NoAliasAnalysis.cpp revision cd81d94322a39503e4a3e87b6ee03d4fcb3465fb 
1//===- NoAliasAnalysis.cpp - Minimal Alias Analysis Impl ------------------===//
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//
10// This file defines the default implementation of the Alias Analysis interface
11// that simply returns "I don't know" for all queries.
12//
13//===----------------------------------------------------------------------===//
14
15#include "llvm/Analysis/Passes.h"
16#include "llvm/Analysis/AliasAnalysis.h"
17#include "llvm/IR/DataLayout.h"
18#include "llvm/IR/LLVMContext.h"
19#include "llvm/Pass.h"
20using namespace llvm;
21
22namespace {
23  /// NoAA - This class implements the -no-aa pass, which always returns "I
24  /// don't know" for alias queries.  NoAA is unlike other alias analysis
25  /// implementations, in that it does not chain to a previous analysis.  As
26  /// such it doesn't follow many of the rules that other alias analyses must.
27  ///
28  struct NoAA : public ImmutablePass, public AliasAnalysis {
29    static char ID; // Class identification, replacement for typeinfo
30    NoAA() : ImmutablePass(ID) {
31      initializeNoAAPass(*PassRegistry::getPassRegistry());
32    }
33
34    void getAnalysisUsage(AnalysisUsage &AU) const override {}
35
36    void initializePass() override {
37      // Note: NoAA does not call InitializeAliasAnalysis because it's
38      // special and does not support chaining.
39      DataLayoutPass *DLP = getAnalysisIfAvailable<DataLayoutPass>();
40      DL = DLP ? &DLP->getDataLayout() : nullptr;
41    }
42
43    AliasResult alias(const Location &LocA, const Location &LocB) override {
44      return MayAlias;
45    }
46
47    ModRefBehavior getModRefBehavior(ImmutableCallSite CS) override {
48      return UnknownModRefBehavior;
49    }
50    ModRefBehavior getModRefBehavior(const Function *F) override {
51      return UnknownModRefBehavior;
52    }
53
54    bool pointsToConstantMemory(const Location &Loc, bool OrLocal) override {
55      return false;
56    }
57    Location getArgLocation(ImmutableCallSite CS, unsigned ArgIdx,
58                            ModRefResult &Mask) override {
59      Mask = ModRef;
60      return Location(CS.getArgument(ArgIdx), UnknownSize,
61                      CS.getInstruction()->getMetadata(LLVMContext::MD_tbaa));
62    }
63
64    ModRefResult getModRefInfo(ImmutableCallSite CS,
65                               const Location &Loc) override {
66      return ModRef;
67    }
68    ModRefResult getModRefInfo(ImmutableCallSite CS1,
69                               ImmutableCallSite CS2) override {
70      return ModRef;
71    }
72
73    void deleteValue(Value *V) override {}
74    void copyValue(Value *From, Value *To) override {}
75    void addEscapingUse(Use &U) override {}
76
77    /// getAdjustedAnalysisPointer - This method is used when a pass implements
78    /// an analysis interface through multiple inheritance.  If needed, it
79    /// should override this to adjust the this pointer as needed for the
80    /// specified pass info.
81    void *getAdjustedAnalysisPointer(const void *ID) override {
82      if (ID == &AliasAnalysis::ID)
83        return (AliasAnalysis*)this;
84      return this;
85    }
86  };
87}  // End of anonymous namespace
88
89// Register this pass...
90char NoAA::ID = 0;
91INITIALIZE_AG_PASS(NoAA, AliasAnalysis, "no-aa",
92                   "No Alias Analysis (always returns 'may' alias)",
93                   true, true, true)
94
95ImmutablePass *llvm::createNoAAPass() { return new NoAA(); }
96