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"
21 class DSAA : public Pass, public AliasAnalysis {
26 //------------------------------------------------
27 // Implement the Pass API
30 // run - Build up the result graph, representing the pointer graph for the
34 InitializeAliasAnalysis(this);
35 TD = &getAnalysis<TDDataStructures>();
39 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
40 AliasAnalysis::getAnalysisUsage(AU);
41 AU.setPreservesAll(); // Does not transform code...
42 AU.addRequired<TDDataStructures>(); // Uses TD Datastructures
43 AU.addRequired<AliasAnalysis>(); // Chains to another AA impl...
46 //------------------------------------------------
47 // Implement the AliasAnalysis API
50 AliasResult alias(const Value *V1, unsigned V1Size,
51 const Value *V2, unsigned V2Size);
53 void getMustAliases(Value *P, std::vector<Value*> &RetVals);
56 DSGraph *getGraphForValue(const Value *V);
59 // Register the pass...
60 RegisterOpt<DSAA> X("ds-aa", "Data Structure Graph Based Alias Analysis");
62 // Register as an implementation of AliasAnalysis
63 RegisterAnalysisGroup<AliasAnalysis, DSAA> Y;
66 // getGraphForValue - Return the DSGraph to use for queries about the specified
69 DSGraph *DSAA::getGraphForValue(const Value *V) {
70 if (const Instruction *I = dyn_cast<Instruction>(V))
71 return &TD->getDSGraph(*I->getParent()->getParent());
72 else if (const Argument *A = dyn_cast<Argument>(V))
73 return &TD->getDSGraph(*A->getParent());
74 else if (const BasicBlock *BB = dyn_cast<BasicBlock>(V))
75 return &TD->getDSGraph(*BB->getParent());
79 // isSinglePhysicalObject - For now, the only case that we know that there is
80 // only one memory object in the node is when there is a single global in the
81 // node, and the only composition bit set is Global.
83 static bool isSinglePhysicalObject(DSNode *N) {
84 assert(N->isComplete() && "Can only tell if this is a complete object!");
85 return N->isGlobalNode() && N->getGlobals().size() == 1 &&
86 !N->isHeapNode() && !N->isAllocaNode() && !N->isUnknownNode();
89 // alias - This is the only method here that does anything interesting...
90 AliasAnalysis::AliasResult DSAA::alias(const Value *V1, unsigned V1Size,
91 const Value *V2, unsigned V2Size) {
92 if (V1 == V2) return MustAlias;
94 DSGraph *G1 = getGraphForValue(V1);
95 DSGraph *G2 = getGraphForValue(V2);
96 assert((!G1 || !G2 || G1 == G2) && "Alias query for 2 different functions?");
98 // Get the graph to use...
99 DSGraph &G = *(G1 ? G1 : (G2 ? G2 : &TD->getGlobalsGraph()));
101 const DSGraph::ScalarMapTy &GSM = G.getScalarMap();
102 DSGraph::ScalarMapTy::const_iterator I = GSM.find((Value*)V1);
103 if (I != GSM.end()) {
104 assert(I->second.getNode() && "Scalar map points to null node?");
105 DSGraph::ScalarMapTy::const_iterator J = GSM.find((Value*)V2);
106 if (J != GSM.end()) {
107 assert(J->second.getNode() && "Scalar map points to null node?");
109 DSNode *N1 = I->second.getNode(), *N2 = J->second.getNode();
110 unsigned O1 = I->second.getOffset(), O2 = J->second.getOffset();
112 // We can only make a judgment of one of the nodes is complete...
113 if (N1->isComplete() || N2->isComplete()) {
115 return NoAlias; // Completely different nodes.
117 #if 0 // This does not correctly handle arrays!
118 // Both point to the same node and same offset, and there is only one
119 // physical memory object represented in the node, return must alias.
121 // FIXME: This isn't correct because we do not handle array indexing
124 if (O1 == O2 && isSinglePhysicalObject(N1))
125 return MustAlias; // Exactly the same object & offset
128 // See if they point to different offsets... if so, we may be able to
129 // determine that they do not alias...
131 if (O2 < O1) { // Ensure that O1 <= O2
134 std::swap(V1Size, V2Size);
137 // FIXME: This is not correct because we do not handle array
138 // indexing correctly with this check!
139 //if (O1+V1Size <= O2) return NoAlias;
145 // FIXME: we could improve on this by checking the globals graph for aliased
147 return getAnalysis<AliasAnalysis>().alias(V1, V1Size, V2, V2Size);
151 /// getMustAliases - If there are any pointers known that must alias this
152 /// pointer, return them now. This allows alias-set based alias analyses to
153 /// perform a form a value numbering (which is exposed by load-vn). If an alias
154 /// analysis supports this, it should ADD any must aliased pointers to the
155 /// specified vector.
157 void DSAA::getMustAliases(Value *P, std::vector<Value*> &RetVals) {
158 #if 0 // This does not correctly handle arrays!
159 // Currently the only must alias information we can provide is to say that
160 // something is equal to a global value. If we already have a global value,
161 // don't get worked up about it.
162 if (!isa<GlobalValue>(P)) {
163 DSGraph *G = getGraphForValue(P);
164 if (!G) G = &TD->getGlobalsGraph();
166 // The only must alias information we can currently determine occurs when
167 // the node for P is a global node with only one entry.
168 const DSGraph::ScalarMapTy &GSM = G->getScalarMap();
169 DSGraph::ScalarMapTy::const_iterator I = GSM.find(P);
170 if (I != GSM.end()) {
171 DSNode *N = I->second.getNode();
172 if (N->isComplete() && isSinglePhysicalObject(N))
173 RetVals.push_back(N->getGlobals()[0]);
177 return getAnalysis<AliasAnalysis>().getMustAliases(P, RetVals);