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      1 //===- BasicAliasAnalysis.h - Stateless, local Alias Analysis ---*- C++ -*-===//
      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 /// \file
     10 /// This is the interface for LLVM's primary stateless and local alias analysis.
     11 ///
     12 //===----------------------------------------------------------------------===//
     13 
     14 #ifndef LLVM_ANALYSIS_BASICALIASANALYSIS_H
     15 #define LLVM_ANALYSIS_BASICALIASANALYSIS_H
     16 
     17 #include "llvm/ADT/DenseMap.h"
     18 #include "llvm/ADT/Optional.h"
     19 #include "llvm/ADT/SmallPtrSet.h"
     20 #include "llvm/ADT/SmallVector.h"
     21 #include "llvm/Analysis/AliasAnalysis.h"
     22 #include "llvm/Analysis/AssumptionCache.h"
     23 #include "llvm/Analysis/MemoryLocation.h"
     24 #include "llvm/IR/CallSite.h"
     25 #include "llvm/IR/PassManager.h"
     26 #include "llvm/Pass.h"
     27 #include <algorithm>
     28 #include <cstdint>
     29 #include <memory>
     30 #include <utility>
     31 
     32 namespace llvm {
     33 
     34 struct AAMDNodes;
     35 class APInt;
     36 class AssumptionCache;
     37 class BasicBlock;
     38 class DataLayout;
     39 class DominatorTree;
     40 class Function;
     41 class GEPOperator;
     42 class LoopInfo;
     43 class PHINode;
     44 class SelectInst;
     45 class TargetLibraryInfo;
     46 class Value;
     47 
     48 /// This is the AA result object for the basic, local, and stateless alias
     49 /// analysis. It implements the AA query interface in an entirely stateless
     50 /// manner. As one consequence, it is never invalidated due to IR changes.
     51 /// While it does retain some storage, that is used as an optimization and not
     52 /// to preserve information from query to query. However it does retain handles
     53 /// to various other analyses and must be recomputed when those analyses are.
     54 class BasicAAResult : public AAResultBase<BasicAAResult> {
     55   friend AAResultBase<BasicAAResult>;
     56 
     57   const DataLayout &DL;
     58   const TargetLibraryInfo &TLI;
     59   AssumptionCache &AC;
     60   DominatorTree *DT;
     61   LoopInfo *LI;
     62 
     63 public:
     64   BasicAAResult(const DataLayout &DL, const TargetLibraryInfo &TLI,
     65                 AssumptionCache &AC, DominatorTree *DT = nullptr,
     66                 LoopInfo *LI = nullptr)
     67       : AAResultBase(), DL(DL), TLI(TLI), AC(AC), DT(DT), LI(LI) {}
     68 
     69   BasicAAResult(const BasicAAResult &Arg)
     70       : AAResultBase(Arg), DL(Arg.DL), TLI(Arg.TLI), AC(Arg.AC), DT(Arg.DT),
     71         LI(Arg.LI) {}
     72   BasicAAResult(BasicAAResult &&Arg)
     73       : AAResultBase(std::move(Arg)), DL(Arg.DL), TLI(Arg.TLI), AC(Arg.AC),
     74         DT(Arg.DT), LI(Arg.LI) {}
     75 
     76   /// Handle invalidation events in the new pass manager.
     77   bool invalidate(Function &F, const PreservedAnalyses &PA,
     78                   FunctionAnalysisManager::Invalidator &Inv);
     79 
     80   AliasResult alias(const MemoryLocation &LocA, const MemoryLocation &LocB);
     81 
     82   ModRefInfo getModRefInfo(ImmutableCallSite CS, const MemoryLocation &Loc);
     83 
     84   ModRefInfo getModRefInfo(ImmutableCallSite CS1, ImmutableCallSite CS2);
     85 
     86   /// Chases pointers until we find a (constant global) or not.
     87   bool pointsToConstantMemory(const MemoryLocation &Loc, bool OrLocal);
     88 
     89   /// Get the location associated with a pointer argument of a callsite.
     90   ModRefInfo getArgModRefInfo(ImmutableCallSite CS, unsigned ArgIdx);
     91 
     92   /// Returns the behavior when calling the given call site.
