known == NoModRef)
386 return NoModRef; // No need to query other mod/ref analyses
387 return ModRefResult(Known & AliasAnalysis::getModRefInfo(CS, P, Size));
391 //===----------------------------------------------------------------------===//
392 // Methods to update the analysis as a result of the client transformation.
394 void GlobalsModRef::deleteValue(Value *V) {
395 if (GlobalValue *GV = dyn_cast<GlobalValue>(V))
396 NonAddressTakenGlobals.erase(GV);
399 void GlobalsModRef::copyValue(Value *From, Value *To) {