1 //===- AliasAnalysis.cpp - Generic Alias Analysis Interface Implementation -==//
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 file implements the generic AliasAnalysis interface which is used as the
11 // common interface used by all clients and implementations of alias analysis.
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,
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
25 //===----------------------------------------------------------------------===//
27 #include "llvm/Analysis/AliasAnalysis.h"
28 #include "llvm/BasicBlock.h"
29 #include "llvm/iMemory.h"
30 #include "llvm/Target/TargetData.h"
33 // Register the AliasAnalysis interface, providing a nice name to refer to.
35 RegisterAnalysisGroup<AliasAnalysis> Z("Alias Analysis");
38 AliasAnalysis::ModRefResult
39 AliasAnalysis::getModRefInfo(LoadInst *L, Value *P, unsigned Size) {
40 return alias(L->getOperand(0), TD->getTypeSize(L->getType()),
41 P, Size) ? Ref : NoModRef;
44 AliasAnalysis::ModRefResult
45 AliasAnalysis::getModRefInfo(StoreInst *S, Value *P, unsigned Size) {
46 // If the stored address cannot alias the pointer in question, then the
47 // pointer cannot be modified by the store.
48 if (!alias(S->getOperand(1), TD->getTypeSize(S->getOperand(0)->getType()),
52 // If the pointer is a pointer to constant memory, then it could not have been
53 // modified by this store.
54 return pointsToConstantMemory(P) ? NoModRef : Mod;
57 AliasAnalysis::ModRefResult
58 AliasAnalysis::getModRefInfo(CallSite CS, Value *P, unsigned Size) {
59 if (Function *F = CS.getCalledFunction())
60 if (onlyReadsMemory(F)) {
61 if (doesNotAccessMemory(F)) return NoModRef;
65 // If P points to a constant memory location, the call definitely could not
66 // modify the memory location.
67 return pointsToConstantMemory(P) ? Ref : ModRef;
70 AliasAnalysis::ModRefResult
71 AliasAnalysis::getModRefInfo(CallSite CS1, CallSite CS2) {
72 // FIXME: could probably do better.
77 // AliasAnalysis destructor: DO NOT move this to the header file for
78 // AliasAnalysis or else clients of the AliasAnalysis class may not depend on
79 // the AliasAnalysis.o file in the current .a file, causing alias analysis
80 // support to not be included in the tool correctly!
82 AliasAnalysis::~AliasAnalysis() {}
84 /// setTargetData - Subclasses must call this method to initialize the
85 /// AliasAnalysis interface before any other methods are called.
87 void AliasAnalysis::InitializeAliasAnalysis(Pass *P) {
88 TD = &P->getAnalysis<TargetData>();
91 // getAnalysisUsage - All alias analysis implementations should invoke this
92 // directly (using AliasAnalysis::getAnalysisUsage(AU)) to make sure that
93 // TargetData is required by the pass.
94 void AliasAnalysis::getAnalysisUsage(AnalysisUsage &AU) const {
95 AU.addRequired<TargetData>(); // All AA's need TargetData.
98 /// canBasicBlockModify - Return true if it is possible for execution of the
99 /// specified basic block to modify the value pointed to by Ptr.
101 bool AliasAnalysis::canBasicBlockModify(const BasicBlock &BB,
102 const Value *Ptr, unsigned Size) {
103 return canInstructionRangeModify(BB.front(), BB.back(), Ptr, Size);
106 /// canInstructionRangeModify - Return true if it is possible for the execution
107 /// of the specified instructions to modify the value pointed to by Ptr. The
108 /// instructions to consider are all of the instructions in the range of [I1,I2]
109 /// INCLUSIVE. I1 and I2 must be in the same basic block.
111 bool AliasAnalysis::canInstructionRangeModify(const Instruction &I1,
112 const Instruction &I2,
113 const Value *Ptr, unsigned Size) {
114 assert(I1.getParent() == I2.getParent() &&
115 "Instructions not in same basic block!");
116 BasicBlock::iterator I = const_cast<Instruction*>(&I1);
117 BasicBlock::iterator E = const_cast<Instruction*>(&I2);
118 ++E; // Convert from inclusive to exclusive range.
120 for (; I != E; ++I) // Check every instruction in range
121 if (getModRefInfo(I, const_cast<Value*>(Ptr), Size) & Mod)
126 // Because of the way .a files work, we must force the BasicAA implementation to
127 // be pulled in if the AliasAnalysis classes are pulled in. Otherwise we run
128 // the risk of AliasAnalysis being used, but the default implementation not
129 // being linked into the tool that uses it.
131 extern void llvm::BasicAAStub();
132 static IncludeFile INCLUDE_BASICAA_CPP((void*)&BasicAAStub);
136 struct NoAA : public ImmutablePass, public AliasAnalysis {
137 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
138 AliasAnalysis::getAnalysisUsage(AU);
141 virtual void initializePass() {
142 InitializeAliasAnalysis(this);
146 // Register this pass...
148 X("no-aa", "No Alias Analysis (always returns 'may' alias)");
150 // Declare that we implement the AliasAnalysis interface
151 RegisterAnalysisGroup<AliasAnalysis, NoAA> Y;
152 } // End of anonymous namespace