ObjCARCExpand.cpp revision 360784 
1//===- ObjCARCExpand.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 ObjC ARC optimizations. ARC stands for Automatic
10/// Reference Counting and is a system for managing reference counts for objects
11/// in Objective C.
12///
13/// This specific file deals with early optimizations which perform certain
14/// cleanup operations.
15///
16/// WARNING: This file knows about certain library functions. It recognizes them
17/// by name, and hardwires knowledge of their semantics.
18///
19/// WARNING: This file knows about how certain Objective-C library functions are
20/// used. Naive LLVM IR transformations which would otherwise be
21/// behavior-preserving may break these assumptions.
22///
23//===----------------------------------------------------------------------===//
24
25#include "ObjCARC.h"
26#include "llvm/IR/Function.h"
27#include "llvm/IR/InstIterator.h"
28#include "llvm/IR/Instruction.h"
29#include "llvm/IR/Instructions.h"
30#include "llvm/IR/Value.h"
31#include "llvm/InitializePasses.h"
32#include "llvm/Pass.h"
33#include "llvm/PassAnalysisSupport.h"
34#include "llvm/PassRegistry.h"
35#include "llvm/PassSupport.h"
36#include "llvm/Support/Casting.h"
37#include "llvm/Support/Debug.h"
38#include "llvm/Support/raw_ostream.h"
39
40#define DEBUG_TYPE "objc-arc-expand"
41
42namespace llvm {
43  class Module;
44}
45
46using namespace llvm;
47using namespace llvm::objcarc;
48
49namespace {
50  /// Early ARC transformations.
51  class ObjCARCExpand : public FunctionPass {
52    void getAnalysisUsage(AnalysisUsage &AU) const override;
53    bool doInitialization(Module &M) override;
54    bool runOnFunction(Function &F) override;
55
56    /// A flag indicating whether this optimization pass should run.
57    bool Run;
58
59  public:
60    static char ID;
61    ObjCARCExpand() : FunctionPass(ID) {
62      initializeObjCARCExpandPass(*PassRegistry::getPassRegistry());
63    }
64  };
65}
66
67char ObjCARCExpand::ID = 0;
68INITIALIZE_PASS(ObjCARCExpand,
69                "objc-arc-expand", "ObjC ARC expansion", false, false)
70
71Pass *llvm::createObjCARCExpandPass() {
72  return new ObjCARCExpand();
73}
74
75void ObjCARCExpand::getAnalysisUsage(AnalysisUsage &AU) const {
76  AU.setPreservesCFG();
77}
78
79bool ObjCARCExpand::doInitialization(Module &M) {
80  Run = ModuleHasARC(M);
81  return false;
82}
83
84bool ObjCARCExpand::runOnFunction(Function &F) {
85  if (!EnableARCOpts)
86    return false;
87
88  // If nothing in the Module uses ARC, don't do anything.
89  if (!Run)
90    return false;
91
92  bool Changed = false;
93
94  LLVM_DEBUG(dbgs() << "ObjCARCExpand: Visiting Function: " << F.getName()
95                    << "\n");
96
97  for (inst_iterator I = inst_begin(&F), E = inst_end(&F); I != E; ++I) {
98    Instruction *Inst = &*I;
99
100    LLVM_DEBUG(dbgs() << "ObjCARCExpand: Visiting: " << *Inst << "\n");
101
102    switch (GetBasicARCInstKind(Inst)) {
103    case ARCInstKind::Retain:
104    case ARCInstKind::RetainRV:
105    case ARCInstKind::Autorelease:
106    case ARCInstKind::AutoreleaseRV:
107    case ARCInstKind::FusedRetainAutorelease:
108    case ARCInstKind::FusedRetainAutoreleaseRV: {
109      // These calls return their argument verbatim, as a low-level
110      // optimization. However, this makes high-level optimizations
111      // harder. Undo any uses of this optimization that the front-end
112      // emitted here. We'll redo them in the contract pass.
113      Changed = true;
114      Value *Value = cast<CallInst>(Inst)->getArgOperand(0);
115      LLVM_DEBUG(dbgs() << "ObjCARCExpand: Old = " << *Inst
116                        << "\n"
117                           "               New = "
118                        << *Value << "\n");
119      Inst->replaceAllUsesWith(Value);
120      break;
121    }
122    default:
123      break;
124    }
125  }
126
127  LLVM_DEBUG(dbgs() << "ObjCARCExpand: Finished List.\n\n");
128
129  return Changed;
130}
131