lib/Analysis/ObjCARCAliasAnalysis.cpp

   1 //===- ObjCARCAliasAnalysis.cpp - ObjC ARC Optimization -------------------===//
   2 //
   3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
   4 // See https://llvm.org/LICENSE.txt for license information.
   5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
   6 //
   7 //===----------------------------------------------------------------------===//
   8 /// \file
   9 /// This file defines a simple ARC-aware AliasAnalysis using special knowledge
  10 /// of Objective C to enhance other optimization passes which rely on the Alias
  11 /// Analysis infrastructure.
  12 ///
  13 /// WARNING: This file knows about certain library functions. It recognizes them
  14 /// by name, and hardwires knowledge of their semantics.
  15 ///
  16 /// WARNING: This file knows about how certain Objective-C library functions are
  17 /// used. Naive LLVM IR transformations which would otherwise be
  18 /// behavior-preserving may break these assumptions.
  19 ///
  20 /// TODO: Theoretically we could check for dependencies between objc_* calls
  21 /// and FMRB_OnlyAccessesArgumentPointees calls or other well-behaved calls.
  22 ///
  23 //===----------------------------------------------------------------------===//
  24
  25 #include "llvm/Analysis/ObjCARCAliasAnalysis.h"
  26 #include "llvm/Analysis/ObjCARCAnalysisUtils.h"
  27 #include "llvm/IR/Function.h"
  28 #include "llvm/IR/Instruction.h"
  29 #include "llvm/IR/Value.h"
  30 #include "llvm/InitializePasses.h"
  31 #include "llvm/PassAnalysisSupport.h"
  32 #include "llvm/PassSupport.h"
  33
  34 #define DEBUG_TYPE "objc-arc-aa"
  35
  36 using namespace llvm;
  37 using namespace llvm::objcarc;
  38
  39 AliasResult ObjCARCAAResult::alias(const MemoryLocation &LocA,
  40                                    const MemoryLocation &LocB) {
  41   if (!EnableARCOpts)
  42     return AAResultBase::alias(LocA, LocB);
  43
  44   // First, strip off no-ops, including ObjC-specific no-ops, and try making a
  45   // precise alias query.
  46   const Value *SA = GetRCIdentityRoot(LocA.Ptr);
  47   const Value *SB = GetRCIdentityRoot(LocB.Ptr);
  48   AliasResult Result =
  49       AAResultBase::alias(MemoryLocation(SA, LocA.Size, LocA.AATags),
  50                           MemoryLocation(SB, LocB.Size, LocB.AATags));
  51   if (Result != MayAlias)
  52     return Result;
  53
  54   // If that failed, climb to the underlying object, including climbing through
  55   // ObjC-specific no-ops, and try making an imprecise alias query.
  56   const Value *UA = GetUnderlyingObjCPtr(SA, DL);
  57   const Value *UB = GetUnderlyingObjCPtr(SB, DL);
  58   if (UA != SA || UB != SB) {
  59     Result = AAResultBase::alias(MemoryLocation(UA), MemoryLocation(UB));
  60     // We can't use MustAlias or PartialAlias results here because
  61     // GetUnderlyingObjCPtr may return an offsetted pointer value.
  62     if (Result == NoAlias)
  63       return NoAlias;
  64   }
  65
  66   // If that failed, fail. We don't need to chain here, since that's covered
  67   // by the earlier precise query.
  68   return MayAlias;
  69 }
  70
  71 bool ObjCARCAAResult::pointsToConstantMemory(const MemoryLocation &Loc,
  72                                              bool OrLocal) {
  73   if (!EnableARCOpts)
  74     return AAResultBase::pointsToConstantMemory(Loc, OrLocal);
  75
  76   // First, strip off no-ops, including ObjC-specific no-ops, and try making
  77   // a precise alias query.
  78   const Value *S = GetRCIdentityRoot(Loc.Ptr);
  79   if (AAResultBase::pointsToConstantMemory(
  80           MemoryLocation(S, Loc.Size, Loc.AATags), OrLocal))
  81     return true;
  82
  83   // If that failed, climb to the underlying object, including climbing through
  84   // ObjC-specific no-ops, and try making an imprecise alias query.
  85   const Value *U = GetUnderlyingObjCPtr(S, DL);
  86   if (U != S)
  87     return AAResultBase::pointsToConstantMemory(MemoryLocation(U), OrLocal);
  88
  89   // If that failed, fail. We don't need to chain here, since that's covered
  90   // by the earlier precise query.
  91   return false;
  92 }
  93
  94 FunctionModRefBehavior ObjCARCAAResult::getModRefBehavior(const Function *F) {
  95   if (!EnableARCOpts)
  96     return AAResultBase::getModRefBehavior(F);
  97
  98   switch (GetFunctionClass(F)) {
  99   case ARCInstKind::NoopCast:
 100     return FMRB_DoesNotAccessMemory;
 101   default:
 102     break;
 103   }
 104
 105   return AAResultBase::getModRefBehavior(F);
 106 }
 107
 108 ModRefInfo ObjCARCAAResult::getModRefInfo(const CallBase *Call,
 109                                           const MemoryLocation &Loc) {
 110   if (!EnableARCOpts)
 111     return AAResultBase::getModRefInfo(Call, Loc);
 112
 113   switch (GetBasicARCInstKind(Call)) {
 114   case ARCInstKind::Retain:
 115   case ARCInstKind::RetainRV:
 116   case ARCInstKind::Autorelease:
 117   case ARCInstKind::AutoreleaseRV:
 118   case ARCInstKind::NoopCast:
 119   case ARCInstKind::AutoreleasepoolPush:
 120   case ARCInstKind::FusedRetainAutorelease:
 121   case ARCInstKind::FusedRetainAutoreleaseRV:
 122     // These functions don't access any memory visible to the compiler.
 123     // Note that this doesn't include objc_retainBlock, because it updates
 124     // pointers when it copies block data.
 125     return ModRefInfo::NoModRef;
 126   default:
 127     break;
 128   }
 129
 130   return AAResultBase::getModRefInfo(Call, Loc);
 131 }
 132
 133 ObjCARCAAResult ObjCARCAA::run(Function &F, FunctionAnalysisManager &AM) {
 134   return ObjCARCAAResult(F.getParent()->getDataLayout());
 135 }
 136
 137 char ObjCARCAAWrapperPass::ID = 0;
 138 INITIALIZE_PASS(ObjCARCAAWrapperPass, "objc-arc-aa",
 139                 "ObjC-ARC-Based Alias Analysis", false, true)
 140
 141 ImmutablePass *llvm::createObjCARCAAWrapperPass() {
 142   return new ObjCARCAAWrapperPass();
 143 }
 144
 145 ObjCARCAAWrapperPass::ObjCARCAAWrapperPass() : ImmutablePass(ID) {
 146   initializeObjCARCAAWrapperPassPass(*PassRegistry::getPassRegistry());
 147 }
 148
 149 bool ObjCARCAAWrapperPass::doInitialization(Module &M) {
 150   Result.reset(new ObjCARCAAResult(M.getDataLayout()));
 151   return false;
 152 }
 153
 154 bool ObjCARCAAWrapperPass::doFinalization(Module &M) {
 155   Result.reset();
 156   return false;
 157 }
 158
 159 void ObjCARCAAWrapperPass::getAnalysisUsage(AnalysisUsage &AU) const {
 160   AU.setPreservesAll();
 161 }