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/Module.h"
16 #include "llvm/Analysis/AliasAnalysis.h"
17 #include "llvm/Analysis/DataStructure/DataStructure.h"
18 #include "llvm/Analysis/DataStructure/DSGraph.h"
22 class DSAA : public Pass, public AliasAnalysis {
28 //------------------------------------------------
29 // Implement the Pass API
32 // run - Build up the result graph, representing the pointer graph for the
36 InitializeAliasAnalysis(this);
37 TD = &getAnalysis<TDDataStructures>();
38 BU = &getAnalysis<BUDataStructures>();
42 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
43 AliasAnalysis::getAnalysisUsage(AU);
44 AU.setPreservesAll(); // Does not transform code
45 AU.addRequiredTransitive<TDDataStructures>(); // Uses TD Datastructures
46 AU.addRequiredTransitive<BUDataStructures>(); // Uses BU Datastructures
49 //------------------------------------------------
50 // Implement the AliasAnalysis API
53 AliasResult alias(const Value *V1, unsigned V1Size,
54 const Value *V2, unsigned V2Size);
56 void getMustAliases(Value *P, std::vector<Value*> &RetVals);
58 ModRefResult getModRefInfo(CallSite CS, Value *P, unsigned Size);
61 DSGraph *getGraphForValue(const Value *V);
64 // Register the pass...
65 RegisterOpt<DSAA> X("ds-aa", "Data Structure Graph Based Alias Analysis");
67 // Register as an implementation of AliasAnalysis
68 RegisterAnalysisGroup<AliasAnalysis, DSAA> Y;
71 // getGraphForValue - Return the DSGraph to use for queries about the specified
74 DSGraph *DSAA::getGraphForValue(const Value *V) {
75 if (const Instruction *I = dyn_cast<Instruction>(V))
76 return &TD->getDSGraph(*I->getParent()->getParent());
77 else if (const Argument *A = dyn_cast<Argument>(V))
78 return &TD->getDSGraph(*A->getParent());
79 else if (const BasicBlock *BB = dyn_cast<BasicBlock>(V))
80 return &TD->getDSGraph(*BB->getParent());
84 // isSinglePhysicalObject - For now, the only case that we know that there is
85 // only one memory object in the node is when there is a single global in the
86 // node, and the only composition bit set is Global.
88 static bool isSinglePhysicalObject(DSNode *N) {
89 assert(N->isComplete() && "Can only tell if this is a complete object!");
90 return N->isGlobalNode() && N->getGlobals().size() == 1 &&
91 !N->isHeapNode() && !N->isAllocaNode() && !N->isUnknownNode();
94 // alias - This is the only method here that does anything interesting...
95 AliasAnalysis::AliasResult DSAA::alias(const Value *V1, unsigned V1Size,
96 const Value *V2, unsigned V2Size) {
97 if (V1 == V2) return MustAlias;
99 DSGraph *G1 = getGraphForValue(V1);
100 DSGraph *G2 = getGraphForValue(V2);
101 assert((!G1 || !G2 || G1 == G2) && "Alias query for 2 different functions?");
103 // Get the graph to use...
104 DSGraph &G = *(G1 ? G1 : (G2 ? G2 : &TD->getGlobalsGraph()));
106 const DSGraph::ScalarMapTy &GSM = G.getScalarMap();
107 DSGraph::ScalarMapTy::const_iterator I = GSM.find((Value*)V1);
108 if (I == GSM.end()) return NoAlias;
110 assert(I->second.getNode() && "Scalar map points to null node?");
111 DSGraph::ScalarMapTy::const_iterator J = GSM.find((Value*)V2);
112 if (J == GSM.end()) return NoAlias;
114 assert(J->second.getNode() && "Scalar map points to null node?");
116 DSNode *N1 = I->second.getNode(), *N2 = J->second.getNode();
117 unsigned O1 = I->second.getOffset(), O2 = J->second.getOffset();
119 // We can only make a judgment of one of the nodes is complete...
120 if (N1->isComplete() || N2->isComplete()) {
122 return NoAlias; // Completely different nodes.
124 #if 0 // This does not correctly handle arrays!
125 // Both point to the same node and same offset, and there is only one
126 // physical memory object represented in the node, return must alias.
128 // FIXME: This isn't correct because we do not handle array indexing
131 if (O1 == O2 && isSinglePhysicalObject(N1))
132 return MustAlias; // Exactly the same object & offset
135 // See if they point to different offsets... if so, we may be able to
136 // determine that they do not alias...
138 if (O2 < O1) { // Ensure that O1 <= O2
141 std::swap(V1Size, V2Size);
144 // FIXME: This is not correct because we do not handle array
145 // indexing correctly with this check!
146 //if (O1+V1Size <= O2) return NoAlias;
150 // FIXME: we could improve on this by checking the globals graph for aliased
152 return AliasAnalysis::alias(V1, V1Size, V2, V2Size);
155 /// getModRefInfo - does a callsite modify or reference a value?
157 AliasAnalysis::ModRefResult
158 DSAA::getModRefInfo(CallSite CS, Value *P, unsigned Size) {
159 Function *F = CS.getCalledFunction();
160 if (!F) return pointsToConstantMemory(P) ? Ref : ModRef;
161 if (F->isExternal()) return ModRef;
163 // Clone the function TD graph, clearing off Mod/Ref flags
164 const Function *csParent = CS.getInstruction()->getParent()->getParent();
165 DSGraph TDGraph(TD->getDSGraph(*csParent));
166 TDGraph.maskNodeTypes(0);
168 // Insert the callee's BU graph into the TD graph
169 const DSGraph &BUGraph = BU->getDSGraph(*F);
170 TDGraph.mergeInGraph(TDGraph.getDSCallSiteForCallSite(CS),
173 // Report the flags that have been added
174 const DSNodeHandle &DSH = TDGraph.getNodeForValue(P);
175 if (const DSNode *N = DSH.getNode())
177 return N->isRead() ? ModRef : Mod;
179 return N->isRead() ? Ref : NoModRef;
184 /// getMustAliases - If there are any pointers known that must alias this
185 /// pointer, return them now. This allows alias-set based alias analyses to
186 /// perform a form a value numbering (which is exposed by load-vn). If an alias
187 /// analysis supports this, it should ADD any must aliased pointers to the
188 /// specified vector.
190 void DSAA::getMustAliases(Value *P, std::vector<Value*> &RetVals) {
191 #if 0 // This does not correctly handle arrays!
192 // Currently the only must alias information we can provide is to say that
193 // something is equal to a global value. If we already have a global value,
194 // don't get worked up about it.
195 if (!isa<GlobalValue>(P)) {
196 DSGraph *G = getGraphForValue(P);
197 if (!G) G = &TD->getGlobalsGraph();
199 // The only must alias information we can currently determine occurs when
200 // the node for P is a global node with only one entry.
201 DSGraph::ScalarMapTy::const_iterator I = G->getScalarMap().find(P);
202 if (I != G->getScalarMap().end()) {
203 DSNode *N = I->second.getNode();
204 if (N->isComplete() && isSinglePhysicalObject(N))
205 RetVals.push_back(N->getGlobals()[0]);
209 return AliasAnalysis::getMustAliases(P, RetVals);