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 PhiValues;
47 class Value;
48 
49 /// This is the AA result object for the basic, local, and stateless alias
50 /// analysis. It implements the AA query interface in an entirely stateless
51 /// manner. As one consequence, it is never invalidated due to IR changes.
52 /// While it does retain some storage, that is used as an optimization and not
53 /// to preserve information from query to query. However it does retain handles
54 /// to various other analyses and must be recomputed when those analyses are.
55 class BasicAAResult : public AAResultBase<BasicAAResult> {
56   friend AAResultBase<BasicAAResult>;
57 
58   const DataLayout &DL;
59   const Function &F;
60   const TargetLibraryInfo &TLI;
61   AssumptionCache &AC;
62   DominatorTree *DT;
63   LoopInfo *LI;
64   PhiValues *PV;
65 
66 public:
67   BasicAAResult(const DataLayout &DL, const Function &F,
68                 const TargetLibraryInfo &TLI, AssumptionCache &AC,
69                 DominatorTree *DT = nullptr, LoopInfo *LI = nullptr,
70                 PhiValues *PV = nullptr)
AAResultBase()71       : AAResultBase(), DL(DL), F(F), TLI(TLI), AC(AC), DT(DT), LI(LI), PV(PV)
72         {}
73 
BasicAAResult(const BasicAAResult & Arg)74   BasicAAResult(const BasicAAResult &Arg)
75       : AAResultBase(Arg), DL(Arg.DL), F(Arg.F), TLI(Arg.TLI), AC(Arg.AC),
76         DT(Arg.DT),  LI(Arg.LI), PV(Arg.PV) {}
BasicAAResult(BasicAAResult && Arg)77   BasicAAResult(BasicAAResult &&Arg)
78       : AAResultBase(std::move(Arg)), DL(Arg.DL), F(Arg.F), TLI(Arg.TLI),
79         AC(Arg.AC), DT(Arg.DT), LI(Arg.LI), PV(Arg.PV) {}
80 
81   /// Handle invalidation events in the new pass manager.
82   bool invalidate(Function &Fn, const PreservedAnalyses &PA,
83                   FunctionAnalysisManager::Invalidator &Inv);
84 
85   AliasResult alias(const MemoryLocation &LocA, const MemoryLocation &LocB);
86 
87   ModRefInfo getModRefInfo(ImmutableCallSite CS, const MemoryLocation &Loc);
88 
89   ModRefInfo getModRefInfo(ImmutableCallSite CS1, ImmutableCallSite CS2);
90 
91   /// Chases pointers until we find a (constant global) or not.
92   bool pointsToConstantMemory(const MemoryLocation &Loc, bool OrLocal);
93 
94   /// Get the location associated with a pointer argument of a callsite.
95   ModRefInfo getArgModRefInfo(ImmutableCallSite CS, unsigned ArgIdx);
96 
97   /// Returns the behavior when calling the given call site.
98   FunctionModRefBehavior getModRefBehavior(ImmutableCallSite CS);
99 
100   /// Returns the behavior when calling the given function. For use when the
101   /// call site is not known.
102   FunctionModRefBehavior getModRefBehavior(const Function *Fn);
103 
104 private:
105   // A linear transformation of a Value; this class represents ZExt(SExt(V,
106   // SExtBits), ZExtBits) * Scale + Offset.
107   struct VariableGEPIndex {
108     // An opaque Value - we can't decompose this further.
109     const Value *V;
110 
111     // We need to track what extensions we've done as we consider the same Value
112     // with different extensions as different variables in a GEP's linear
113     // expression;
114     // e.g.: if V == -1, then sext(x) != zext(x).
115     unsigned ZExtBits;
116     unsigned SExtBits;
117 
118     int64_t Scale;
119 
120     bool operator==(const VariableGEPIndex &Other) const {
121       return V == Other.V && ZExtBits == Other.ZExtBits &&
122              SExtBits == Other.SExtBits && Scale == Other.Scale;
123     }
124 
125     bool operator!=(const VariableGEPIndex &Other) const {
126       return !operator==(Other);
127     }
128   };
129 
130   // Represents the internal structure of a GEP, decomposed into a base pointer,
131   // constant offsets, and variable scaled indices.
132   struct DecomposedGEP {
133     // Base pointer of the GEP
134     const Value *Base;
135     // Total constant offset w.r.t the base from indexing into structs
136     int64_t StructOffset;
137     // Total constant offset w.r.t the base from indexing through
138     // pointers/arrays/vectors
139     int64_t OtherOffset;
140     // Scaled variable (non-constant) indices.
141     SmallVector<VariableGEPIndex, 4> VarIndices;
142   };
143 
144   /// Track alias queries to guard against recursion.
145   using LocPair = std::pair<MemoryLocation, MemoryLocation>;
146   using AliasCacheTy = SmallDenseMap<LocPair, AliasResult, 8>;
147   AliasCacheTy AliasCache;
148 
149   /// Tracks phi nodes we have visited.
150   ///
151   /// When interpret "Value" pointer equality as value equality we need to make
152   /// sure that the "Value" is not part of a cycle. Otherwise, two uses could
153   /// come from different "iterations" of a cycle and see different values for
154   /// the same "Value" pointer.
155   ///
156   /// The following example shows the problem:
157   ///   %p = phi(%alloca1, %addr2)
158   ///   %l = load %ptr
159   ///   %addr1 = gep, %alloca2, 0, %l
160   ///   %addr2 = gep  %alloca2, 0, (%l + 1)
161   ///      alias(%p, %addr1) -> MayAlias !
