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/Analysis/DataStructure.h"
16 #include "llvm/Analysis/DSGraph.h"
17 #include "llvm/Analysis/AliasAnalysis.h"
18 #include "llvm/Module.h"
22 class DSAA : public Pass, public AliasAnalysis {
27 //------------------------------------------------
28 // Implement the Pass API
31 // run - Build up the result graph, representing the pointer graph for the
35 InitializeAliasAnalysis(this);
36 TD = &getAnalysis<TDDataStructures>();
40 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
41 AliasAnalysis::getAnalysisUsage(AU);
42 AU.setPreservesAll(); // Does not transform code...
43 AU.addRequired<TDDataStructures>(); // Uses TD Datastructures
44 AU.addRequired<AliasAnalysis>(); // Chains to another AA impl...
47 //------------------------------------------------
48 // Implement the AliasAnalysis API
51 AliasResult alias(const Value *V1, unsigned V1Size,
52 const Value *V2, unsigned V2Size);
54 void getMustAliases(Value *P, std::vector<Value*> &RetVals);
56 bool pointsToConstantMemory(const Value *P) {
57 return getAnalysis<AliasAnalysis>().pointsToConstantMemory(P);
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()) {
109 assert(I->second.getNode() && "Scalar map points to null node?");
110 DSGraph::ScalarMapTy::const_iterator J = GSM.find((Value*)V2);
111 if (J != GSM.end()) {
112 assert(J->second.getNode() && "Scalar map points to null node?");
114 DSNode *N1 = I->second.getNode(), *N2 = J->second.getNode();
115 unsigned O1 = I->second.getOffset(), O2 = J->second.getOffset();
117 // We can only make a judgment of one of the nodes is complete...
118 if (N1->isComplete() || N2->isComplete()) {
120 return NoAlias; // Completely different nodes.
122 #if 0 // This does not correctly handle arrays!
123 // Both point to the same node and same offset, and there is only one
124 // physical memory object represented in the node, return must alias.
126 // FIXME: This isn't correct because we do not handle array indexing
129 if (O1 == O2 && isSinglePhysicalObject(N1))
130 return MustAlias; // Exactly the same object & offset
133 // See if they point to different offsets... if so, we may be able to
134 // determine that they do not alias...
136 if (O2 < O1) { // Ensure that O1 <= O2
139 std::swap(V1Size, V2Size);
142 // FIXME: This is not correct because we do not handle array
143 // indexing correctly with this check!
144 //if (O1+V1Size <= O2) return NoAlias;
150 // FIXME: we could improve on this by checking the globals graph for aliased
152 return getAnalysis<AliasAnalysis>().alias(V1, V1Size, V2, V2Size);
156 /// getMustAliases - If there are any pointers known that must alias this
157 /// pointer, return them now. This allows alias-set based alias analyses to
158 /// perform a form a value numbering (which is exposed by load-vn). If an alias
159 /// analysis supports this, it should ADD any must aliased pointers to the
160 /// specified vector.
162 void DSAA::getMustAliases(Value *P, std::vector<Value*> &RetVals) {
163 #if 0 // This does not correctly handle arrays!
164 // Currently the only must alias information we can provide is to say that
165 // something is equal to a global value. If we already have a global value,
166 // don't get worked up about it.
167 if (!isa<GlobalValue>(P)) {
168 DSGraph *G = getGraphForValue(P);
169 if (!G) G = &TD->getGlobalsGraph();
171 // The only must alias information we can currently determine occurs when
172 // the node for P is a global node with only one entry.
173 DSGraph::ScalarMapTy::const_iterator I = G->getScalarMap().find(P);
174 if (I != G->getScalarMap().end()) {
175 DSNode *N = I->second.getNode();
176 if (N->isComplete() && isSinglePhysicalObject(N))
177 RetVals.push_back(N->getGlobals()[0]);
181 return getAnalysis<AliasAnalysis>().getMustAliases(P, RetVals);