AliasAnalysis.cpp revision 22d8cd61436755a8bc15ff5dc660426ca1cb08d7
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/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" 27#include "llvm/Pass.h" 28 29// Register the AliasAnalysis interface, providing a nice name to refer to. 30static RegisterAnalysisGroup<AliasAnalysis> X("Alias Analysis"); 31 32// CanModify - Define a little visitor class that is used to check to see if 33// arbitrary chunks of code can modify a specified pointer. 34// 35namespace { 36 struct CanModify : public InstVisitor<CanModify, bool> { 37 const AliasAnalysis &AA; 38 const Value *Ptr; 39 40 CanModify(const AliasAnalysis *aa, const Value *ptr) 41 : AA(*aa), Ptr(ptr) {} 42 43 bool visitInvokeInst(InvokeInst &II) { 44 return AA.canInvokeModify(II, Ptr); 45 } 46 bool visitCallInst(CallInst &CI) { 47 return AA.canCallModify(CI, Ptr); 48 } 49 bool visitStoreInst(StoreInst &SI) { 50 assert(!SI.hasIndices() && "Only support stores without indexing!"); 51 return AA.alias(Ptr, SI.getOperand(1)); 52 } 53 54 // Other instructions do not alias anything. 55 bool visitInstruction(Instruction &I) { return false; } 56 }; 57} 58 59// AliasAnalysis destructor: DO NOT move this to the header file for 60// AliasAnalysis or else clients of the AliasAnalysis class may not depend on 61// the AliasAnalysis.o file in the current .a file, causing alias analysis 62// support to not be included in the tool correctly! 63// 64AliasAnalysis::~AliasAnalysis() {} 65 66// canBasicBlockModify - Return true if it is possible for execution of the 67// specified basic block to modify the value pointed to by Ptr. 68// 69bool AliasAnalysis::canBasicBlockModify(const BasicBlock &bb, 70 const Value *Ptr) const { 71 CanModify CM(this, Ptr); 72 BasicBlock &BB = const_cast<BasicBlock&>(bb); 73 74 for (BasicBlock::iterator I = BB.begin(), E = BB.end(); I != E; ++I) 75 if (CM.visit(I)) // Check every instruction in the basic block... 76 return true; 77 78 return false; 79} 80 81// canInstructionRangeModify - Return true if it is possible for the execution 82// of the specified instructions to modify the value pointed to by Ptr. The 83// instructions to consider are all of the instructions in the range of [I1,I2] 84// INCLUSIVE. I1 and I2 must be in the same basic block. 85// 86bool AliasAnalysis::canInstructionRangeModify(const Instruction &I1, 87 const Instruction &I2, 88 const Value *Ptr) const { 89 assert(I1.getParent() == I2.getParent() && 90 "Instructions not in same basic block!"); 91 CanModify CM(this, Ptr); 92 BasicBlock::iterator I = const_cast<Instruction*>(&I1); 93 BasicBlock::iterator E = const_cast<Instruction*>(&I2); 94 ++E; // Convert from inclusive to exclusive range. 95 96 for (; I != E; ++I) 97 if (CM.visit(I)) // Check every instruction in the basic block... 98 return true; 99 100 return false; 101} 102 103//===----------------------------------------------------------------------===// 104// BasicAliasAnalysis Pass Implementation 105//===----------------------------------------------------------------------===// 106// 107// Because of the way .a files work, the implementation of the 108// BasicAliasAnalysis class MUST be in the AliasAnalysis file itself, or else we 109// run the risk of AliasAnalysis being used, but the default implementation not 110// being linked into the tool that uses it. As such, we register and implement 111// the class here. 112// 113namespace { 114 // Register this pass... 115 RegisterOpt<BasicAliasAnalysis> 116 X("basicaa", "Basic Alias Analysis (default AA impl)"); 117 118 // Declare that we implement the AliasAnalysis interface 119 RegisterAnalysisGroup<AliasAnalysis, BasicAliasAnalysis, true> Y; 120} // End of anonymous namespace 121 122 123 124// hasUniqueAddress - Return true if the 125static inline bool hasUniqueAddress(const Value *V) { 126 return isa<GlobalValue>(V) || isa<MallocInst>(V) || isa<AllocaInst>(V); 127} 128 129AliasAnalysis::Result BasicAliasAnalysis::alias(const Value *V1, 130 const Value *V2) const { 131 // Strip off constant pointer refs if they exist 132 if (const ConstantPointerRef *CPR = dyn_cast<ConstantPointerRef>(V1)) 133 V1 = CPR->getValue(); 134 if (const ConstantPointerRef *CPR = dyn_cast<ConstantPointerRef>(V2)) 135 V2 = CPR->getValue(); 136 137 // Are we checking for alias of the same value? 138 if (V1 == V2) return MustAlias; 139 140 if (!isa<PointerType>(V1->getType()) || !isa<PointerType>(V2->getType())) 141 return NoAlias; // Scalars cannot alias each other 142 143 bool V1Unique = hasUniqueAddress(V1); 144 bool V2Unique = hasUniqueAddress(V2); 145 146 if (V1Unique && V2Unique) 147 return NoAlias; // Can't alias if they are different unique values 148 149 if ((V1Unique && isa<ConstantPointerNull>(V2)) || 150 (V2Unique && isa<ConstantPointerNull>(V1))) 151 return NoAlias; // Unique values don't alias null 152 153 // TODO: Handle getelementptr with nonzero offset 154 155 return MayAlias; 156} 157