lib/Analysis/AliasAnalysis.cpp

   1 //===- AliasAnalysis.cpp - Generic Alias Analysis Interface Implementation -==//
   2 //
   3 //                     The LLVM Compiler Infrastructure
   4 //
   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.
   7 //
   8 //===----------------------------------------------------------------------===//
   9 //
  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.
  12 //
  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,
  17 // etc.
  18 //
  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
  23 // easy cases.
  24 //
  25 //===----------------------------------------------------------------------===//
  26
  27 #include "llvm/Analysis/AliasAnalysis.h"
  28 #include "llvm/Pass.h"
  29 #include "llvm/BasicBlock.h"
  30 #include "llvm/Instructions.h"
  31 #include "llvm/Type.h"
  32 #include "llvm/Target/TargetData.h"
  33 using namespace llvm;
  34
  35 // Register the AliasAnalysis interface, providing a nice name to refer to.
  36 namespace {
  37   RegisterAnalysisGroup<AliasAnalysis> Z("Alias Analysis");
  38 }
  39 char AliasAnalysis::ID = 0;
  40
  41 //===----------------------------------------------------------------------===//
  42 // Default chaining methods
  43 //===----------------------------------------------------------------------===//
  44
  45 AliasAnalysis::AliasResult
  46 AliasAnalysis::alias(const Value *V1, unsigned V1Size,
  47                      const Value *V2, unsigned V2Size) {
  48   assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!");
  49   return AA->alias(V1, V1Size, V2, V2Size);
  50 }
  51
  52 void AliasAnalysis::getMustAliases(Value *P, std::vector<Value*> &RetVals) {
  53   assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!");
  54   return AA->getMustAliases(P, RetVals);
  55 }
  56
  57 bool AliasAnalysis::pointsToConstantMemory(const Value *P) {
  58   assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!");
  59   return AA->pointsToConstantMemory(P);
  60 }
  61
  62 AliasAnalysis::ModRefBehavior
  63 AliasAnalysis::getModRefBehavior(Function *F, CallSite CS,
  64                                  std::vector<PointerAccessInfo> *Info) {
  65   assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!");
  66   return AA->getModRefBehavior(F, CS, Info);
  67 }
  68
  69 bool AliasAnalysis::hasNoModRefInfoForCalls() const {
  70   assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!");
  71   return AA->hasNoModRefInfoForCalls();
  72 }
  73
  74 void AliasAnalysis::deleteValue(Value *V) {
  75   assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!");
  76   AA->deleteValue(V);
  77 }
  78
  79 void AliasAnalysis::copyValue(Value *From, Value *To) {
  80   assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!");
  81   AA->copyValue(From, To);
  82 }
  83
  84 AliasAnalysis::ModRefResult
  85 AliasAnalysis::getModRefInfo(CallSite CS1, CallSite CS2) {
  86   // FIXME: we can do better.
  87   assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!");
  88   return AA->getModRefInfo(CS1, CS2);
  89 }
  90
  91
  92 //===----------------------------------------------------------------------===//
  93 // AliasAnalysis non-virtual helper method implementation
  94 //===----------------------------------------------------------------------===//
  95
  96 AliasAnalysis::ModRefResult
  97 AliasAnalysis::getModRefInfo(LoadInst *L, Value *P, unsigned Size) {
  98   return alias(L->getOperand(0), TD->getTypeSize(L->getType()),
  99                P, Size) ? Ref : NoModRef;
 100 }
 101
 102 AliasAnalysis::ModRefResult
 103 AliasAnalysis::getModRefInfo(StoreInst *S, Value *P, unsigned Size) {
 104   // If the stored address cannot alias the pointer in question, then the
 105   // pointer cannot be modified by the store.
 106   if (!alias(S->getOperand(1), TD->getTypeSize(S->getOperand(0)->getType()),
 107              P, Size))
 108     return NoModRef;
 109
 110   // If the pointer is a pointer to constant memory, then it could not have been
 111   // modified by this store.
