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      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/IR/Module.h"
     20 #include "llvm/Pass.h"
     21 using namespace llvm;
     22 
     23 namespace {
     24   /// NoAA - This class implements the -no-aa pass, which always returns "I
     25   /// don't know" for alias queries.  NoAA is unlike other alias analysis
     26   /// implementations, in that it does not chain to a previous analysis.  As
     27   /// such it doesn't follow many of the rules that other alias analyses must.
     28   ///
     29   struct NoAA : public ImmutablePass, public AliasAnalysis {
     30     static char ID; // Class identification, replacement for typeinfo
     31     NoAA() : ImmutablePass(ID) {
     32       initializeNoAAPass(*PassRegistry::getPassRegistry());
     33     }
     34 
     35     void getAnalysisUsage(AnalysisUsage &AU) const override {}
     36 
     37     bool doInitialization(Module &M) override {
     38       // Note: NoAA does not call InitializeAliasAnalysis because it's
     39       // special and does not support chaining.
     40       DL = &M.getDataLayout();
     41       return true;
     42     }
     43 
     44     AliasResult alias(const Location &LocA, const Location &LocB) override {
     45       return MayAlias;
     46     }
     47 
     48     ModRefBehavior getModRefBehavior(ImmutableCallSite CS) override {
     49       return UnknownModRefBehavior;
     50     }
     51     ModRefBehavior getModRefBehavior(const Function *F) override {
     52       return UnknownModRefBehavior;
     53     }
     54 
     55     bool pointsToConstantMemory(const Location &Loc, bool OrLocal) override {
     56       return false;
     57     }
     58     Location getArgLocation(ImmutableCallSite CS, unsigned ArgIdx,
     59                             ModRefResult &Mask) override {
     60       Mask = ModRef;
     61       AAMDNodes AATags;
     62       CS->getAAMetadata(AATags);
     63       return Location(CS.getArgument(ArgIdx), UnknownSize, AATags);
     64     }
     65 
     66     ModRefResult getModRefInfo(ImmutableCallSite CS,
     67                                const Location &Loc) override {
     68       return ModRef;
     69     }
     70     ModRefResult getModRefInfo(ImmutableCallSite CS1,
     71                                ImmutableCallSite CS2) override {
     72       return ModRef;
     73     }
     74 
     75     void deleteValue(Value *V) override {}
     76     void copyValue(Value *From, Value *To) override {}
     77     void addEscapingUse(Use &U) override {}
     78 
     79     /// getAdjustedAnalysisPointer - This method is used when a pass implements
     80     /// an analysis interface through multiple inheritance.  If needed, it
     81     /// should override this to adjust the this pointer as needed for the
     82     /// specified pass info.
     83     void *getAdjustedAnalysisPointer(const void *ID) override {
     84       if (ID == &AliasAnalysis::ID)
     85         return (AliasAnalysis*)this;
     86       return this;
     87     }
     88   };
     89 }  // End of anonymous namespace
     90 
     91 // Register this pass...
     92 char NoAA::ID = 0;
     93 INITIALIZE_AG_PASS(NoAA, AliasAnalysis, "no-aa",
     94                    "No Alias Analysis (always returns 'may' alias)",
     95                    true, true, true)
     96 
     97 ImmutablePass *llvm::createNoAAPass() { return new NoAA(); }
     98