     93   FunctionModRefBehavior getModRefBehavior(ImmutableCallSite CS);
     94 
     95   /// Returns the behavior when calling the given function. For use when the
     96   /// call site is not known.
     97   FunctionModRefBehavior getModRefBehavior(const Function *F);
     98 
     99 private:
    100   // A linear transformation of a Value; this class represents ZExt(SExt(V,
    101   // SExtBits), ZExtBits) * Scale + Offset.
    102   struct VariableGEPIndex {
    103     // An opaque Value - we can't decompose this further.
    104     const Value *V;
    105 
    106     // We need to track what extensions we've done as we consider the same Value
    107     // with different extensions as different variables in a GEP's linear
    108     // expression;
    109     // e.g.: if V == -1, then sext(x) != zext(x).
    110     unsigned ZExtBits;
    111     unsigned SExtBits;
    112 
    113     int64_t Scale;
    114 
    115     bool operator==(const VariableGEPIndex &Other) const {
    116       return V == Other.V && ZExtBits == Other.ZExtBits &&
    117              SExtBits == Other.SExtBits && Scale == Other.Scale;
    118     }
    119 
    120     bool operator!=(const VariableGEPIndex &Other) const {
    121       return !operator==(Other);
    122     }
    123   };
    124 
    125   // Represents the internal structure of a GEP, decomposed into a base pointer,
    126   // constant offsets, and variable scaled indices.
    127   struct DecomposedGEP {
    128     // Base pointer of the GEP
    129     const Value *Base;
    130     // Total constant offset w.r.t the base from indexing into structs
    131     int64_t StructOffset;
    132     // Total constant offset w.r.t the base from indexing through
    133     // pointers/arrays/vectors
    134     int64_t OtherOffset;
    135     // Scaled variable (non-constant) indices.
    136     SmallVector<VariableGEPIndex, 4> VarIndices;
    137   };
    138 
    139   /// Track alias queries to guard against recursion.
    140   using LocPair = std::pair<MemoryLocation, MemoryLocation>;
    141   using AliasCacheTy = SmallDenseMap<LocPair, AliasResult, 8>;
    142   AliasCacheTy AliasCache;
    143 
    144   /// Tracks phi nodes we have visited.
    145   ///
    146   /// When interpret "Value" pointer equality as value equality we need to make
    147   /// sure that the "Value" is not part of a cycle. Otherwise, two uses could
    148   /// come from different "iterations" of a cycle and see different values for
    149   /// the same "Value" pointer.
    150   ///
    151   /// The following example shows the problem:
    152   ///   %p = phi(%alloca1, %addr2)
    153   ///   %l = load %ptr
    154   ///   %addr1 = gep, %alloca2, 0, %l
    155   ///   %addr2 = gep  %alloca2, 0, (%l + 1)
    156   ///      alias(%p, %addr1) -> MayAlias !
    157   ///   store %l, ...
    158   SmallPtrSet<const BasicBlock *, 8> VisitedPhiBBs;
    159 
    160   /// Tracks instructions visited by pointsToConstantMemory.
    161   SmallPtrSet<const Value *, 16> Visited;
    162 
    163   static const Value *
    164   GetLinearExpression(const Value *V, APInt &Scale, APInt &Offset,
    165                       unsigned &ZExtBits, unsigned &SExtBits,
    166                       const DataLayout &DL, unsigned Depth, AssumptionCache *AC,
    167                       DominatorTree *DT, bool &NSW, bool &NUW);
    168 
    169   static bool DecomposeGEPExpression(const Value *V, DecomposedGEP &Decomposed,
    170       const DataLayout &DL, AssumptionCache *AC, DominatorTree *DT);
    171 
    172   static bool isGEPBaseAtNegativeOffset(const GEPOperator *GEPOp,
    173       const DecomposedGEP &DecompGEP, const DecomposedGEP &DecompObject,
    174       uint64_t ObjectAccessSize);
    175 
    176   /// \brief A Heuristic for aliasGEP that searches for a constant offset
    177   /// between the variables.
    178   ///
    179   /// GetLinearExpression has some limitations, as generally zext(%x + 1)
    180   /// != zext(%x) + zext(1) if the arithmetic overflows. GetLinearExpression
    181   /// will therefore conservatively refuse to decompose these expressions.
