1 //===- DataStructureAA.cpp - Data Structure Based Alias Analysis ----------===//
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 pass uses the top-down data structure graphs to implement a simple
11 // context sensitive alias analysis.
13 //===----------------------------------------------------------------------===//
15 #include "llvm/Constants.h"
16 #include "llvm/DerivedTypes.h"
17 #include "llvm/Module.h"
18 #include "llvm/Analysis/AliasAnalysis.h"
19 #include "llvm/Analysis/Passes.h"
20 #include "llvm/Analysis/DataStructure/DataStructure.h"
21 #include "llvm/Analysis/DataStructure/DSGraph.h"
25 class DSAA : public ModulePass, public AliasAnalysis {
31 //------------------------------------------------
32 // Implement the Pass API
35 // run - Build up the result graph, representing the pointer graph for the
38 bool runOnModule(Module &M) {
39 InitializeAliasAnalysis(this);
40 TD = &getAnalysis<TDDataStructures>();
41 BU = &getAnalysis<BUDataStructures>();
45 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
46 AliasAnalysis::getAnalysisUsage(AU);
47 AU.setPreservesAll(); // Does not transform code
48 AU.addRequiredTransitive<TDDataStructures>(); // Uses TD Datastructures
49 AU.addRequiredTransitive<BUDataStructures>(); // Uses BU Datastructures
52 //------------------------------------------------
53 // Implement the AliasAnalysis API
56 AliasResult alias(const Value *V1, unsigned V1Size,
57 const Value *V2, unsigned V2Size);
59 void getMustAliases(Value *P, std::vector<Value*> &RetVals);
61 ModRefResult getModRefInfo(CallSite CS, Value *P, unsigned Size);
62 ModRefResult getModRefInfo(CallSite CS1, CallSite CS2) {
63 return AliasAnalysis::getModRefInfo(CS1,CS2);
66 virtual void deleteValue(Value *V) {
71 virtual void copyValue(Value *From, Value *To) {
72 if (From == To) return;
73 BU->copyValue(From, To);
74 TD->copyValue(From, To);
78 DSGraph *getGraphForValue(const Value *V);
81 // Register the pass...
82 RegisterOpt<DSAA> X("ds-aa", "Data Structure Graph Based Alias Analysis");
84 // Register as an implementation of AliasAnalysis
85 RegisterAnalysisGroup<AliasAnalysis, DSAA> Y;
88 ModulePass *llvm::createDSAAPass() { return new DSAA(); }
90 // getGraphForValue - Return the DSGraph to use for queries about the specified
93 DSGraph *DSAA::getGraphForValue(const Value *V) {
94 if (const Instruction *I = dyn_cast<Instruction>(V))
95 return &TD->getDSGraph(*I->getParent()->getParent());
96 else if (const Argument *A = dyn_cast<Argument>(V))
97 return &TD->getDSGraph(*A->getParent());
98 else if (const BasicBlock *BB = dyn_cast<BasicBlock>(V))
99 return &TD->getDSGraph(*BB->getParent());
104 // isSinglePhysicalObject - For now, the only case that we know that there is
105 // only one memory object in the node is when there is a single global in the
106 // node, and the only composition bit set is Global.
108 static bool isSinglePhysicalObject(DSNode *N) {
109 assert(N->isComplete() && "Can only tell if this is a complete object!");
110 return N->isGlobalNode() && N->getGlobals().size() == 1 &&
111 !N->isHeapNode() && !N->isAllocaNode() && !N->isUnknownNode();
115 // alias - This is the only method here that does anything interesting...
116 AliasAnalysis::AliasResult DSAA::alias(const Value *V1, unsigned V1Size,
117 const Value *V2, unsigned V2Size) {
118 if (V1 == V2) return MustAlias;
120 DSGraph *G1 = getGraphForValue(V1);
121 DSGraph *G2 = getGraphForValue(V2);
122 assert((!G1 || !G2 || G1 == G2) && "Alias query for 2 different functions?");
124 // Get the graph to use...
125 DSGraph &G = *(G1 ? G1 : (G2 ? G2 : &TD->getGlobalsGraph()));
127 const DSGraph::ScalarMapTy &GSM = G.getScalarMap();
128 DSGraph::ScalarMapTy::const_iterator I = GSM.find((Value*)V1);
129 if (I == GSM.end()) return NoAlias;
131 DSGraph::ScalarMapTy::const_iterator J = GSM.find((Value*)V2);
132 if (J == GSM.end()) return NoAlias;
134 DSNode *N1 = I->second.getNode(), *N2 = J->second.getNode();
135 unsigned O1 = I->second.getOffset(), O2 = J->second.getOffset();
136 if (N1 == 0 || N2 == 0)
137 return MayAlias; // Can't tell whether anything aliases null.
139 // We can only make a judgment of one of the nodes is complete...
140 if (N1->isComplete() || N2->isComplete()) {
142 return NoAlias; // Completely different nodes.
144 #if 0 // This does not correctly handle arrays!
145 // Both point to the same node and same offset, and there is only one
146 // physical memory object represented in the node, return must alias.
148 // FIXME: This isn't correct because we do not handle array indexing
151 if (O1 == O2 && isSinglePhysicalObject(N1))
152 return MustAlias; // Exactly the same object & offset
155 // See if they point to different offsets... if so, we may be able to
156 // determine that they do not alias...
