LLVM API Documentation

AliasAnalysis.cpp

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00001 //===- AliasAnalysis.cpp - Generic Alias Analysis Interface Implementation -==//
00002 //
00003 //                     The LLVM Compiler Infrastructure
00004 //
00005 // This file was developed by the LLVM research group and is distributed under
00006 // the University of Illinois Open Source License. See LICENSE.TXT for details.
00007 //
00008 //===----------------------------------------------------------------------===//
00009 //
00010 // This file implements the generic AliasAnalysis interface which is used as the
00011 // common interface used by all clients and implementations of alias analysis.
00012 //
00013 // This file also implements the default version of the AliasAnalysis interface
00014 // that is to be used when no other implementation is specified.  This does some
00015 // simple tests that detect obvious cases: two different global pointers cannot
00016 // alias, a global cannot alias a malloc, two different mallocs cannot alias,
00017 // etc.
00018 //
00019 // This alias analysis implementation really isn't very good for anything, but
00020 // it is very fast, and makes a nice clean default implementation.  Because it
00021 // handles lots of little corner cases, other, more complex, alias analysis
00022 // implementations may choose to rely on this pass to resolve these simple and
00023 // easy cases.
00024 //
00025 //===----------------------------------------------------------------------===//
00026 
00027 #include "llvm/Analysis/AliasAnalysis.h"
00028 #include "llvm/Pass.h"
00029 #include "llvm/BasicBlock.h"
00030 #include "llvm/Instructions.h"
00031 #include "llvm/Type.h"
00032 #include "llvm/Target/TargetData.h"
00033 #include <iostream>
00034 using namespace llvm;
00035 
00036 // Register the AliasAnalysis interface, providing a nice name to refer to.
00037 namespace {
00038   RegisterAnalysisGroup<AliasAnalysis> Z("Alias Analysis");
00039 }
00040 
00041 //===----------------------------------------------------------------------===//
00042 // Default chaining methods
00043 //===----------------------------------------------------------------------===//
00044 
00045 AliasAnalysis::AliasResult
00046 AliasAnalysis::alias(const Value *V1, unsigned V1Size,
00047                      const Value *V2, unsigned V2Size) {
00048   assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!");
00049   return AA->alias(V1, V1Size, V2, V2Size);
00050 }
00051 
00052 void AliasAnalysis::getMustAliases(Value *P, std::vector<Value*> &RetVals) {
00053   assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!");
00054   return AA->getMustAliases(P, RetVals);
00055 }
00056 
00057 bool AliasAnalysis::pointsToConstantMemory(const Value *P) {
00058   assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!");
00059   return AA->pointsToConstantMemory(P);
00060 }
00061 
00062 AliasAnalysis::ModRefBehavior
00063 AliasAnalysis::getModRefBehavior(Function *F, CallSite CS,
00064                                  std::vector<PointerAccessInfo> *Info) {
00065   assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!");
00066   return AA->getModRefBehavior(F, CS, Info);
00067 }
00068 
00069 bool AliasAnalysis::hasNoModRefInfoForCalls() const {
00070   assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!");
00071   return AA->hasNoModRefInfoForCalls();
00072 }
00073 
00074 void AliasAnalysis::deleteValue(Value *V) {
00075   assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!");
00076   AA->deleteValue(V);
00077 }
00078 
00079 void AliasAnalysis::copyValue(Value *From, Value *To) {
00080   assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!");
00081   AA->copyValue(From, To);
00082 }
00083 
00084 AliasAnalysis::ModRefResult
00085 AliasAnalysis::getModRefInfo(CallSite CS1, CallSite CS2) {
00086   // FIXME: we can do better.
00087   assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!");
00088   return AA->getModRefInfo(CS1, CS2);
00089 }
00090 
00091 
00092 //===----------------------------------------------------------------------===//
00093 // AliasAnalysis non-virtual helper method implementation
00094 //===----------------------------------------------------------------------===//
00095 
00096 AliasAnalysis::ModRefResult
00097 AliasAnalysis::getModRefInfo(LoadInst *L, Value *P, unsigned Size) {
00098   return alias(L->getOperand(0), TD->getTypeSize(L->getType()),
00099                P, Size) ? Ref : NoModRef;
00100 }
00101 
00102 AliasAnalysis::ModRefResult
00103 AliasAnalysis::getModRefInfo(StoreInst *S, Value *P, unsigned Size) {
00104   // If the stored address cannot alias the pointer in question, then the
00105   // pointer cannot be modified by the store.
