lib/Analysis/ScalarEvolutionAliasAnalysis.cpp

   1 //===- ScalarEvolutionAliasAnalysis.cpp - SCEV-based Alias Analysis -------===//
   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 defines the ScalarEvolutionAliasAnalysis pass, which implements a
  11 // simple alias analysis implemented in terms of ScalarEvolution queries.
  12 //
  13 // This differs from traditional loop dependence analysis in that it tests
  14 // for dependencies within a single iteration of a loop, rather than
  15 // dependencies between different iterations.
  16 //
  17 // ScalarEvolution has a more complete understanding of pointer arithmetic
  18 // than BasicAliasAnalysis' collection of ad-hoc analyses.
  19 //
  20 //===----------------------------------------------------------------------===//
  21
  22 #include "llvm/Analysis/AliasAnalysis.h"
  23 #include "llvm/Analysis/ScalarEvolutionExpressions.h"
  24 #include "llvm/Analysis/Passes.h"
  25 #include "llvm/Pass.h"
  26 using namespace llvm;
  27
  28 namespace {
  29   /// ScalarEvolutionAliasAnalysis - This is a simple alias analysis
  30   /// implementation that uses ScalarEvolution to answer queries.
  31   class ScalarEvolutionAliasAnalysis : public FunctionPass,
  32                                        public AliasAnalysis {
  33     ScalarEvolution *SE;
  34
  35   public:
  36     static char ID; // Class identification, replacement for typeinfo
  37     ScalarEvolutionAliasAnalysis() : FunctionPass(ID), SE(0) {
  38       initializeScalarEvolutionAliasAnalysisPass(
  39         *PassRegistry::getPassRegistry());
  40     }
  41
  42     /// getAdjustedAnalysisPointer - This method is used when a pass implements
  43     /// an analysis interface through multiple inheritance.  If needed, it
  44     /// should override this to adjust the this pointer as needed for the
  45     /// specified pass info.
  46     virtual void *getAdjustedAnalysisPointer(AnalysisID PI) {
  47       if (PI == &AliasAnalysis::ID)
  48         return (AliasAnalysis*)this;
  49       return this;
  50     }
  51
  52   private:
  53     virtual void getAnalysisUsage(AnalysisUsage &AU) const;
  54     virtual bool runOnFunction(Function &F);
  55     virtual AliasResult alias(const Location &LocA, const Location &LocB);
  56
  57     Value *GetBaseValue(const SCEV *S);
  58   };
  59 }  // End of anonymous namespace
  60
  61 // Register this pass...
  62 char ScalarEvolutionAliasAnalysis::ID = 0;
  63 INITIALIZE_AG_PASS_BEGIN(ScalarEvolutionAliasAnalysis, AliasAnalysis, "scev-aa",
  64                    "ScalarEvolution-based Alias Analysis", false, true, false)
  65 INITIALIZE_PASS_DEPENDENCY(ScalarEvolution)
  66 INITIALIZE_AG_PASS_END(ScalarEvolutionAliasAnalysis, AliasAnalysis, "scev-aa",
  67                     "ScalarEvolution-based Alias Analysis", false, true, false)
  68
  69 FunctionPass *llvm::createScalarEvolutionAliasAnalysisPass() {
  70   return new ScalarEvolutionAliasAnalysis();
  71 }
  72
  73 void
  74 ScalarEvolutionAliasAnalysis::getAnalysisUsage(AnalysisUsage &AU) const {
  75   AU.addRequiredTransitive<ScalarEvolution>();
  76   AU.setPreservesAll();
  77   AliasAnalysis::getAnalysisUsage(AU);
  78 }
  79
  80 bool
  81 ScalarEvolutionAliasAnalysis::runOnFunction(Function &F) {
  82   InitializeAliasAnalysis(this);
  83   SE = &getAnalysis<ScalarEvolution>();
  84   return false;
  85 }
  86
  87 /// GetBaseValue - Given an expression, try to find a
  88 /// base value. Return null is none was found.
  89 Value *
  90 ScalarEvolutionAliasAnalysis::GetBaseValue(const SCEV *S) {
  91   if (const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(S)) {
  92     // In an addrec, assume that the base will be in the start, rather
  93     // than the step.
