lib/Analysis/ScalarEvolutionAliasAnalysis.cpp

   1 //===- ScalarEvolutionAliasAnalysis.cpp - SCEV-based Alias Analysis -------===//
   2 //
   3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
   4 // See https://llvm.org/LICENSE.txt for license information.
   5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
   6 //
   7 //===----------------------------------------------------------------------===//
   8 //
   9 // This file defines the ScalarEvolutionAliasAnalysis pass, which implements a
  10 // simple alias analysis implemented in terms of ScalarEvolution queries.
  11 //
  12 // This differs from traditional loop dependence analysis in that it tests
  13 // for dependencies within a single iteration of a loop, rather than
  14 // dependencies between different iterations.
  15 //
  16 // ScalarEvolution has a more complete understanding of pointer arithmetic
  17 // than BasicAliasAnalysis' collection of ad-hoc analyses.
  18 //
  19 //===----------------------------------------------------------------------===//
  20
  21 #include "llvm/Analysis/ScalarEvolutionAliasAnalysis.h"
  22 using namespace llvm;
  23
  24 AliasResult SCEVAAResult::alias(const MemoryLocation &LocA,
  25                                 const MemoryLocation &LocB, AAQueryInfo &AAQI) {
  26   // If either of the memory references is empty, it doesn't matter what the
  27   // pointer values are. This allows the code below to ignore this special
  28   // case.
  29   if (LocA.Size.isZero() || LocB.Size.isZero())
  30     return NoAlias;
  31
  32   // This is SCEVAAResult. Get the SCEVs!
  33   const SCEV *AS = SE.getSCEV(const_cast<Value *>(LocA.Ptr));
  34   const SCEV *BS = SE.getSCEV(const_cast<Value *>(LocB.Ptr));
  35
  36   // If they evaluate to the same expression, it's a MustAlias.
  37   if (AS == BS)
  38     return MustAlias;
  39
  40   // If something is known about the difference between the two addresses,
  41   // see if it's enough to prove a NoAlias.
  42   if (SE.getEffectiveSCEVType(AS->getType()) ==
  43       SE.getEffectiveSCEVType(BS->getType())) {
  44     unsigned BitWidth = SE.getTypeSizeInBits(AS->getType());
  45     APInt ASizeInt(BitWidth, LocA.Size.hasValue()
  46                                  ? LocA.Size.getValue()
  47                                  : MemoryLocation::UnknownSize);
  48     APInt BSizeInt(BitWidth, LocB.Size.hasValue()
  49                                  ? LocB.Size.getValue()
  50                                  : MemoryLocation::UnknownSize);
  51
  52     // Compute the difference between the two pointers.
  53     const SCEV *BA = SE.getMinusSCEV(BS, AS);
  54
  55     // Test whether the difference is known to be great enough that memory of
  56     // the given sizes don't overlap. This assumes that ASizeInt and BSizeInt
  57     // are non-zero, which is special-cased above.
  58     if (ASizeInt.ule(SE.getUnsignedRange(BA).getUnsignedMin()) &&
  59         (-BSizeInt).uge(SE.getUnsignedRange(BA).getUnsignedMax()))
  60       return NoAlias;
  61
  62     // Folding the subtraction while preserving range information can be tricky
  63     // (because of INT_MIN, etc.); if the prior test failed, swap AS and BS
  64     // and try again to see if things fold better that way.
  65
  66     // Compute the difference between the two pointers.
  67     const SCEV *AB = SE.getMinusSCEV(AS, BS);
  68
  69     // Test whether the difference is known to be great enough that memory of
  70     // the given sizes don't overlap. This assumes that ASizeInt and BSizeInt
  71     // are non-zero, which is special-cased above.
  72     if (BSizeInt.ule(SE.getUnsignedRange(AB).getUnsignedMin()) &&
  73         (-ASizeInt).uge(SE.getUnsignedRange(AB).getUnsignedMax()))
  74       return NoAlias;
  75   }
  76
  77   // If ScalarEvolution can find an underlying object, form a new query.
  78   // The correctness of this depends on ScalarEvolution not recognizing
  79   // inttoptr and ptrtoint operators.
  80   Value *AO = GetBaseValue(AS);
  81   Value *BO = GetBaseValue(BS);
  82   if ((AO && AO != LocA.Ptr) || (BO && BO != LocB.Ptr))
  83     if (alias(MemoryLocation(AO ? AO : LocA.Ptr,
  84                              AO ? LocationSize::unknown() : LocA.Size,
  85                              AO ? AAMDNodes() : LocA.AATags),
  86               MemoryLocation(BO ? BO : LocB.Ptr,
  87                              BO ? LocationSize::unknown() : LocB.Size,
  88                              BO ? AAMDNodes() : LocB.AATags),
  89               AAQI) == NoAlias)
  90       return NoAlias;
  91
  92   // Forward the query to the next analysis.
  93   return AAResultBase::alias(LocA, LocB, AAQI);
  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 }