llvm/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 #include "llvm/Analysis/ScalarEvolution.h"
  23 #include "llvm/Analysis/ScalarEvolutionExpressions.h"
  24 #include "llvm/InitializePasses.h"
  25 using namespace llvm;
  26
  27 static bool canComputePointerDiff(ScalarEvolution &SE,
  28                                   const SCEV *A, const SCEV *B) {
  29   if (SE.getEffectiveSCEVType(A->getType()) !=
  30       SE.getEffectiveSCEVType(B->getType()))
  31     return false;
  32
  33   return SE.instructionCouldExistWithOperands(A, B);
  34 }
  35
  36 AliasResult SCEVAAResult::alias(const MemoryLocation &LocA,
  37                                 const MemoryLocation &LocB, AAQueryInfo &AAQI,
  38                                 const Instruction *) {
  39   // If either of the memory references is empty, it doesn't matter what the
  40   // pointer values are. This allows the code below to ignore this special
  41   // case.
  42   if (LocA.Size.isZero() || LocB.Size.isZero())
  43     return AliasResult::NoAlias;
  44
  45   // This is SCEVAAResult. Get the SCEVs!
  46   const SCEV *AS = SE.getSCEV(const_cast<Value *>(LocA.Ptr));
  47   const SCEV *BS = SE.getSCEV(const_cast<Value *>(LocB.Ptr));
  48
  49   // If they evaluate to the same expression, it's a MustAlias.
  50   if (AS == BS)
  51     return AliasResult::MustAlias;
  52
  53   // If something is known about the difference between the two addresses,
  54   // see if it's enough to prove a NoAlias.
  55   if (canComputePointerDiff(SE, AS, BS)) {
  56     unsigned BitWidth = SE.getTypeSizeInBits(AS->getType());
  57     APInt ASizeInt(BitWidth, LocA.Size.hasValue()
  58                                  ? static_cast<uint64_t>(LocA.Size.getValue())
  59                                  : MemoryLocation::UnknownSize);
  60     APInt BSizeInt(BitWidth, LocB.Size.hasValue()
  61                                  ? static_cast<uint64_t>(LocB.Size.getValue())
  62                                  : MemoryLocation::UnknownSize);
  63
  64     // Compute the difference between the two pointers.
  65     const SCEV *BA = SE.getMinusSCEV(BS, AS);
  66
  67     // Test whether the difference is known to be great enough that memory of
  68     // the given sizes don't overlap. This assumes that ASizeInt and BSizeInt
  69     // are non-zero, which is special-cased above.
  70     if (!isa<SCEVCouldNotCompute>(BA) &&
  71         ASizeInt.ule(SE.getUnsignedRange(BA).getUnsignedMin()) &&
  72         (-BSizeInt).uge(SE.getUnsignedRange(BA).getUnsignedMax()))
  73       return AliasResult::NoAlias;
  74
  75     // Folding the subtraction while preserving range information can be tricky
  76     // (because of INT_MIN, etc.); if the prior test failed, swap AS and BS
  77     // and try again to see if things fold better that way.
  78
  79     // Compute the difference between the two pointers.
  80     const SCEV *AB = SE.getMinusSCEV(AS, BS);
  81
  82     // Test whether the difference is known to be great enough that memory of
  83     // the given sizes don't overlap. This assumes that ASizeInt and BSizeInt
  84     // are non-zero, which is special-cased above.
  85     if (!isa<SCEVCouldNotCompute>(AB) &&
  86         BSizeInt.ule(SE.getUnsignedRange(AB).getUnsignedMin()) &&
  87         (-ASizeInt).uge(SE.getUnsignedRange(AB).getUnsignedMax()))
  88       return AliasResult::NoAlias;
  89   }
  90
  91   // If ScalarEvolution can find an underlying object, form a new query.
  92   // The correctness of this depends on ScalarEvolution not recognizing
  93   // inttoptr and ptrtoint operators.
