1 //===- AliasAnalysis.cpp - Generic Alias Analysis Interface Implementation -==//
2 //
3 //                     The LLVM Compiler Infrastructure
4 //
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.
7 //
8 //===----------------------------------------------------------------------===//
9 //
10 // This file implements the generic AliasAnalysis interface which is used as the
11 // common interface used by all clients and implementations of alias analysis.
12 //
13 // This file also implements the default version of the AliasAnalysis interface
14 // that is to be used when no other implementation is specified.  This does some
15 // simple tests that detect obvious cases: two different global pointers cannot
16 // alias, a global cannot alias a malloc, two different mallocs cannot alias,
17 // etc.
18 //
19 // This alias analysis implementation really isn't very good for anything, but
20 // it is very fast, and makes a nice clean default implementation.  Because it
21 // handles lots of little corner cases, other, more complex, alias analysis
22 // implementations may choose to rely on this pass to resolve these simple and
23 // easy cases.
24 //
25 //===----------------------------------------------------------------------===//
26 
27 #include "llvm/Analysis/AliasAnalysis.h"
28 #include "llvm/BasicBlock.h"
29 #include "llvm/Instructions.h"
30 #include "llvm/Type.h"
31 #include "llvm/Target/TargetData.h"
32 #include <iostream>
33 using namespace llvm;
34 
35 // Register the AliasAnalysis interface, providing a nice name to refer to.
36 namespace {
37   RegisterAnalysisGroup<AliasAnalysis> Z("Alias Analysis");
38 }
39 
40 //===----------------------------------------------------------------------===//
41 // Default chaining methods
42 //===----------------------------------------------------------------------===//
43 
44 AliasAnalysis::AliasResult
45 AliasAnalysis::alias(const Value *V1, unsigned V1Size,
46                      const Value *V2, unsigned V2Size) {
47   assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!");
48   return AA->alias(V1, V1Size, V2, V2Size);
49 }
50 
51 void AliasAnalysis::getMustAliases(Value *P, std::vector<Value*> &RetVals) {
52   assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!");
53   return AA->getMustAliases(P, RetVals);
54 }
55 
56 bool AliasAnalysis::pointsToConstantMemory(const Value *P) {
57   assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!");
58   return AA->pointsToConstantMemory(P);
59 }
60 
61 AliasAnalysis::ModRefBehavior
62 AliasAnalysis::getModRefBehavior(Function *F, CallSite CS,
63                                  std::vector<PointerAccessInfo> *Info) {
64   assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!");
65   return AA->getModRefBehavior(F, CS, Info);
66 }
67 
68 bool AliasAnalysis::hasNoModRefInfoForCalls() const {
69   assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!");
70   return AA->hasNoModRefInfoForCalls();
71 }
72 
73 void AliasAnalysis::deleteValue(Value *V) {
74   assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!");
75   AA->deleteValue(V);
76 }
77 
78 void AliasAnalysis::copyValue(Value *From, Value *To) {
79   assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!");
80   AA->copyValue(From, To);
81 }
82 
83 AliasAnalysis::ModRefResult
84 AliasAnalysis::getModRefInfo(CallSite CS1, CallSite CS2) {
85   // FIXME: we can do better.
86   assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!");
87   return AA->getModRefInfo(CS1, CS2);
88 }
89 
90 
91 //===----------------------------------------------------------------------===//
92 // AliasAnalysis non-virtual helper method implementation
93 //===----------------------------------------------------------------------===//
94 
95 AliasAnalysis::ModRefResult
96 AliasAnalysis::getModRefInfo(LoadInst *L, Value *P, unsigned Size) {
97   return alias(L->getOperand(0), TD->getTypeSize(L->getType()),
98                P, Size) ? Ref : NoModRef;
99 }
100 
101 AliasAnalysis::ModRefResult
102 AliasAnalysis::getModRefInfo(StoreInst *S, Value *P, unsigned Size) {
103   // If the stored address cannot alias the pointer in question, then the
104   // pointer cannot be modified by the store.
