lib/Analysis/AliasAnalysis.cpp

   1 //===- AliasAnalysis.cpp - Generic Alias Analysis Interface Implementation -==//
   2 //
   3 //                     The LLVM Compiler Infrastructure
   4 //
   5 // This file is distributed under the University of Illinois Open Source
   6 // 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/Function.h"
  31 #include "llvm/IntrinsicInst.h"
  32 #include "llvm/Instructions.h"
  33 #include "llvm/Type.h"
  34 #include "llvm/Target/TargetData.h"
  35 using namespace llvm;
  36
  37 // Register the AliasAnalysis interface, providing a nice name to refer to.
  38 static RegisterAnalysisGroup<AliasAnalysis> Z("Alias Analysis");
  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 bool AliasAnalysis::hasNoModRefInfoForCalls() const {
  63   assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!");
  64   return AA->hasNoModRefInfoForCalls();
  65 }
  66
  67 void AliasAnalysis::deleteValue(Value *V) {
  68   assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!");
  69   AA->deleteValue(V);
  70 }
  71
  72 void AliasAnalysis::copyValue(Value *From, Value *To) {
  73   assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!");
  74   AA->copyValue(From, To);
  75 }
  76
  77 AliasAnalysis::ModRefResult
  78 AliasAnalysis::getModRefInfo(CallSite CS1, CallSite CS2) {
  79   // FIXME: we can do better.
  80   assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!");
  81   return AA->getModRefInfo(CS1, CS2);
  82 }
  83
  84
  85 //===----------------------------------------------------------------------===//
  86 // AliasAnalysis non-virtual helper method implementation
  87 //===----------------------------------------------------------------------===//
  88
  89 AliasAnalysis::ModRefResult
  90 AliasAnalysis::getModRefInfo(LoadInst *L, Value *P, unsigned Size) {
  91   return alias(L->getOperand(0), TD->getTypeStoreSize(L->getType()),
  92                P, Size) ? Ref : NoModRef;
  93 }
  94
  95 AliasAnalysis::ModRefResult
  96 AliasAnalysis::getModRefInfo(StoreInst *S, Value *P, unsigned Size) {
  97   // If the stored address cannot alias the pointer in question, then the
  98   // pointer cannot be modified by the store.
  99   if (!alias(S->getOperand(1),
 100              TD->getTypeStoreSize(S->getOperand(0)->getType()), P, Size))
 101     return NoModRef;
 102
 103   // If the pointer is a pointer to constant memory, then it could not have been
 104   // modified by this store.
 105   return pointsToConstantMemory(P) ? NoModRef : Mod;
 106 }
 107
 108 AliasAnalysis::ModRefBehavior
 109 AliasAnalysis::getModRefBehavior(CallSite CS,
 110                                  std::vector<PointerAccessInfo> *Info) {
 111   if (CS.doesNotAccessMemory())
 112     // Can't do better than this.
 113     return DoesNotAccessMemory;
 114   ModRefBehavior MRB = getModRefBehavior(CS.getCalledFunction(), Info);
 115   if (MRB != DoesNotAccessMemory && CS.onlyReadsMemory())
 116     return OnlyReadsMemory;
 117   return MRB;
 118 }
 119
 120 AliasAnalysis::ModRefBehavior
 121 AliasAnalysis::getModRefBehavior(Function *F,
 122                                  std::vector<PointerAccessInfo> *Info) {
 123   if (F) {
 124     if (F->doesNotAccessMemory())
 125       // Can't do better than this.
 126       return DoesNotAccessMemory;
 127     if (F->onlyReadsMemory())
 128       return OnlyReadsMemory;
 129     if (unsigned id = F->getIntrinsicID()) {
 130 #define GET_INTRINSIC_MODREF_BEHAVIOR
 131 #include "llvm/Intrinsics.gen"
 132 #undef GET_INTRINSIC_MODREF_BEHAVIOR
 133     }
 134   }
 135   return UnknownModRefBehavior;
 136 }
 137
 138 AliasAnalysis::ModRefResult
 139 AliasAnalysis::getModRefInfo(CallSite CS, Value *P, unsigned Size) {
 140   ModRefResult Mask = ModRef;
 141   ModRefBehavior MRB = getModRefBehavior(CS);
 142   if (MRB == DoesNotAccessMemory)
 143     return NoModRef;
 144   else if (MRB == OnlyReadsMemory)
 145     Mask = Ref;
 146   else if (MRB == AliasAnalysis::AccessesArguments) {
 147     bool doesAlias = false;
 148     for (CallSite::arg_iterator AI = CS.arg_begin(), AE = CS.arg_end();
 149          AI != AE; ++AI)
 150       if (alias(*AI, ~0U, P, Size) != NoAlias) {
 151         doesAlias = true;
 152         break;
 153       }
 154
 155     if (!doesAlias)
 156       return NoModRef;
 157   }
 158
 159   if (!AA) return Mask;
 160
 161   // If P points to a constant memory location, the call definitely could not
 162   // modify the memory location.