  94   Value *AO = GetBaseValue(AS);
  95   Value *BO = GetBaseValue(BS);
  96   if ((AO && AO != LocA.Ptr) || (BO && BO != LocB.Ptr))
  97     if (alias(MemoryLocation(AO ? AO : LocA.Ptr,
  98                              AO ? LocationSize::beforeOrAfterPointer()
  99                                 : LocA.Size,
 100                              AO ? AAMDNodes() : LocA.AATags),
 101               MemoryLocation(BO ? BO : LocB.Ptr,
 102                              BO ? LocationSize::beforeOrAfterPointer()
 103                                 : LocB.Size,
 104                              BO ? AAMDNodes() : LocB.AATags),
 105               AAQI, nullptr) == AliasResult::NoAlias)
 106       return AliasResult::NoAlias;
 107
 108   return AliasResult::MayAlias;
 109 }
 110
 111 /// Given an expression, try to find a base value.
 112 ///
 113 /// Returns null if none was found.
 114 Value *SCEVAAResult::GetBaseValue(const SCEV *S) {
 115   if (const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(S)) {
 116     // In an addrec, assume that the base will be in the start, rather
 117     // than the step.
 118     return GetBaseValue(AR->getStart());
 119   } else if (const SCEVAddExpr *A = dyn_cast<SCEVAddExpr>(S)) {
 120     // If there's a pointer operand, it'll be sorted at the end of the list.
 121     const SCEV *Last = A->getOperand(A->getNumOperands() - 1);
 122     if (Last->getType()->isPointerTy())
 123       return GetBaseValue(Last);
 124   } else if (const SCEVUnknown *U = dyn_cast<SCEVUnknown>(S)) {
 125     // This is a leaf node.
 126     return U->getValue();
 127   }
 128   // No Identified object found.
 129   return nullptr;
 130 }
 131
 132 bool SCEVAAResult::invalidate(Function &Fn, const PreservedAnalyses &PA,
 133                               FunctionAnalysisManager::Invalidator &Inv) {
 134   // We don't care if this analysis itself is preserved, it has no state. But
 135   // we need to check that the analyses it depends on have been.
 136   return Inv.invalidate<ScalarEvolutionAnalysis>(Fn, PA);
 137 }
 138
 139 AnalysisKey SCEVAA::Key;
 140
 141 SCEVAAResult SCEVAA::run(Function &F, FunctionAnalysisManager &AM) {
 142   return SCEVAAResult(AM.getResult<ScalarEvolutionAnalysis>(F));
 143 }
 144
 145 char SCEVAAWrapperPass::ID = 0;
 146 INITIALIZE_PASS_BEGIN(SCEVAAWrapperPass, "scev-aa",
 147                       "ScalarEvolution-based Alias Analysis", false, true)
 148 INITIALIZE_PASS_DEPENDENCY(ScalarEvolutionWrapperPass)
 149 INITIALIZE_PASS_END(SCEVAAWrapperPass, "scev-aa",
 150                     "ScalarEvolution-based Alias Analysis", false, true)
 151
 152 FunctionPass *llvm::createSCEVAAWrapperPass() {
 153   return new SCEVAAWrapperPass();
 154 }
 155
 156 SCEVAAWrapperPass::SCEVAAWrapperPass() : FunctionPass(ID) {
 157   initializeSCEVAAWrapperPassPass(*PassRegistry::getPassRegistry());
 158 }
 159
 160 bool SCEVAAWrapperPass::runOnFunction(Function &F) {
 161   Result.reset(
 162       new SCEVAAResult(getAnalysis<ScalarEvolutionWrapperPass>().getSE()));
 163   return false;
 164 }
 165
 166 void SCEVAAWrapperPass::getAnalysisUsage(AnalysisUsage &AU) const {
 167   AU.setPreservesAll();
 168   AU.addRequired<ScalarEvolutionWrapperPass>();
 169 }