1 //===- AliasAnalysis.cpp - Generic Alias Analysis Interface Implementation -==//
2 //
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.
5 //
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,
10 // etc.
11 //
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
16 // easy cases.
17 //
18 //===----------------------------------------------------------------------===//
19 
20 #include "llvm/Analysis/AliasAnalysis.h"
21 #include "llvm/BasicBlock.h"
22 #include "llvm/iMemory.h"
23 #include "llvm/Target/TargetData.h"
24 
25 // Register the AliasAnalysis interface, providing a nice name to refer to.
26 namespace {
27   RegisterAnalysisGroup<AliasAnalysis> Z("Alias Analysis");
28 }
29 
30 AliasAnalysis::ModRefResult
31 AliasAnalysis::getModRefInfo(LoadInst *L, Value *P, unsigned Size) {
32   return alias(L->getOperand(0), TD->getTypeSize(L->getType()),
33                P, Size) ? Ref : NoModRef;
34 }
35 
36 AliasAnalysis::ModRefResult
37 AliasAnalysis::getModRefInfo(StoreInst *S, Value *P, unsigned Size) {
38   return alias(S->getOperand(1), TD->getTypeSize(S->getOperand(0)->getType()),
39                P, Size) ? Mod : NoModRef;
40 }
41 
42 
43 // AliasAnalysis destructor: DO NOT move this to the header file for
44 // AliasAnalysis or else clients of the AliasAnalysis class may not depend on
45 // the AliasAnalysis.o file in the current .a file, causing alias analysis
46 // support to not be included in the tool correctly!
47 //
48 AliasAnalysis::~AliasAnalysis() {}
49 
50 /// setTargetData - Subclasses must call this method to initialize the
51 /// AliasAnalysis interface before any other methods are called.
52 ///
53 void AliasAnalysis::InitializeAliasAnalysis(Pass *P) {
54   TD = &P->getAnalysis<TargetData>();
55 }
56 
57 // getAnalysisUsage - All alias analysis implementations should invoke this
58 // directly (using AliasAnalysis::getAnalysisUsage(AU)) to make sure that
59 // TargetData is required by the pass.
60 void AliasAnalysis::getAnalysisUsage(AnalysisUsage &AU) const {
61   AU.addRequired<TargetData>();            // All AA's need TargetData.
62 }
63 
64 /// canBasicBlockModify - Return true if it is possible for execution of the
65 /// specified basic block to modify the value pointed to by Ptr.
66 ///
67 bool AliasAnalysis::canBasicBlockModify(const BasicBlock &BB,
68                                         const Value *Ptr, unsigned Size) {
69   return canInstructionRangeModify(BB.front(), BB.back(), Ptr, Size);
70 }
71 
72 /// canInstructionRangeModify - Return true if it is possible for the execution
73 /// of the specified instructions to modify the value pointed to by Ptr.  The
74 /// instructions to consider are all of the instructions in the range of [I1,I2]
75 /// INCLUSIVE.  I1 and I2 must be in the same basic block.
76 ///
77 bool AliasAnalysis::canInstructionRangeModify(const Instruction &I1,
78                                               const Instruction &I2,
79                                               const Value *Ptr, unsigned Size) {
80   assert(I1.getParent() == I2.getParent() &&
81          "Instructions not in same basic block!");
82   BasicBlock::iterator I = const_cast<Instruction*>(&I1);
83   BasicBlock::iterator E = const_cast<Instruction*>(&I2);
84   ++E;  // Convert from inclusive to exclusive range.
85 
86   for (; I != E; ++I) // Check every instruction in range
87     if (getModRefInfo(I, const_cast<Value*>(Ptr), Size) & Mod)
88       return true;
89   return false;
90 }
91 
92 // Because of the way .a files work, we must force the BasicAA implementation to
93 // be pulled in if the AliasAnalysis classes are pulled in.  Otherwise we run
94 // the risk of AliasAnalysis being used, but the default implementation not
95 // being linked into the tool that uses it.
96 //
97 extern void BasicAAStub();
98 static IncludeFile INCLUDE_BASICAA_CPP((void*)&BasicAAStub);
99 
100 
101 namespace {
102   struct NoAA : public ImmutablePass, public AliasAnalysis {
103     virtual void getAnalysisUsage(AnalysisUsage &AU) const {
104       AliasAnalysis::getAnalysisUsage(AU);
105     }
106 
107     virtual void initializePass() {
108       InitializeAliasAnalysis(this);
109     }
110   };
111 
112   // Register this pass...
113   RegisterOpt<NoAA>
114   X("no-aa", "No Alias Analysis (always returns 'may' alias)");
115 
116   // Declare that we implement the AliasAnalysis interface
117   RegisterAnalysisGroup<AliasAnalysis, NoAA> Y;
118 }  // End of anonymous namespace
119