 112   return pointsToConstantMemory(P) ? NoModRef : Mod;
 113 }
 114
 115 AliasAnalysis::ModRefResult
 116 AliasAnalysis::getModRefInfo(CallSite CS, Value *P, unsigned Size) {
 117   ModRefResult Mask = ModRef;
 118   if (Function *F = CS.getCalledFunction()) {
 119     ModRefBehavior MRB = getModRefBehavior(F, CallSite());
 120     if (MRB == OnlyReadsMemory)
 121       Mask = Ref;
 122     else if (MRB == DoesNotAccessMemory)
 123       return NoModRef;
 124   }
 125
 126   if (!AA) return Mask;
 127
 128   // If P points to a constant memory location, the call definitely could not
 129   // modify the memory location.
 130   if ((Mask & Mod) && AA->pointsToConstantMemory(P))
 131     Mask = ModRefResult(Mask & ~Mod);
 132
 133   return ModRefResult(Mask & AA->getModRefInfo(CS, P, Size));
 134 }
 135
 136 // AliasAnalysis destructor: DO NOT move this to the header file for
 137 // AliasAnalysis or else clients of the AliasAnalysis class may not depend on
 138 // the AliasAnalysis.o file in the current .a file, causing alias analysis
 139 // support to not be included in the tool correctly!
 140 //
 141 AliasAnalysis::~AliasAnalysis() {}
 142
 143 /// setTargetData - Subclasses must call this method to initialize the
 144 /// AliasAnalysis interface before any other methods are called.
 145 ///
 146 void AliasAnalysis::InitializeAliasAnalysis(Pass *P) {
 147   TD = &P->getAnalysis<TargetData>();
 148   AA = &P->getAnalysis<AliasAnalysis>();
 149 }
 150
 151 // getAnalysisUsage - All alias analysis implementations should invoke this
 152 // directly (using AliasAnalysis::getAnalysisUsage(AU)) to make sure that
 153 // TargetData is required by the pass.
 154 void AliasAnalysis::getAnalysisUsage(AnalysisUsage &AU) const {
 155   AU.addRequired<TargetData>();            // All AA's need TargetData.
 156   AU.addRequired<AliasAnalysis>();         // All AA's chain
 157 }
 158
 159 /// canBasicBlockModify - Return true if it is possible for execution of the
 160 /// specified basic block to modify the value pointed to by Ptr.
 161 ///
 162 bool AliasAnalysis::canBasicBlockModify(const BasicBlock &BB,
 163                                         const Value *Ptr, unsigned Size) {
 164   return canInstructionRangeModify(BB.front(), BB.back(), Ptr, Size);
 165 }
 166
 167 /// canInstructionRangeModify - Return true if it is possible for the execution
 168 /// of the specified instructions to modify the value pointed to by Ptr.  The
 169 /// instructions to consider are all of the instructions in the range of [I1,I2]
 170 /// INCLUSIVE.  I1 and I2 must be in the same basic block.
 171 ///
 172 bool AliasAnalysis::canInstructionRangeModify(const Instruction &I1,
 173                                               const Instruction &I2,
 174                                               const Value *Ptr, unsigned Size) {
 175   assert(I1.getParent() == I2.getParent() &&
 176          "Instructions not in same basic block!");
 177   BasicBlock::iterator I = const_cast<Instruction*>(&I1);
 178   BasicBlock::iterator E = const_cast<Instruction*>(&I2);
 179   ++E;  // Convert from inclusive to exclusive range.
 180
 181   for (; I != E; ++I) // Check every instruction in range
 182     if (getModRefInfo(I, const_cast<Value*>(Ptr), Size) & Mod)
 183       return true;
 184   return false;
 185 }
 186
 187 // Because of the way .a files work, we must force the BasicAA implementation to
 188 // be pulled in if the AliasAnalysis classes are pulled in.  Otherwise we run
 189 // the risk of AliasAnalysis being used, but the default implementation not
 190 // being linked into the tool that uses it.
 191 DEFINING_FILE_FOR(AliasAnalysis)