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 bool doesNotAccessMemory(Function *F) {
105 if (FunctionRecord *FR = getFunctionInfo(F))
106 if (FR->FunctionEffect == 0)
108 return AliasAnalysis::doesNotAccessMemory(F);
110 bool onlyReadsMemory(Function *F) {
111 if (FunctionRecord *FR = getFunctionInfo(F))
112 if ((FR->FunctionEffect & Mod) == 0)
114 return AliasAnalysis::onlyReadsMemory(F);
118 virtual void deleteValue(Value *V);
119 virtual void copyValue(Value *From, Value *To);
122 /// getFunctionInfo - Return the function info for the function, or null if
123 /// the function calls an external function (in which case we don't have
124 /// anything useful to say about it).
125 FunctionRecord *getFunctionInfo(Function *F) {
126 std::map<Function*, FunctionRecord>::iterator I = FunctionInfo.find(F);
127 if (I != FunctionInfo.end())
132 void AnalyzeGlobals(Module &M);
133 void AnalyzeCallGraph(CallGraph &CG, Module &M);
134 void AnalyzeSCC(std::vector<CallGraphNode *> &SCC);
135 bool AnalyzeUsesOfGlobal(Value *V, std::vector<Function*> &Readers,
136 std::vector<Function*> &Writers);
139 RegisterOpt<GlobalsModRef> X("globalsmodref-aa",
140 "Simple mod/ref analysis for globals");
141 RegisterAnalysisGroup<AliasAnalysis, GlobalsModRef> Y;
144 Pass *llvm::createGlobalsModRefPass() { return new GlobalsModRef(); }
147 /// AnalyzeGlobalUses - Scan through the users of all of the internal
148 /// GlobalValue's in the program. If none of them have their "Address taken"
149 /// (really, their address passed to something nontrivial), record this fact,
150 /// and record the functions that they are used directly in.
151 void GlobalsModRef::AnalyzeGlobals(Module &M) {
152 std::vector<Function*> Readers, Writers;
153 for (Module::iterator I = M.begin(), E = M.end(); I != E; ++I)
154 if (I->hasInternalLinkage()) {
155 if (!AnalyzeUsesOfGlobal(I, Readers, Writers)) {
156 // Remember that we are tracking this global.
157 NonAddressTakenGlobals.insert(I);
158 ++NumNonAddrTakenFunctions;
160 Readers.clear(); Writers.clear();
163 for (Module::giterator I = M.gbegin(), E = M.gend(); I != E; ++I)
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 (CallInst *CI = dyn_cast<CallInst>(*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 = 1, e = CI->getNumOperands(); i != e; ++i)
206 if (CI->getOperand(i) == V) return true;
207 } else if (InvokeInst *II = dyn_cast<InvokeInst>(*UI)) {
208 // Make sure that this is just the function being called, not that it is
209 // passing into the function.
210 for (unsigned i = 3, e = II->getNumOperands(); i != e; ++i)
211 if (II->getOperand(i) == V) return true;
212 } else if (ConstantExpr *CE = dyn_cast<ConstantExpr>(*UI)) {
213 if (CE->getOpcode() == Instruction::GetElementPtr ||
214 CE->getOpcode() == Instruction::Cast) {
215 if (AnalyzeUsesOfGlobal(CE, Readers, Writers))
220 } else if (GlobalValue *GV = dyn_cast<GlobalValue>(*UI)) {
221 if (AnalyzeUsesOfGlobal(GV, Readers, Writers)) return true;
228 /// AnalyzeCallGraph - At this point, we know the functions where globals are
229 /// immediately stored to and read from. Propagate this information up the call
230 /// graph to all callers and compute the mod/ref info for all memory for each
232 void GlobalsModRef::AnalyzeCallGraph(CallGraph &CG, Module &M) {
233 // We do a bottom-up SCC traversal of the call graph. In other words, we
234 // visit all callees before callers (leaf-first).
235 for (scc_iterator<CallGraph*> I = scc_begin(&CG), E = scc_end(&CG); I!=E; ++I)
236 if ((*I).size() != 1) {
238 } else if (Function *F = (*I)[0]->getFunction()) {
239 if (!F->isExternal()) {
240 // Nonexternal function.
243 // Otherwise external function. Handle intrinsics and other special
245 if (getAnalysis<AliasAnalysis>().doesNotAccessMemory(F))
246 // If it does not access memory, process the function, causing us to
247 // realize it doesn't do anything (the body is empty).
250 // Otherwise, don't process it. This will cause us to conservatively
255 // Do not process the external node, assume the worst.
259 void GlobalsModRef::AnalyzeSCC(std::vector<CallGraphNode *> &SCC) {
260 assert(!SCC.empty() && "SCC with no functions?");
261 FunctionRecord &FR = FunctionInfo[SCC[0]->getFunction()];
263 bool CallsExternal = false;
264 unsigned FunctionEffect = 0;
266 // Collect the mod/ref properties due to called functions. We only compute
268 for (unsigned i = 0, e = SCC.size(); i != e && !CallsExternal; ++i)
269 for (CallGraphNode::iterator CI = SCC[i]->begin(), E = SCC[i]->end();
271 if (Function *Callee = (*CI)->getFunction()) {
272 if (FunctionRecord *CalleeFR = getFunctionInfo(Callee)) {
273 // Propagate function effect up.