105   if (!alias(S->getOperand(1), TD->getTypeSize(S->getOperand(0)->getType()),
106              P, Size))
107     return NoModRef;
108 
109   // If the pointer is a pointer to constant memory, then it could not have been
110   // modified by this store.
111   return pointsToConstantMemory(P) ? NoModRef : Mod;
112 }
113 
114 AliasAnalysis::ModRefResult
115 AliasAnalysis::getModRefInfo(CallSite CS, Value *P, unsigned Size) {
116   ModRefResult Mask = ModRef;
117   if (Function *F = CS.getCalledFunction()) {
118     ModRefBehavior MRB = getModRefBehavior(F, CallSite());
119     if (MRB == OnlyReadsMemory)
120       Mask = Ref;
121     else if (MRB == DoesNotAccessMemory)
122       return NoModRef;
123   }
124 
125   if (!AA) return Mask;
126 
127   // If P points to a constant memory location, the call definitely could not
128   // modify the memory location.
129   if ((Mask & Mod) && AA->pointsToConstantMemory(P))
130     Mask = ModRefResult(Mask & ~Mod);
131 
132   return ModRefResult(Mask & AA->getModRefInfo(CS, P, Size));
133 }
134 
135 // AliasAnalysis destructor: DO NOT move this to the header file for
136 // AliasAnalysis or else clients of the AliasAnalysis class may not depend on
137 // the AliasAnalysis.o file in the current .a file, causing alias analysis
138 // support to not be included in the tool correctly!
139 //
140 AliasAnalysis::~AliasAnalysis() {}
141 
142 /// setTargetData - Subclasses must call this method to initialize the
143 /// AliasAnalysis interface before any other methods are called.
144 ///
145 void AliasAnalysis::InitializeAliasAnalysis(Pass *P) {
146   TD = &P->getAnalysis<TargetData>();
147   AA = &P->getAnalysis<AliasAnalysis>();
148 }
149 
150 // getAnalysisUsage - All alias analysis implementations should invoke this
151 // directly (using AliasAnalysis::getAnalysisUsage(AU)) to make sure that
152 // TargetData is required by the pass.
153 void AliasAnalysis::getAnalysisUsage(AnalysisUsage &AU) const {
154   AU.addRequired<TargetData>();            // All AA's need TargetData.
155   AU.addRequired<AliasAnalysis>();         // All AA's chain
156 }
157 
158 /// canBasicBlockModify - Return true if it is possible for execution of the
159 /// specified basic block to modify the value pointed to by Ptr.
160 ///
161 bool AliasAnalysis::canBasicBlockModify(const BasicBlock &BB,
162                                         const Value *Ptr, unsigned Size) {
163   return canInstructionRangeModify(BB.front(), BB.back(), Ptr, Size);
164 }
165 
166 /// canInstructionRangeModify - Return true if it is possible for the execution
167 /// of the specified instructions to modify the value pointed to by Ptr.  The
168 /// instructions to consider are all of the instructions in the range of [I1,I2]
169 /// INCLUSIVE.  I1 and I2 must be in the same basic block.
170 ///
171 bool AliasAnalysis::canInstructionRangeModify(const Instruction &I1,
172                                               const Instruction &I2,
173                                               const Value *Ptr, unsigned Size) {
174   assert(I1.getParent() == I2.getParent() &&
175          "Instructions not in same basic block!");
176   BasicBlock::iterator I = const_cast<Instruction*>(&I1);
177   BasicBlock::iterator E = const_cast<Instruction*>(&I2);
178   ++E;  // Convert from inclusive to exclusive range.
179 
180   for (; I != E; ++I) // Check every instruction in range
181     if (getModRefInfo(I, const_cast<Value*>(Ptr), Size) & Mod)
182       return true;
183   return false;
184 }
185 
186 // Because of the way .a files work, we must force the BasicAA implementation to
187 // be pulled in if the AliasAnalysis classes are pulled in.  Otherwise we run
188 // the risk of AliasAnalysis being used, but the default implementation not
189 // being linked into the tool that uses it.
190 //
191 namespace llvm {
192   extern void BasicAAStub();
193 }
194 static IncludeFile INCLUDE_BASICAA_CPP((void*)&BasicAAStub);
195