  94     return GetBaseValue(AR->getStart());
  95   } else if (const SCEVAddExpr *A = dyn_cast<SCEVAddExpr>(S)) {
  96     // If there's a pointer operand, it'll be sorted at the end of the list.
  97     const SCEV *Last = A->getOperand(A->getNumOperands()-1);
  98     if (Last->getType()->isPointerTy())
  99       return GetBaseValue(Last);
 100   } else if (const SCEVUnknown *U = dyn_cast<SCEVUnknown>(S)) {
 101     // This is a leaf node.
 102     return U->getValue();
 103   }
 104   // No Identified object found.
 105   return 0;
 106 }
 107
 108 AliasAnalysis::AliasResult
 109 ScalarEvolutionAliasAnalysis::alias(const Location &LocA,
 110                                     const Location &LocB) {
 111   // If either of the memory references is empty, it doesn't matter what the
 112   // pointer values are. This allows the code below to ignore this special
 113   // case.
 114   if (LocA.Size == 0 || LocB.Size == 0)
 115     return NoAlias;
 116
 117   // This is ScalarEvolutionAliasAnalysis. Get the SCEVs!
 118   const SCEV *AS = SE->getSCEV(const_cast<Value *>(LocA.Ptr));
 119   const SCEV *BS = SE->getSCEV(const_cast<Value *>(LocB.Ptr));
 120
 121   // If they evaluate to the same expression, it's a MustAlias.
 122   if (AS == BS) return MustAlias;
 123
 124   // If something is known about the difference between the two addresses,
 125   // see if it's enough to prove a NoAlias.
 126   if (SE->getEffectiveSCEVType(AS->getType()) ==
 127       SE->getEffectiveSCEVType(BS->getType())) {
 128     unsigned BitWidth = SE->getTypeSizeInBits(AS->getType());
 129     APInt ASizeInt(BitWidth, LocA.Size);
 130     APInt BSizeInt(BitWidth, LocB.Size);
 131
 132     // Compute the difference between the two pointers.
 133     const SCEV *BA = SE->getMinusSCEV(BS, AS);
 134
 135     // Test whether the difference is known to be great enough that memory of
 136     // the given sizes don't overlap. This assumes that ASizeInt and BSizeInt
 137     // are non-zero, which is special-cased above.
 138     if (ASizeInt.ule(SE->getUnsignedRange(BA).getUnsignedMin()) &&
 139         (-BSizeInt).uge(SE->getUnsignedRange(BA).getUnsignedMax()))
 140       return NoAlias;
 141
 142     // Folding the subtraction while preserving range information can be tricky
 143     // (because of INT_MIN, etc.); if the prior test failed, swap AS and BS
 144     // and try again to see if things fold better that way.
 145
 146     // Compute the difference between the two pointers.
 147     const SCEV *AB = SE->getMinusSCEV(AS, BS);
 148
 149     // Test whether the difference is known to be great enough that memory of
 150     // the given sizes don't overlap. This assumes that ASizeInt and BSizeInt
 151     // are non-zero, which is special-cased above.
 152     if (BSizeInt.ule(SE->getUnsignedRange(AB).getUnsignedMin()) &&
 153         (-ASizeInt).uge(SE->getUnsignedRange(AB).getUnsignedMax()))
 154       return NoAlias;
 155   }
 156
 157   // If ScalarEvolution can find an underlying object, form a new query.
 158   // The correctness of this depends on ScalarEvolution not recognizing
 159   // inttoptr and ptrtoint operators.
 160   Value *AO = GetBaseValue(AS);
 161   Value *BO = GetBaseValue(BS);
 162   if ((AO && AO != LocA.Ptr) || (BO && BO != LocB.Ptr))
 163     if (alias(Location(AO ? AO : LocA.Ptr,
 164                        AO ? +UnknownSize : LocA.Size,
 165                        AO ? 0 : LocA.TBAATag),
 166               Location(BO ? BO : LocB.Ptr,
 167                        BO ? +UnknownSize : LocB.Size,
 168                        BO ? 0 : LocB.TBAATag)) == NoAlias)
 169       return NoAlias;
 170
 171   // Forward the query to the next analysis.
 172   return AliasAnalysis::alias(LocA, LocB);
 173 }