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/ScalarEvolutionAliasAnalysis.h"
  23 using namespace llvm;
  24
  25 AliasResult SCEVAAResult::alias(const MemoryLocation &LocA,
  26                                 const MemoryLocation &LocB) {
  27   // If either of the memory references is empty, it doesn't matter what the
  28   // pointer values are. This allows the code below to ignore this special
  29   // case.
  30   if (LocA.Size == 0 || LocB.Size == 0)
  31     return NoAlias;
  32
  33   // This is SCEVAAResult. Get the SCEVs!
  34   const SCEV *AS = SE.getSCEV(const_cast<Value *>(LocA.Ptr));
  35   const SCEV *BS = SE.getSCEV(const_cast<Value *>(LocB.Ptr));
  36
  37   // If they evaluate to the same expression, it's a MustAlias.
  38   if (AS == BS)
  39     return MustAlias;
  40
  41   // If something is known about the difference between the two addresses,
  42   // see if it's enough to prove a NoAlias.
  43   if (SE.getEffectiveSCEVType(AS->getType()) ==
  44       SE.getEffectiveSCEVType(BS->getType())) {
  45     unsigned BitWidth = SE.getTypeSizeInBits(AS->getType());
  46     APInt ASizeInt(BitWidth, LocA.Size);
  47     APInt BSizeInt(BitWidth, LocB.Size);
  48
  49     // Compute the difference between the two pointers.
  50     const SCEV *BA = SE.getMinusSCEV(BS, AS);
  51
  52     // Test whether the difference is known to be great enough that memory of
  53     // the given sizes don't overlap. This assumes that ASizeInt and BSizeInt
  54     // are non-zero, which is special-cased above.
  55     if (ASizeInt.ule(SE.getUnsignedRange(BA).getUnsignedMin()) &&
  56         (-BSizeInt).uge(SE.getUnsignedRange(BA).getUnsignedMax()))
  57       return NoAlias;
  58
  59     // Folding the subtraction while preserving range information can be tricky
  60     // (because of INT_MIN, etc.); if the prior test failed, swap AS and BS
  61     // and try again to see if things fold better that way.
  62
  63     // Compute the difference between the two pointers.
  64     const SCEV *AB = SE.getMinusSCEV(AS, BS);
  65
  66     // Test whether the difference is known to be great enough that memory of
  67     // the given sizes don't overlap. This assumes that ASizeInt and BSizeInt
  68     // are non-zero, which is special-cased above.
  69     if (BSizeInt.ule(SE.getUnsignedRange(AB).getUnsignedMin()) &&
  70         (-ASizeInt).uge(SE.getUnsignedRange(AB).getUnsignedMax()))
  71       return NoAlias;
  72   }
  73
  74   // If ScalarEvolution can find an underlying object, form a new query.
  75   // The correctness of this depends on ScalarEvolution not recognizing
  76   // inttoptr and ptrtoint operators.
  77   Value *AO = GetBaseValue(AS);
  78   Value *BO = GetBaseValue(BS);
  79   if ((AO && AO != LocA.Ptr) || (BO && BO != LocB.Ptr))
  80     if (alias(MemoryLocation(AO ? AO : LocA.Ptr,
  81                              AO ? +MemoryLocation::UnknownSize : LocA.Size,
  82                              AO ? AAMDNodes() : LocA.AATags),
  83               MemoryLocation(BO ? BO : LocB.Ptr,
  84                              BO ? +MemoryLocation::UnknownSize : LocB.Size,
  85                              BO ? AAMDNodes() : LocB.AATags)) == NoAlias)
  86       return NoAlias;
  87
  88   // Forward the query to the next analysis.
  89   return AAResultBase::alias(LocA, LocB);
  90 }
  91
  92 /// Given an expression, try to find a base value.
  93 ///
  94 /// Returns null if none was found.
  95 Value *SCEVAAResult::GetBaseValue(const SCEV *S) {
  96   if (const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(S)) {
  97     // In an addrec, assume that the base will be in the start, rather
  98     // than the step.
  99     return GetBaseValue(AR->getStart());
 100   } else if (const SCEVAddExpr *A = dyn_cast<SCEVAddExpr>(S)) {
 101     // If there's a pointer operand, it'll be sorted at the end of the list.
 102     const SCEV *Last = A->getOperand(A->getNumOperands() - 1);
 103     if (Last->getType()->isPointerTy())
 104       return GetBaseValue(Last);
 105   } else if (const SCEVUnknown *U = dyn_cast<SCEVUnknown>(S)) {
 106     // This is a leaf node.
 107     return U->getValue();
 108   }
 109   // No Identified object found.
 110   return nullptr;
 111 }
 112
 113 AnalysisKey SCEVAA::Key;
 114
 115 SCEVAAResult SCEVAA::run(Function &F, FunctionAnalysisManager &AM) {
 116   return SCEVAAResult(AM.getResult<ScalarEvolutionAnalysis>(F));
 117 }
 118
 119 char SCEVAAWrapperPass::ID = 0;
 120 INITIALIZE_PASS_BEGIN(SCEVAAWrapperPass, "scev-aa",
 121                       "ScalarEvolution-based Alias Analysis", false, true)
 122 INITIALIZE_PASS_DEPENDENCY(ScalarEvolutionWrapperPass)
 123 INITIALIZE_PASS_END(SCEVAAWrapperPass, "scev-aa",
 124                     "ScalarEvolution-based Alias Analysis", false, true)
 125
 126 FunctionPass *llvm::createSCEVAAWrapperPass() {
 127   return new SCEVAAWrapperPass();
 128 }
 129
 130 SCEVAAWrapperPass::SCEVAAWrapperPass() : FunctionPass(ID) {
 131   initializeSCEVAAWrapperPassPass(*PassRegistry::getPassRegistry());
 132 }
 133
 134 bool SCEVAAWrapperPass::runOnFunction(Function &F) {
 135   Result.reset(
 136       new SCEVAAResult(getAnalysis<ScalarEvolutionWrapperPass>().getSE()));
 137   return false;
 138 }
 139
 140 void SCEVAAWrapperPass::getAnalysisUsage(AnalysisUsage &AU) const {
 141   AU.setPreservesAll();
 142   AU.addRequired<ScalarEvolutionWrapperPass>();
 143 }