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 
42 namespace llvm {
43   class Module;
44 }
45 
46 using namespace llvm;
47 using namespace llvm::objcarc;
48 
49 namespace {
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 
67 char ObjCARCExpand::ID = 0;
68 INITIALIZE_PASS(ObjCARCExpand,
69                 "objc-arc-expand", "ObjC ARC expansion", false, false)
70 
71 Pass *llvm::createObjCARCExpandPass() {
72   return new ObjCARCExpand();
73 }
74 
75 void ObjCARCExpand::getAnalysisUsage(AnalysisUsage &AU) const {
76   AU.setPreservesCFG();
77 }
78 
79 bool ObjCARCExpand::doInitialization(Module &M) {
80   Run = ModuleHasARC(M);
81   return false;
82 }
83 
84 bool 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