1 //===- AliasAnalysis.cpp - Generic Alias Analysis Interface Implementation -==//
3 // This file implements the generic AliasAnalysis interface which is used as the
4 // common interface used by all clients and implementations of alias analysis.
6 // This file also implements the default version of the AliasAnalysis interface
7 // that is to be used when no other implementation is specified. This does some
8 // simple tests that detect obvious cases: two different global pointers cannot
9 // alias, a global cannot alias a malloc, two different mallocs cannot alias,
12 // This alias analysis implementation really isn't very good for anything, but
13 // it is very fast, and makes a nice clean default implementation. Because it
14 // handles lots of little corner cases, other, more complex, alias analysis
15 // implementations may choose to rely on this pass to resolve these simple and
18 //===----------------------------------------------------------------------===//
20 #include "llvm/Analysis/BasicAliasAnalysis.h"
21 #include "llvm/BasicBlock.h"
22 #include "llvm/Support/InstVisitor.h"
23 #include "llvm/iMemory.h"
24 #include "llvm/Constants.h"
25 #include "llvm/GlobalValue.h"
26 #include "llvm/DerivedTypes.h"
28 // Register the AliasAnalysis interface, providing a nice name to refer to.
29 static RegisterAnalysisGroup<AliasAnalysis> X("Alias Analysis");
31 // CanModify - Define a little visitor class that is used to check to see if
32 // arbitrary chunks of code can modify a specified pointer.
35 struct CanModify : public InstVisitor<CanModify, bool> {
36 const AliasAnalysis &AA;
39 CanModify(const AliasAnalysis *aa, const Value *ptr)
40 : AA(*aa), Ptr(ptr) {}
42 bool visitInvokeInst(InvokeInst &II) {
43 return AA.canInvokeModify(II, Ptr);
45 bool visitCallInst(CallInst &CI) {
46 return AA.canCallModify(CI, Ptr);
48 bool visitStoreInst(StoreInst &SI) {
49 return AA.alias(Ptr, SI.getOperand(1));
52 // Other instructions do not alias anything.
53 bool visitInstruction(Instruction &I) { return false; }
57 // AliasAnalysis destructor: DO NOT move this to the header file for
58 // AliasAnalysis or else clients of the AliasAnalysis class may not depend on
59 // the AliasAnalysis.o file in the current .a file, causing alias analysis
60 // support to not be included in the tool correctly!
62 AliasAnalysis::~AliasAnalysis() {}
64 /// canBasicBlockModify - Return true if it is possible for execution of the
65 /// specified basic block to modify the value pointed to by Ptr.
67 bool AliasAnalysis::canBasicBlockModify(const BasicBlock &bb,
68 const Value *Ptr) const {
69 CanModify CM(this, Ptr);
70 BasicBlock &BB = const_cast<BasicBlock&>(bb);
72 for (BasicBlock::iterator I = BB.begin(), E = BB.end(); I != E; ++I)
73 if (CM.visit(I)) // Check every instruction in the basic block...
79 /// canInstructionRangeModify - Return true if it is possible for the execution
80 /// of the specified instructions to modify the value pointed to by Ptr. The
81 /// instructions to consider are all of the instructions in the range of [I1,I2]
82 /// INCLUSIVE. I1 and I2 must be in the same basic block.
84 bool AliasAnalysis::canInstructionRangeModify(const Instruction &I1,
85 const Instruction &I2,
86 const Value *Ptr) const {
87 assert(I1.getParent() == I2.getParent() &&
88 "Instructions not in same basic block!");
89 CanModify CM(this, Ptr);
90 BasicBlock::iterator I = const_cast<Instruction*>(&I1);
91 BasicBlock::iterator E = const_cast<Instruction*>(&I2);
92 ++E; // Convert from inclusive to exclusive range.
95 if (CM.visit(I)) // Check every instruction in the basic block...
101 //===----------------------------------------------------------------------===//
102 // BasicAliasAnalysis Pass Implementation
103 //===----------------------------------------------------------------------===//
105 // Because of the way .a files work, the implementation of the
106 // BasicAliasAnalysis class MUST be in the AliasAnalysis file itself, or else we
107 // run the risk of AliasAnalysis being used, but the default implementation not
108 // being linked into the tool that uses it. As such, we register and implement
112 // Register this pass...
113 RegisterOpt<BasicAliasAnalysis>
114 X("basicaa", "Basic Alias Analysis (default AA impl)");
116 // Declare that we implement the AliasAnalysis interface
117 RegisterAnalysisGroup<AliasAnalysis, BasicAliasAnalysis, true> Y;
118 } // End of anonymous namespace
122 // hasUniqueAddress - Return true if the
123 static inline bool hasUniqueAddress(const Value *V) {
124 return isa<GlobalValue>(V) || isa<MallocInst>(V) || isa<AllocaInst>(V);
127 AliasAnalysis::Result BasicAliasAnalysis::alias(const Value *V1,
128 const Value *V2) const {
129 // Strip off constant pointer refs if they exist
130 if (const ConstantPointerRef *CPR = dyn_cast<ConstantPointerRef>(V1))
131 V1 = CPR->getValue();
132 if (const ConstantPointerRef *CPR = dyn_cast<ConstantPointerRef>(V2))
133 V2 = CPR->getValue();
135 // Are we checking for alias of the same value?
136 if (V1 == V2) return MustAlias;
138 if (!isa<PointerType>(V1->getType()) || !isa<PointerType>(V2->getType()))
139 return NoAlias; // Scalars cannot alias each other
141 bool V1Unique = hasUniqueAddress(V1);
142 bool V2Unique = hasUniqueAddress(V2);
144 if (V1Unique && V2Unique)
145 return NoAlias; // Can't alias if they are different unique values
147 if ((V1Unique && isa<ConstantPointerNull>(V2)) ||
148 (V2Unique && isa<ConstantPointerNull>(V1)))
149 return NoAlias; // Unique values don't alias null
151 // TODO: Handle getelementptr with nonzero offset