162   ///   store %l, ...
163   SmallPtrSet<const BasicBlock *, 8> VisitedPhiBBs;
164 
165   /// Tracks instructions visited by pointsToConstantMemory.
166   SmallPtrSet<const Value *, 16> Visited;
167 
168   static const Value *
169   GetLinearExpression(const Value *V, APInt &Scale, APInt &Offset,
170                       unsigned &ZExtBits, unsigned &SExtBits,
171                       const DataLayout &DL, unsigned Depth, AssumptionCache *AC,
172                       DominatorTree *DT, bool &NSW, bool &NUW);
173 
174   static bool DecomposeGEPExpression(const Value *V, DecomposedGEP &Decomposed,
175       const DataLayout &DL, AssumptionCache *AC, DominatorTree *DT);
176 
177   static bool isGEPBaseAtNegativeOffset(const GEPOperator *GEPOp,
178       const DecomposedGEP &DecompGEP, const DecomposedGEP &DecompObject,
179       LocationSize ObjectAccessSize);
180 
181   /// A Heuristic for aliasGEP that searches for a constant offset
182   /// between the variables.
183   ///
184   /// GetLinearExpression has some limitations, as generally zext(%x + 1)
185   /// != zext(%x) + zext(1) if the arithmetic overflows. GetLinearExpression
186   /// will therefore conservatively refuse to decompose these expressions.
187   /// However, we know that, for all %x, zext(%x) != zext(%x + 1), even if
188   /// the addition overflows.
189   bool
190   constantOffsetHeuristic(const SmallVectorImpl<VariableGEPIndex> &VarIndices,
191                           LocationSize V1Size, LocationSize V2Size,
192                           int64_t BaseOffset, AssumptionCache *AC,
193                           DominatorTree *DT);
194 
195   bool isValueEqualInPotentialCycles(const Value *V1, const Value *V2);
196 
197   void GetIndexDifference(SmallVectorImpl<VariableGEPIndex> &Dest,
198                           const SmallVectorImpl<VariableGEPIndex> &Src);
199 
200   AliasResult aliasGEP(const GEPOperator *V1, LocationSize V1Size,
201                        const AAMDNodes &V1AAInfo, const Value *V2,
202                        LocationSize V2Size, const AAMDNodes &V2AAInfo,
203                        const Value *UnderlyingV1, const Value *UnderlyingV2);
204 
205   AliasResult aliasPHI(const PHINode *PN, LocationSize PNSize,
206                        const AAMDNodes &PNAAInfo, const Value *V2,
207                        LocationSize V2Size, const AAMDNodes &V2AAInfo,
208                        const Value *UnderV2);
209 
210   AliasResult aliasSelect(const SelectInst *SI, LocationSize SISize,
211                           const AAMDNodes &SIAAInfo, const Value *V2,
212                           LocationSize V2Size, const AAMDNodes &V2AAInfo,
213                           const Value *UnderV2);
214 
215   AliasResult aliasCheck(const Value *V1, LocationSize V1Size,
216                          AAMDNodes V1AATag, const Value *V2,
217                          LocationSize V2Size, AAMDNodes V2AATag,
218                          const Value *O1 = nullptr, const Value *O2 = nullptr);
219 };
220 
221 /// Analysis pass providing a never-invalidated alias analysis result.
222 class BasicAA : public AnalysisInfoMixin<BasicAA> {
223   friend AnalysisInfoMixin<BasicAA>;
224 
225   static AnalysisKey Key;
226 
227 public:
228   using Result = BasicAAResult;
229 
230   BasicAAResult run(Function &F, FunctionAnalysisManager &AM);
231 };
232 
233 /// Legacy wrapper pass to provide the BasicAAResult object.
234 class BasicAAWrapperPass : public FunctionPass {
235   std::unique_ptr<BasicAAResult> Result;
236 
237   virtual void anchor();
238 
239 public:
240   static char ID;
241 
242   BasicAAWrapperPass();
243 
getResult()244   BasicAAResult &getResult() { return *Result; }
getResult()245   const BasicAAResult &getResult() const { return *Result; }
246 
247   bool runOnFunction(Function &F) override;
248   void getAnalysisUsage(AnalysisUsage &AU) const override;
249 };
250 
251 FunctionPass *createBasicAAWrapperPass();
252 
253 /// A helper for the legacy pass manager to create a \c BasicAAResult object
254 /// populated to the best of our ability for a particular function when inside
255 /// of a \c ModulePass or a \c CallGraphSCCPass.
256 BasicAAResult createLegacyPMBasicAAResult(Pass &P, Function &F);
257 
258 /// This class is a functor to be used in legacy module or SCC passes for
259 /// computing AA results for a function. We store the results in fields so that
260 /// they live long enough to be queried, but we re-use them each time.
261 class LegacyAARGetter {
262   Pass &P;
263   Optional<BasicAAResult> BAR;
264   Optional<AAResults> AAR;
265 
266 public:
LegacyAARGetter(Pass & P)267   LegacyAARGetter(Pass &P) : P(P) {}
operator()268   AAResults &operator()(Function &F) {
269     BAR.emplace(createLegacyPMBasicAAResult(P, F));
270     AAR.emplace(createLegacyPMAAResults(P, F, *BAR));
271     return *AAR;
272   }
273 };
274 
275 } // end namespace llvm
276 
277 #endif // LLVM_ANALYSIS_BASICALIASANALYSIS_H
278