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annotate lib/Analysis/ScalarEvolutionAliasAnalysis.cpp @ 77:54457678186b LLVM3.6

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LLVM 3.6
author Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
date Mon, 08 Sep 2014 22:06:00 +0900
parents 95c75e76d11b
children afa8332a0e37
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1 //===- ScalarEvolutionAliasAnalysis.cpp - SCEV-based Alias Analysis -------===//
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2 //
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3 // The LLVM Compiler Infrastructure
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4 //
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5 // This file is distributed under the University of Illinois Open Source
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6 // License. See LICENSE.TXT for details.
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7 //
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8 //===----------------------------------------------------------------------===//
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9 //
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10 // This file defines the ScalarEvolutionAliasAnalysis pass, which implements a
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11 // simple alias analysis implemented in terms of ScalarEvolution queries.
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12 //
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13 // This differs from traditional loop dependence analysis in that it tests
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14 // for dependencies within a single iteration of a loop, rather than
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15 // dependencies between different iterations.
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16 //
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17 // ScalarEvolution has a more complete understanding of pointer arithmetic
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18 // than BasicAliasAnalysis' collection of ad-hoc analyses.
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19 //
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20 //===----------------------------------------------------------------------===//
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21
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22 #include "llvm/Analysis/Passes.h"
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23 #include "llvm/Analysis/AliasAnalysis.h"
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24 #include "llvm/Analysis/ScalarEvolutionExpressions.h"
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25 #include "llvm/Pass.h"
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26 using namespace llvm;
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27
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28 namespace {
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29 /// ScalarEvolutionAliasAnalysis - This is a simple alias analysis
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30 /// implementation that uses ScalarEvolution to answer queries.
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31 class ScalarEvolutionAliasAnalysis : public FunctionPass,
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32 public AliasAnalysis {
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33 ScalarEvolution *SE;
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34
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35 public:
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36 static char ID; // Class identification, replacement for typeinfo
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37 ScalarEvolutionAliasAnalysis() : FunctionPass(ID), SE(nullptr) {
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38 initializeScalarEvolutionAliasAnalysisPass(
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39 *PassRegistry::getPassRegistry());
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40 }
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41
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42 /// getAdjustedAnalysisPointer - This method is used when a pass implements
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43 /// an analysis interface through multiple inheritance. If needed, it
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44 /// should override this to adjust the this pointer as needed for the
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45 /// specified pass info.
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46 void *getAdjustedAnalysisPointer(AnalysisID PI) override {
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47 if (PI == &AliasAnalysis::ID)
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48 return (AliasAnalysis*)this;
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49 return this;
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50 }
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51
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52 private:
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53 void getAnalysisUsage(AnalysisUsage &AU) const override;
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54 bool runOnFunction(Function &F) override;
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55 AliasResult alias(const Location &LocA, const Location &LocB) override;
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56
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57 Value *GetBaseValue(const SCEV *S);
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58 };
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59 } // End of anonymous namespace
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60
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61 // Register this pass...
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62 char ScalarEvolutionAliasAnalysis::ID = 0;
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63 INITIALIZE_AG_PASS_BEGIN(ScalarEvolutionAliasAnalysis, AliasAnalysis, "scev-aa",
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64 "ScalarEvolution-based Alias Analysis", false, true, false)
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65 INITIALIZE_PASS_DEPENDENCY(ScalarEvolution)
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66 INITIALIZE_AG_PASS_END(ScalarEvolutionAliasAnalysis, AliasAnalysis, "scev-aa",
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67 "ScalarEvolution-based Alias Analysis", false, true, false)
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68
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69 FunctionPass *llvm::createScalarEvolutionAliasAnalysisPass() {
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70 return new ScalarEvolutionAliasAnalysis();
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71 }
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72
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73 void
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74 ScalarEvolutionAliasAnalysis::getAnalysisUsage(AnalysisUsage &AU) const {
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75 AU.addRequiredTransitive<ScalarEvolution>();
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76 AU.setPreservesAll();
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77 AliasAnalysis::getAnalysisUsage(AU);
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78 }
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79
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80 bool
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81 ScalarEvolutionAliasAnalysis::runOnFunction(Function &F) {
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82 InitializeAliasAnalysis(this);
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83 SE = &getAnalysis<ScalarEvolution>();
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84 return false;
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85 }
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86
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87 /// GetBaseValue - Given an expression, try to find a
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88 /// base value. Return null is none was found.