158 if (O2 < O1) { // Ensure that O1 <= O2
161 std::swap(V1Size, V2Size);
169 // FIXME: we could improve on this by checking the globals graph for aliased
171 return AliasAnalysis::alias(V1, V1Size, V2, V2Size);
174 /// getModRefInfo - does a callsite modify or reference a value?
176 AliasAnalysis::ModRefResult
177 DSAA::getModRefInfo(CallSite CS, Value *P, unsigned Size) {
178 AliasAnalysis::ModRefResult Result =AliasAnalysis::getModRefInfo(CS, P, Size);
179 Function *F = CS.getCalledFunction();
181 if (!F || Result == NoModRef)
184 if (F->isExternal()) {
185 // If we are calling an external function, and if this global doesn't escape
186 // the portion of the program we have analyzed, we can draw conclusions
187 // based on whether the global escapes the program.
188 Function *Caller = CS.getInstruction()->getParent()->getParent();
189 DSGraph *G = &TD->getDSGraph(*Caller);
190 DSScalarMap::iterator NI = G->getScalarMap().find(P);
191 if (NI == G->getScalarMap().end()) {
192 // If it wasn't in the local function graph, check the global graph. This
193 // can occur for globals who are locally reference but hoisted out to the
194 // globals graph despite that.
195 G = G->getGlobalsGraph();
196 NI = G->getScalarMap().find(P);
197 if (NI == G->getScalarMap().end())
201 // If we found a node and it's complete, it cannot be passed out to the
203 if (NI->second.getNode()->isComplete())
208 // Get the graphs for the callee and caller. Note that we want the BU graph
209 // for the callee because we don't want all caller's effects incorporated!
210 const Function *Caller = CS.getInstruction()->getParent()->getParent();
211 DSGraph &CallerTDGraph = TD->getDSGraph(*Caller);
212 DSGraph &CalleeBUGraph = BU->getDSGraph(*F);
214 // Figure out which node in the TD graph this pointer corresponds to.
215 DSScalarMap &CallerSM = CallerTDGraph.getScalarMap();
216 DSScalarMap::iterator NI = CallerSM.find(P);
217 if (NI == CallerSM.end()) {
218 if (isa<ConstantPointerNull>(P) || isa<UndefValue>(P))
219 Result = NoModRef; // null is never modified :)
221 assert(isa<GlobalVariable>(P) &&
222 cast<GlobalVariable>(P)->getType()->getElementType()->isFirstClassType() &&
223 "This isn't a global that DSA inconsiderately dropped "
226 DSGraph &GG = *CallerTDGraph.getGlobalsGraph();
227 DSScalarMap::iterator NI = GG.getScalarMap().find(P);
228 if (NI != GG.getScalarMap().end() && !NI->second.isNull()) {
229 // Otherwise, if the node is only M or R, return this. This can be
230 // useful for globals that should be marked const but are not.
231 DSNode *N = NI->second.getNode();
232 if (!N->isModified())
233 Result = (ModRefResult)(Result & ~Mod);
235 Result = (ModRefResult)(Result & ~Ref);
241 const DSNode *N = NI->second.getNode();
242 assert(N && "Null pointer in scalar map??");
244 // Compute the mapping from nodes in the callee graph to the nodes in the
245 // caller graph for this call site.
246 DSGraph::NodeMapTy CalleeCallerMap;
247 DSCallSite DSCS = CallerTDGraph.getDSCallSiteForCallSite(CS);
248 CallerTDGraph.computeCalleeCallerMapping(DSCS, *F, CalleeBUGraph,
251 // Loop over all of the nodes in the callee that correspond to "N", keeping
252 // track of aggregate mod/ref info.
253 bool NeverReads = true, NeverWrites = true;
254 for (DSGraph::NodeMapTy::iterator I = CalleeCallerMap.begin(),
255 E = CalleeCallerMap.end(); I != E; ++I)
256 if (I->second.getNode() == N) {
257 if (I->first->isModified())
259 if (I->first->isRead())
261 if (NeverReads == false && NeverWrites == false)
265 if (NeverWrites) // We proved it was not modified.
266 Result = ModRefResult(Result & ~Mod);
267 if (NeverReads) // We proved it was not read.
268 Result = ModRefResult(Result & ~Ref);
274 /// getMustAliases - If there are any pointers known that must alias this
275 /// pointer, return them now. This allows alias-set based alias analyses to
276 /// perform a form a value numbering (which is exposed by load-vn). If an alias
277 /// analysis supports this, it should ADD any must aliased pointers to the
278 /// specified vector.
280 void DSAA::getMustAliases(Value *P, std::vector<Value*> &RetVals) {
281 #if 0 // This does not correctly handle arrays!
282 // Currently the only must alias information we can provide is to say that
283 // something is equal to a global value. If we already have a global value,
284 // don't get worked up about it.
285 if (!isa<GlobalValue>(P)) {
286 DSGraph *G = getGraphForValue(P);
287 if (!G) G = &TD->getGlobalsGraph();
289 // The only must alias information we can currently determine occurs when
290 // the node for P is a global node with only one entry.
291 DSGraph::ScalarMapTy::const_iterator I = G->getScalarMap().find(P);
292 if (I != G->getScalarMap().end()) {
293 DSNode *N = I->second.getNode();
294 if (N->isComplete() && isSinglePhysicalObject(N))
295 RetVals.push_back(N->getGlobals()[0]);
299 return AliasAnalysis::getMustAliases(P, RetVals);