00106   if (!alias(S->getOperand(1), TD->getTypeSize(S->getOperand(0)->getType()),
00107              P, Size))
00108     return NoModRef;
00109 
00110   // If the pointer is a pointer to constant memory, then it could not have been
00111   // modified by this store.
00112   return pointsToConstantMemory(P) ? NoModRef : Mod;
00113 }
00114 
00115 AliasAnalysis::ModRefResult
00116 AliasAnalysis::getModRefInfo(CallSite CS, Value *P, unsigned Size) {
00117   ModRefResult Mask = ModRef;
00118   if (Function *F = CS.getCalledFunction()) {
00119     ModRefBehavior MRB = getModRefBehavior(F, CallSite());
00120     if (MRB == OnlyReadsMemory)
00121       Mask = Ref;
00122     else if (MRB == DoesNotAccessMemory)
00123       return NoModRef;
00124   }
00125 
00126   if (!AA) return Mask;
00127 
00128   // If P points to a constant memory location, the call definitely could not
00129   // modify the memory location.
00130   if ((Mask & Mod) && AA->pointsToConstantMemory(P))
00131     Mask = ModRefResult(Mask & ~Mod);
00132 
00133   return ModRefResult(Mask & AA->getModRefInfo(CS, P, Size));
00134 }
00135 
00136 // AliasAnalysis destructor: DO NOT move this to the header file for
00137 // AliasAnalysis or else clients of the AliasAnalysis class may not depend on
00138 // the AliasAnalysis.o file in the current .a file, causing alias analysis
00139 // support to not be included in the tool correctly!
00140 //
00141 AliasAnalysis::~AliasAnalysis() {}
00142 
00143 /// setTargetData - Subclasses must call this method to initialize the
00144 /// AliasAnalysis interface before any other methods are called.
00145 ///
00146 void AliasAnalysis::InitializeAliasAnalysis(Pass *P) {
00147   TD = &P->getAnalysis<TargetData>();
00148   AA = &P->getAnalysis<AliasAnalysis>();
00149 }
00150 
00151 // getAnalysisUsage - All alias analysis implementations should invoke this
00152 // directly (using AliasAnalysis::getAnalysisUsage(AU)) to make sure that
00153 // TargetData is required by the pass.
00154 void AliasAnalysis::getAnalysisUsage(AnalysisUsage &AU) const {
00155   AU.addRequired<TargetData>();            // All AA's need TargetData.
00156   AU.addRequired<AliasAnalysis>();         // All AA's chain
00157 }
00158 
00159 /// canBasicBlockModify - Return true if it is possible for execution of the
00160 /// specified basic block to modify the value pointed to by Ptr.
00161 ///
00162 bool AliasAnalysis::canBasicBlockModify(const BasicBlock &BB,
00163                                         const Value *Ptr, unsigned Size) {
00164   return canInstructionRangeModify(BB.front(), BB.back(), Ptr, Size);
00165 }
00166 
00167 /// canInstructionRangeModify - Return true if it is possible for the execution
00168 /// of the specified instructions to modify the value pointed to by Ptr.  The
00169 /// instructions to consider are all of the instructions in the range of [I1,I2]
00170 /// INCLUSIVE.  I1 and I2 must be in the same basic block.
00171 ///
00172 bool AliasAnalysis::canInstructionRangeModify(const Instruction &I1,
00173                                               const Instruction &I2,
00174                                               const Value *Ptr, unsigned Size) {
00175   assert(I1.getParent() == I2.getParent() &&
00176          "Instructions not in same basic block!");
00177   BasicBlock::iterator I = const_cast<Instruction*>(&I1);
00178   BasicBlock::iterator E = const_cast<Instruction*>(&I2);
00179   ++E;  // Convert from inclusive to exclusive range.
00180 
00181   for (; I != E; ++I) // Check every instruction in range
00182     if (getModRefInfo(I, const_cast<Value*>(Ptr), Size) & Mod)
00183       return true;
00184   return false;
00185 }
00186 
00187 // Because of the way .a files work, we must force the BasicAA implementation to
00188 // be pulled in if the AliasAnalysis classes are pulled in.  Otherwise we run
00189 // the risk of AliasAnalysis being used, but the default implementation not
00190 // being linked into the tool that uses it.
00191 DEFINING_FILE_FOR(AliasAnalysis)