    182   /// However, we know that, for all %x, zext(%x) != zext(%x + 1), even if
    183   /// the addition overflows.
    184   bool
    185   constantOffsetHeuristic(const SmallVectorImpl<VariableGEPIndex> &VarIndices,
    186                           uint64_t V1Size, uint64_t V2Size, int64_t BaseOffset,
    187                           AssumptionCache *AC, DominatorTree *DT);
    188 
    189   bool isValueEqualInPotentialCycles(const Value *V1, const Value *V2);
    190 
    191   void GetIndexDifference(SmallVectorImpl<VariableGEPIndex> &Dest,
    192                           const SmallVectorImpl<VariableGEPIndex> &Src);
    193 
    194   AliasResult aliasGEP(const GEPOperator *V1, uint64_t V1Size,
    195                        const AAMDNodes &V1AAInfo, const Value *V2,
    196                        uint64_t V2Size, const AAMDNodes &V2AAInfo,
    197                        const Value *UnderlyingV1, const Value *UnderlyingV2);
    198 
    199   AliasResult aliasPHI(const PHINode *PN, uint64_t PNSize,
    200                        const AAMDNodes &PNAAInfo, const Value *V2,
    201                        uint64_t V2Size, const AAMDNodes &V2AAInfo,
    202                        const Value *UnderV2);
    203 
    204   AliasResult aliasSelect(const SelectInst *SI, uint64_t SISize,
    205                           const AAMDNodes &SIAAInfo, const Value *V2,
    206                           uint64_t V2Size, const AAMDNodes &V2AAInfo,
    207                           const Value *UnderV2);
    208 
    209   AliasResult aliasCheck(const Value *V1, uint64_t V1Size, AAMDNodes V1AATag,
    210                          const Value *V2, uint64_t V2Size, AAMDNodes V2AATag,
    211                          const Value *O1 = nullptr, const Value *O2 = nullptr);
    212 };
    213 
    214 /// Analysis pass providing a never-invalidated alias analysis result.
    215 class BasicAA : public AnalysisInfoMixin<BasicAA> {
    216   friend AnalysisInfoMixin<BasicAA>;
    217 
    218   static AnalysisKey Key;
    219 
    220 public:
    221   using Result = BasicAAResult;
    222 
    223   BasicAAResult run(Function &F, FunctionAnalysisManager &AM);
    224 };
    225 
    226 /// Legacy wrapper pass to provide the BasicAAResult object.
    227 class BasicAAWrapperPass : public FunctionPass {
    228   std::unique_ptr<BasicAAResult> Result;
    229 
    230   virtual void anchor();
    231 
    232 public:
    233   static char ID;
    234 
    235   BasicAAWrapperPass();
    236 
    237   BasicAAResult &getResult() { return *Result; }
    238   const BasicAAResult &getResult() const { return *Result; }
    239 
    240   bool runOnFunction(Function &F) override;
    241   void getAnalysisUsage(AnalysisUsage &AU) const override;
    242 };
    243 
    244 FunctionPass *createBasicAAWrapperPass();
    245 
    246 /// A helper for the legacy pass manager to create a \c BasicAAResult object
    247 /// populated to the best of our ability for a particular function when inside
    248 /// of a \c ModulePass or a \c CallGraphSCCPass.
    249 BasicAAResult createLegacyPMBasicAAResult(Pass &P, Function &F);
    250 
    251 /// This class is a functor to be used in legacy module or SCC passes for
    252 /// computing AA results for a function. We store the results in fields so that
    253 /// they live long enough to be queried, but we re-use them each time.
    254 class LegacyAARGetter {
    255   Pass &P;
    256   Optional<BasicAAResult> BAR;
    257   Optional<AAResults> AAR;
    258 
    259 public:
    260   LegacyAARGetter(Pass &P) : P(P) {}
    261   AAResults &operator()(Function &F) {
    262     BAR.emplace(createLegacyPMBasicAAResult(P, F));
    263     AAR.emplace(createLegacyPMAAResults(P, F, *BAR));
    264     return *AAR;
    265   }
    266 };
    267 
    268 } // end namespace llvm
    269 
    270 #endif // LLVM_ANALYSIS_BASICALIASANALYSIS_H
    271