274 FunctionEffect |= CalleeFR->FunctionEffect;
276 // Incorporate callee's effects on globals into our info.
277 for (std::map<GlobalValue*, unsigned>::iterator GI =
278 CalleeFR->GlobalInfo.begin(), E = CalleeFR->GlobalInfo.end();
280 FR.GlobalInfo[GI->first] |= GI->second;
283 CallsExternal = true;
287 CallsExternal = true;
291 // If this SCC calls an external function, we can't say anything about it, so
292 // remove all SCC functions from the FunctionInfo map.
294 for (unsigned i = 0, e = SCC.size(); i != e; ++i)
295 FunctionInfo.erase(SCC[i]->getFunction());
299 // Otherwise, unless we already know that this function mod/refs memory, scan
300 // the function bodies to see if there are any explicit loads or stores.
301 if (FunctionEffect != ModRef) {
302 for (unsigned i = 0, e = SCC.size(); i != e && FunctionEffect != ModRef;++i)
303 for (inst_iterator II = inst_begin(SCC[i]->getFunction()),
304 E = inst_end(SCC[i]->getFunction());
305 II != E && FunctionEffect != ModRef; ++II)
306 if (isa<LoadInst>(*II))
307 FunctionEffect |= Ref;
308 else if (isa<StoreInst>(*II))
309 FunctionEffect |= Mod;
312 if ((FunctionEffect & Mod) == 0)
313 ++NumReadMemFunctions;
314 if (FunctionEffect == 0)
316 FR.FunctionEffect = FunctionEffect;
318 // Finally, now that we know the full effect on this SCC, clone the
319 // information to each function in the SCC.
320 for (unsigned i = 1, e = SCC.size(); i != e; ++i)
321 FunctionInfo[SCC[i]->getFunction()] = FR;
326 /// getUnderlyingObject - This traverses the use chain to figure out what object
327 /// the specified value points to. If the value points to, or is derived from,
328 /// a global object, return it.
329 static const GlobalValue *getUnderlyingObject(const Value *V) {
330 if (!isa<PointerType>(V->getType())) return 0;
332 // If we are at some type of object... return it.
333 if (const GlobalValue *GV = dyn_cast<GlobalValue>(V)) return GV;
335 // Traverse through different addressing mechanisms...
336 if (const Instruction *I = dyn_cast<Instruction>(V)) {
337 if (isa<CastInst>(I) || isa<GetElementPtrInst>(I))
338 return getUnderlyingObject(I->getOperand(0));
339 } else if (const ConstantExpr *CE = dyn_cast<ConstantExpr>(V)) {
340 if (CE->getOpcode() == Instruction::Cast ||
341 CE->getOpcode() == Instruction::GetElementPtr)
342 return getUnderlyingObject(CE->getOperand(0));
347 /// alias - If one of the pointers is to a global that we are tracking, and the
348 /// other is some random pointer, we know there cannot be an alias, because the
349 /// address of the global isn't taken.
350 AliasAnalysis::AliasResult
351 GlobalsModRef::alias(const Value *V1, unsigned V1Size,
352 const Value *V2, unsigned V2Size) {
353 GlobalValue *GV1 = const_cast<GlobalValue*>(getUnderlyingObject(V1));
354 GlobalValue *GV2 = const_cast<GlobalValue*>(getUnderlyingObject(V2));
356 // If the global's address is taken, pretend we don't know it's a pointer to
358 if (GV1 && !NonAddressTakenGlobals.count(GV1)) GV1 = 0;
359 if (GV2 && !NonAddressTakenGlobals.count(GV2)) GV2 = 0;
361 if ((GV1 || GV2) && GV1 != GV2)
364 return AliasAnalysis::alias(V1, V1Size, V2, V2Size);
367 AliasAnalysis::ModRefResult
368 GlobalsModRef::getModRefInfo(CallSite CS, Value *P, unsigned Size) {
369 unsigned Known = ModRef;
371 // If we are asking for mod/ref info of a direct call with a pointer to a
372 // global we are tracking, return information if we have it.
373 if (GlobalValue *GV = const_cast<GlobalValue*>(getUnderlyingObject(P)))
374 if (GV->hasInternalLinkage())
375 if (Function *F = CS.getCalledFunction())
376 if (NonAddressTakenGlobals.count(GV))
377 if (FunctionRecord *FR = getFunctionInfo(F))
378 Known = FR->getInfoForGlobal(GV);
380 if (Known == NoModRef)
381 return NoModRef; // No need to query other mod/ref analyses
382 return ModRefResult(Known & AliasAnalysis::getModRefInfo(CS, P, Size));
386 //===----------------------------------------------------------------------===//
387 // Methods to update the analysis as a result of the client transformation.
389 void GlobalsModRef::deleteValue(Value *V) {
390 if (GlobalValue *GV = dyn_cast<GlobalValue>(V))
391 NonAddressTakenGlobals.erase(GV);
394 void GlobalsModRef::copyValue(Value *From, Value *To) {