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/PassRegistry.h"
34 #include "llvm/Support/Casting.h"
35 #include "llvm/Support/Debug.h"
36 #include "llvm/Support/raw_ostream.h"
37 
38 #define DEBUG_TYPE "objc-arc-expand"
39 
40 namespace llvm {
41   class Module;
42 }
43 
44 using namespace llvm;
45 using namespace llvm::objcarc;
46 
47 namespace {
48   /// Early ARC transformations.
49   class ObjCARCExpand : public FunctionPass {
50     void getAnalysisUsage(AnalysisUsage &AU) const override;
51     bool doInitialization(Module &M) override;
52     bool runOnFunction(Function &F) override;
53 
54     /// A flag indicating whether this optimization pass should run.
55     bool Run;
56 
57   public:
58     static char ID;
59     ObjCARCExpand() : FunctionPass(ID) {
60       initializeObjCARCExpandPass(*PassRegistry::getPassRegistry());
61     }
62   };
63 }
64 
65 char ObjCARCExpand::ID = 0;
66 INITIALIZE_PASS(ObjCARCExpand,
67                 "objc-arc-expand", "ObjC ARC expansion", false, false)
68 
69 Pass *llvm::createObjCARCExpandPass() {
70   return new ObjCARCExpand();
71 }
72 
73 void ObjCARCExpand::getAnalysisUsage(AnalysisUsage &AU) const {
74   AU.setPreservesCFG();
75 }
76 
77 bool ObjCARCExpand::doInitialization(Module &M) {
78   Run = ModuleHasARC(M);
79   return false;
80 }
81 
82 bool ObjCARCExpand::runOnFunction(Function &F) {
83   if (!EnableARCOpts)
84     return false;
85 
86   // If nothing in the Module uses ARC, don't do anything.
87   if (!Run)
88     return false;
89 
90   bool Changed = false;
91 
92   LLVM_DEBUG(dbgs() << "ObjCARCExpand: Visiting Function: " << F.getName()
93                     << "\n");
94 
95   for (inst_iterator I = inst_begin(&F), E = inst_end(&F); I != E; ++I) {
96     Instruction *Inst = &*I;
97 
98     LLVM_DEBUG(dbgs() << "ObjCARCExpand: Visiting: " << *Inst << "\n");
99 
100     switch (GetBasicARCInstKind(Inst)) {
101     case ARCInstKind::Retain:
102     case ARCInstKind::RetainRV:
103     case ARCInstKind::Autorelease:
104     case ARCInstKind::AutoreleaseRV:
105     case ARCInstKind::FusedRetainAutorelease:
106     case ARCInstKind::FusedRetainAutoreleaseRV: {
107       // These calls return their argument verbatim, as a low-level
108       // optimization. However, this makes high-level optimizations
109       // harder. Undo any uses of this optimization that the front-end
110       // emitted here. We'll redo them in the contract pass.
111       Changed = true;
112       Value *Value = cast<CallInst>(Inst)->getArgOperand(0);
113       LLVM_DEBUG(dbgs() << "ObjCARCExpand: Old = " << *Inst
114                         << "\n"
115                            "               New = "
116                         << *Value << "\n");
117       Inst->replaceAllUsesWith(Value);
118       break;
119     }
120     default:
121       break;
122     }
123   }
124 
125   LLVM_DEBUG(dbgs() << "ObjCARCExpand: Finished List.\n\n");
126 
127   return Changed;
128 }
129