src/llvm/lib/Transforms/ObjCARC/ObjCARCAliasAnalysis.cpp

   1 //===- ObjCARCAliasAnalysis.cpp - ObjC ARC Optimization -------------------===//
   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 file defines a simple ARC-aware AliasAnalysis using special knowledge
  11 /// of Objective C to enhance other optimization passes which rely on the Alias
  12 /// Analysis infrastructure.
  13 ///
  14 /// WARNING: This file knows about certain library functions. It recognizes them
  15 /// by name, and hardwires knowledge of their semantics.
  16 ///
  17 /// WARNING: This file knows about how certain Objective-C library functions are
  18 /// used. Naive LLVM IR transformations which would otherwise be
  19 /// behavior-preserving may break these assumptions.
  20 ///
  21 //===----------------------------------------------------------------------===//
  22
  23 #include "ObjCARC.h"
  24 #include "ObjCARCAliasAnalysis.h"
  25 #include "llvm/IR/Instruction.h"
  26 #include "llvm/InitializePasses.h"
  27 #include "llvm/PassAnalysisSupport.h"
  28 #include "llvm/PassSupport.h"
  29
  30 #define DEBUG_TYPE "objc-arc-aa"
  31
  32 namespace llvm {
  33   class Function;
  34   class Value;
  35 }
  36
  37 using namespace llvm;
  38 using namespace llvm::objcarc;
  39
  40 // Register this pass...
  41 char ObjCARCAliasAnalysis::ID = 0;
  42 INITIALIZE_AG_PASS(ObjCARCAliasAnalysis, AliasAnalysis, "objc-arc-aa",
  43                    "ObjC-ARC-Based Alias Analysis", false, true, false)
  44
  45 ImmutablePass *llvm::createObjCARCAliasAnalysisPass() {
  46   return new ObjCARCAliasAnalysis();
  47 }
  48
  49 void
  50 ObjCARCAliasAnalysis::getAnalysisUsage(AnalysisUsage &AU) const {
  51   AU.setPreservesAll();
  52   AliasAnalysis::getAnalysisUsage(AU);
  53 }
  54
  55 AliasAnalysis::AliasResult
  56 ObjCARCAliasAnalysis::alias(const Location &LocA, const Location &LocB) {
  57   if (!EnableARCOpts)
  58     return AliasAnalysis::alias(LocA, LocB);
  59
  60   // First, strip off no-ops, including ObjC-specific no-ops, and try making a
  61   // precise alias query.
  62   const Value *SA = StripPointerCastsAndObjCCalls(LocA.Ptr);
  63   const Value *SB = StripPointerCastsAndObjCCalls(LocB.Ptr);
  64   AliasResult Result =
  65     AliasAnalysis::alias(Location(SA, LocA.Size, LocA.AATags),
  66                          Location(SB, LocB.Size, LocB.AATags));
  67   if (Result != MayAlias)
  68     return Result;
  69
  70   // If that failed, climb to the underlying object, including climbing through
  71   // ObjC-specific no-ops, and try making an imprecise alias query.
  72   const Value *UA = GetUnderlyingObjCPtr(SA);
  73   const Value *UB = GetUnderlyingObjCPtr(SB);
  74   if (UA != SA || UB != SB) {
  75     Result = AliasAnalysis::alias(Location(UA), Location(UB));
  76     // We can't use MustAlias or PartialAlias results here because
  77     // GetUnderlyingObjCPtr may return an offsetted pointer value.
  78     if (Result == NoAlias)
  79       return NoAlias;
  80   }
  81
  82   // If that failed, fail. We don't need to chain here, since that's covered
  83   // by the earlier precise query.
  84   return MayAlias;
  85 }
  86
  87 bool
  88 ObjCARCAliasAnalysis::pointsToConstantMemory(const Location &Loc,
  89                                              bool OrLocal) {
  90   if (!EnableARCOpts)
  91     return AliasAnalysis::pointsToConstantMemory(Loc, OrLocal);
  92
  93   // First, strip off no-ops, including ObjC-specific no-ops, and try making
  94   // a precise alias query.
  95   const Value *S = StripPointerCastsAndObjCCalls(Loc.Ptr);
  96   if (AliasAnalysis::pointsToConstantMemory(Location(S, Loc.Size, Loc.AATags),
  97                                             OrLocal))
  98     return true;
  99
 100   // If that failed, climb to the underlying object, including climbing through
 101   // ObjC-specific no-ops, and try making an imprecise alias query.
 102   const Value *U = GetUnderlyingObjCPtr(S);
 103   if (U != S)
 104     return AliasAnalysis::pointsToConstantMemory(Location(U), OrLocal);
 105
 106   // If that failed, fail. We don't need to chain here, since that's covered
 107   // by the earlier precise query.
 108   return false;
 109 }
 110
 111 AliasAnalysis::ModRefBehavior
 112 ObjCARCAliasAnalysis::getModRefBehavior(ImmutableCallSite CS) {
 113   // We have nothing to do. Just chain to the next AliasAnalysis.
 114   return AliasAnalysis::getModRefBehavior(CS);
 115 }
 116
 117 AliasAnalysis::ModRefBehavior
 118 ObjCARCAliasAnalysis::getModRefBehavior(const Function *F) {
 119   if (!EnableARCOpts)
 120     return AliasAnalysis::getModRefBehavior(F);
 121
 122   switch (GetFunctionClass(F)) {
 123   case IC_NoopCast:
 124     return DoesNotAccessMemory;
 125   default:
 126     break;
 127   }
 128
 129   return AliasAnalysis::getModRefBehavior(F);
 130 }
 131
 132 AliasAnalysis::ModRefResult
 133 ObjCARCAliasAnalysis::getModRefInfo(ImmutableCallSite CS, const Location &Loc) {
 134   if (!EnableARCOpts)
 135     return AliasAnalysis::getModRefInfo(CS, Loc);
 136
 137   switch (GetBasicInstructionClass(CS.getInstruction())) {
 138   case IC_Retain:
 139   case IC_RetainRV:
 140   case IC_Autorelease:
 141   case IC_AutoreleaseRV:
 142   case IC_NoopCast:
 143   case IC_AutoreleasepoolPush:
 144   case IC_FusedRetainAutorelease:
 145   case IC_FusedRetainAutoreleaseRV:
 146     // These functions don't access any memory visible to the compiler.
 147     // Note that this doesn't include objc_retainBlock, because it updates
 148     // pointers when it copies block data.
 149     return NoModRef;
 150   default:
 151     break;
 152   }
 153
 154   return AliasAnalysis::getModRefInfo(CS, Loc);
 155 }
 156
 157 AliasAnalysis::ModRefResult
 158 ObjCARCAliasAnalysis::getModRefInfo(ImmutableCallSite CS1,
 159                                     ImmutableCallSite CS2) {
 160   // TODO: Theoretically we could check for dependencies between objc_* calls
 161   // and OnlyAccessesArgumentPointees calls or other well-behaved calls.
 162   return AliasAnalysis::getModRefInfo(CS1, CS2);
 163 }