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.hasValue()
  47                                  ? LocA.Size.getValue()
  48                                  : MemoryLocation::UnknownSize);
  49     APInt BSizeInt(BitWidth, LocB.Size.hasValue()
  50                                  ? LocB.Size.getValue()
  51                                  : MemoryLocation::UnknownSize);
  52
  53     // Compute the difference between the two pointers.
  54     const SCEV *BA = SE.getMinusSCEV(BS, AS);
  55
  56     // Test whether the difference is known to be great enough that memory of
  57     // the given sizes don't overlap. This assumes that ASizeInt and BSizeInt
  58     // are non-zero, which is special-cased above.
  59     if (ASizeInt.ule(SE.getUnsignedRange(BA).getUnsignedMin()) &&
  60         (-BSizeInt).uge(SE.getUnsignedRange(BA).getUnsignedMax()))
  61       return NoAlias;
  62
  63     // Folding the subtraction while preserving range information can be tricky
  64     // (because of INT_MIN, etc.); if the prior test failed, swap AS and BS
  65     // and try again to see if things fold better that way.
  66
  67     // Compute the difference between the two pointers.
  68     const SCEV *AB = SE.getMinusSCEV(AS, BS);
  69
  70     // Test whether the difference is known to be great enough that memory of
  71     // the given sizes don't overlap. This assumes that ASizeInt and BSizeInt
  72     // are non-zero, which is special-cased above.
  73     if (BSizeInt.ule(SE.getUnsignedRange(AB).getUnsignedMin()) &&
  74         (-ASizeInt).uge(SE.getUnsignedRange(AB).getUnsignedMax()))
  75       return NoAlias;
  76   }
  77
  78   // If ScalarEvolution can find an underlying object, form a new query.
  79   // The correctness of this depends on ScalarEvolution not recognizing
  80   // inttoptr and ptrtoint operators.
  81   Value *AO = GetBaseValue(AS);
  82   Value *BO = GetBaseValue(BS);
  83   if ((AO && AO != LocA.Ptr) || (BO && BO != LocB.Ptr))
  84     if (alias(MemoryLocation(AO ? AO : LocA.Ptr,
  85                              AO ? +MemoryLocation::UnknownSize : LocA.Size,
  86                              AO ? AAMDNodes() : LocA.AATags),
  87               MemoryLocation(BO ? BO : LocB.Ptr,
  88                              BO ? +MemoryLocation::UnknownSize : LocB.Size,
  89                              BO ? AAMDNodes() : LocB.AATags)) == NoAlias)
  90       return NoAlias;
  91
  92   // Forward the query to the next analysis.
  93   return AAResultBase::alias(LocA, LocB);
  94 }
  95
  96 /// Given an expression, try to find a base value.
  97 ///
  98 /// Returns null if none was found.
  99 Value *SCEVAAResult::GetBaseValue(const SCEV *S) {
 100   if (const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(S)) {
 101     // In an addrec, assume that the base will be in the start, rather
 102     // than the step.
 103     return GetBaseValue(AR->getStart());
 104   } else if (const SCEVAddExpr *A = dyn_cast<SCEVAddExpr>(S)) {
 105     // If there's a pointer operand, it'll be sorted at the end of the list.
 106     const SCEV *Last = A->getOperand(A->getNumOperands() - 1);
 107     if (Last->getType()->isPointerTy())
 108       return GetBaseValue(Last);
 109   } else if (const SCEVUnknown *U = dyn_cast<SCEVUnknown>(S)) {
 110     // This is a leaf node.
 111     return U->getValue();
 112   }
 113   // No Identified object found.
 114   return nullptr;
 115 }
 116
 117 AnalysisKey SCEVAA::Key;
 118
 119 SCEVAAResult SCEVAA::run(Function &F, FunctionAnalysisManager &AM) {
 120   return SCEVAAResult(AM.getResult<ScalarEvolutionAnalysis>(F));
 121 }
 122
 123 char SCEVAAWrapperPass::ID = 0;
 124 INITIALIZE_PASS_BEGIN(SCEVAAWrapperPass, "scev-aa",
 125                       "ScalarEvolution-based Alias Analysis", false, true)
 126 INITIALIZE_PASS_DEPENDENCY(ScalarEvolutionWrapperPass)
 127 INITIALIZE_PASS_END(SCEVAAWrapperPass, "scev-aa",
 128                     "ScalarEvolution-based Alias Analysis", false, true)
 129
 130 FunctionPass *llvm::createSCEVAAWrapperPass() {
 131   return new SCEVAAWrapperPass();
 132 }
 133
 134 SCEVAAWrapperPass::SCEVAAWrapperPass() : FunctionPass(ID) {
 135   initializeSCEVAAWrapperPassPass(*PassRegistry::getPassRegistry());
 136 }
 137
 138 bool SCEVAAWrapperPass::runOnFunction(Function &F) {
 139   Result.reset(
 140       new SCEVAAResult(getAnalysis<ScalarEvolutionWrapperPass>().getSE()));
 141   return false;
 142 }
 143
 144 void SCEVAAWrapperPass::getAnalysisUsage(AnalysisUsage &AU) const {
 145   AU.setPreservesAll();
 146   AU.addRequired<ScalarEvolutionWrapperPass>();
 147 }