 163   if ((Mask & Mod) && AA->pointsToConstantMemory(P))
 164     Mask = ModRefResult(Mask & ~Mod);
 165
 166   return ModRefResult(Mask & AA->getModRefInfo(CS, P, Size));
 167 }
 168
 169 // AliasAnalysis destructor: DO NOT move this to the header file for
 170 // AliasAnalysis or else clients of the AliasAnalysis class may not depend on
 171 // the AliasAnalysis.o file in the current .a file, causing alias analysis
 172 // support to not be included in the tool correctly!
 173 //
 174 AliasAnalysis::~AliasAnalysis() {}
 175
 176 /// InitializeAliasAnalysis - Subclasses must call this method to initialize the
 177 /// AliasAnalysis interface before any other methods are called.
 178 ///
 179 void AliasAnalysis::InitializeAliasAnalysis(Pass *P) {
 180   TD = &P->getAnalysis<TargetData>();
 181   AA = &P->getAnalysis<AliasAnalysis>();
 182 }
 183
 184 // getAnalysisUsage - All alias analysis implementations should invoke this
 185 // directly (using AliasAnalysis::getAnalysisUsage(AU)) to make sure that
 186 // TargetData is required by the pass.
 187 void AliasAnalysis::getAnalysisUsage(AnalysisUsage &AU) const {
 188   AU.addRequired<TargetData>();            // All AA's need TargetData.
 189   AU.addRequired<AliasAnalysis>();         // All AA's chain
 190 }
 191
 192 /// canBasicBlockModify - Return true if it is possible for execution of the
 193 /// specified basic block to modify the value pointed to by Ptr.
 194 ///
 195 bool AliasAnalysis::canBasicBlockModify(const BasicBlock &BB,
 196                                         const Value *Ptr, unsigned Size) {
 197   return canInstructionRangeModify(BB.front(), BB.back(), Ptr, Size);
 198 }
 199
 200 /// canInstructionRangeModify - Return true if it is possible for the execution
 201 /// of the specified instructions to modify the value pointed to by Ptr.  The
 202 /// instructions to consider are all of the instructions in the range of [I1,I2]
 203 /// INCLUSIVE.  I1 and I2 must be in the same basic block.
 204 ///
 205 bool AliasAnalysis::canInstructionRangeModify(const Instruction &I1,
 206                                               const Instruction &I2,
 207                                               const Value *Ptr, unsigned Size) {
 208   assert(I1.getParent() == I2.getParent() &&
 209          "Instructions not in same basic block!");
 210   BasicBlock::iterator I = const_cast<Instruction*>(&I1);
 211   BasicBlock::iterator E = const_cast<Instruction*>(&I2);
 212   ++E;  // Convert from inclusive to exclusive range.
 213
 214   for (; I != E; ++I) // Check every instruction in range
 215     if (getModRefInfo(I, const_cast<Value*>(Ptr), Size) & Mod)
 216       return true;
 217   return false;
 218 }
 219
 220 /// isNoAliasCall - Return true if this pointer is returned by a noalias
 221 /// function.
 222 bool llvm::isNoAliasCall(const Value *V) {
 223   if (isa<CallInst>(V) || isa<InvokeInst>(V))
 224     return CallSite(const_cast<Instruction*>(cast<Instruction>(V)))
 225       .paramHasAttr(0, Attribute::NoAlias);
 226   return false;
 227 }
 228
 229 /// isIdentifiedObject - Return true if this pointer refers to a distinct and
 230 /// identifiable object.  This returns true for:
 231 ///    Global Variables and Functions
 232 ///    Allocas and Mallocs
 233 ///    ByVal and NoAlias Arguments
 234 ///    NoAlias returns
 235 ///
 236 bool llvm::isIdentifiedObject(const Value *V) {
 237   if (isa<GlobalValue>(V) || isa<AllocationInst>(V) || isNoAliasCall(V))
 238     return true;
 239   if (const Argument *A = dyn_cast<Argument>(V))
 240     return A->hasNoAliasAttr() || A->hasByValAttr();
 241   return false;
 242 }
 243
 244 // Because of the way .a files work, we must force the BasicAA implementation to
 245 // be pulled in if the AliasAnalysis classes are pulled in.  Otherwise we run
 246 // the risk of AliasAnalysis being used, but the default implementation not
 247 // being linked into the tool that uses it.
 248 DEFINING_FILE_FOR(AliasAnalysis)