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