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89 Value *
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90 ScalarEvolutionAliasAnalysis::GetBaseValue(const SCEV *S) {
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91 if (const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(S)) {
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92 // In an addrec, assume that the base will be in the start, rather
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93 // than the step.
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94 return GetBaseValue(AR->getStart());
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95 } else if (const SCEVAddExpr *A = dyn_cast<SCEVAddExpr>(S)) {
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96 // If there's a pointer operand, it'll be sorted at the end of the list.
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97 const SCEV *Last = A->getOperand(A->getNumOperands()-1);
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98 if (Last->getType()->isPointerTy())
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99 return GetBaseValue(Last);
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100 } else if (const SCEVUnknown *U = dyn_cast<SCEVUnknown>(S)) {
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101 // This is a leaf node.
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102 return U->getValue();
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103 }
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104 // No Identified object found.
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105 return nullptr;
0
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106 }
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107
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108 AliasAnalysis::AliasResult
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109 ScalarEvolutionAliasAnalysis::alias(const Location &LocA,
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110 const Location &LocB) {
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111 // If either of the memory references is empty, it doesn't matter what the
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112 // pointer values are. This allows the code below to ignore this special
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113 // case.
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114 if (LocA.Size == 0 || LocB.Size == 0)
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115 return NoAlias;
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116
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117 // This is ScalarEvolutionAliasAnalysis. Get the SCEVs!
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118 const SCEV *AS = SE->getSCEV(const_cast<Value *>(LocA.Ptr));
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119 const SCEV *BS = SE->getSCEV(const_cast<Value *>(LocB.Ptr));
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120
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121 // If they evaluate to the same expression, it's a MustAlias.
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122 if (AS == BS) return MustAlias;
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123
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124 // If something is known about the difference between the two addresses,
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125 // see if it's enough to prove a NoAlias.
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126 if (SE->getEffectiveSCEVType(AS->getType()) ==
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127 SE->getEffectiveSCEVType(BS->getType())) {
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128 unsigned BitWidth = SE->getTypeSizeInBits(AS->getType());
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129 APInt ASizeInt(BitWidth, LocA.Size);
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130 APInt BSizeInt(BitWidth, LocB.Size);
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131
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132 // Compute the difference between the two pointers.
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133 const SCEV *BA = SE->getMinusSCEV(BS, AS);
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134
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135 // Test whether the difference is known to be great enough that memory of
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136 // the given sizes don't overlap. This assumes that ASizeInt and BSizeInt
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137 // are non-zero, which is special-cased above.
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138 if (ASizeInt.ule(SE->getUnsignedRange(BA).getUnsignedMin()) &&
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139 (-BSizeInt).uge(SE->getUnsignedRange(BA).getUnsignedMax()))
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140 return NoAlias;
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141
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142 // Folding the subtraction while preserving range information can be tricky
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143 // (because of INT_MIN, etc.); if the prior test failed, swap AS and BS
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144 // and try again to see if things fold better that way.
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145
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146 // Compute the difference between the two pointers.
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147 const SCEV *AB = SE->getMinusSCEV(AS, BS);
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148
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149 // Test whether the difference is known to be great enough that memory of
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150 // the given sizes don't overlap. This assumes that ASizeInt and BSizeInt
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151 // are non-zero, which is special-cased above.
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152 if (BSizeInt.ule(SE->getUnsignedRange(AB).getUnsignedMin()) &&
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153 (-ASizeInt).uge(SE->getUnsignedRange(AB).getUnsignedMax()))
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154 return NoAlias;
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155 }
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156
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157 // If ScalarEvolution can find an underlying object, form a new query.
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158 // The correctness of this depends on ScalarEvolution not recognizing
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159 // inttoptr and ptrtoint operators.
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160 Value *AO = GetBaseValue(AS);
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161 Value *BO = GetBaseValue(BS);
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162 if ((AO && AO != LocA.Ptr) || (BO && BO != LocB.Ptr))
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163 if (alias(Location(AO ? AO : LocA.Ptr,
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164 AO ? +UnknownSize : LocA.Size,
77
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165 AO ? AAMDNodes() : LocA.AATags),
0
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166 Location(BO ? BO : LocB.Ptr,
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167 BO ? +UnknownSize : LocB.Size,
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168 BO ? AAMDNodes() : LocB.AATags)) == NoAlias)
0
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169 return NoAlias;
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170
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171 // Forward the query to the next analysis.
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172 return AliasAnalysis::alias(LocA, LocB);
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173 }