1 //===- ObjCARCAPElim.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 ///
11 /// This file defines ObjC ARC optimizations. ARC stands for Automatic
12 /// Reference Counting and is a system for managing reference counts for objects
13 /// in Objective C.
14 ///
15 /// This specific file implements optimizations which remove extraneous
16 /// autorelease pools.
17 ///
18 /// WARNING: This file knows about certain library functions. It recognizes them
19 /// by name, and hardwires knowledge of their semantics.
20 ///
21 /// WARNING: This file knows about how certain Objective-C library functions are
22 /// used. Naive LLVM IR transformations which would otherwise be
23 /// behavior-preserving may break these assumptions.
24 ///
25 //===----------------------------------------------------------------------===//
26
27 #include "ObjCARC.h"
28 #include "llvm/ADT/STLExtras.h"
29 #include "llvm/IR/Constants.h"
30 #include "llvm/Support/Debug.h"
31 #include "llvm/Support/raw_ostream.h"
32
33 using namespace llvm;
34 using namespace llvm::objcarc;
35
36 #define DEBUG_TYPE "objc-arc-ap-elim"
37
38 namespace {
39 /// \brief Autorelease pool elimination.
40 class ObjCARCAPElim : public ModulePass {
41 void getAnalysisUsage(AnalysisUsage &AU) const override;
42 bool runOnModule(Module &M) override;
43
44 static bool MayAutorelease(ImmutableCallSite CS, unsigned Depth = 0);
45 static bool OptimizeBB(BasicBlock *BB);
46
47 public:
48 static char ID;
ObjCARCAPElim()49 ObjCARCAPElim() : ModulePass(ID) {
50 initializeObjCARCAPElimPass(*PassRegistry::getPassRegistry());
51 }
52 };
53 }
54
55 char ObjCARCAPElim::ID = 0;
56 INITIALIZE_PASS(ObjCARCAPElim,
57 "objc-arc-apelim",
58 "ObjC ARC autorelease pool elimination",
59 false, false)
60
createObjCARCAPElimPass()61 Pass *llvm::createObjCARCAPElimPass() {
62 return new ObjCARCAPElim();
63 }
64
getAnalysisUsage(AnalysisUsage & AU) const65 void ObjCARCAPElim::getAnalysisUsage(AnalysisUsage &AU) const {
66 AU.setPreservesCFG();
67 }
68
69 /// Interprocedurally determine if calls made by the given call site can
70 /// possibly produce autoreleases.
MayAutorelease(ImmutableCallSite CS,unsigned Depth)71 bool ObjCARCAPElim::MayAutorelease(ImmutableCallSite CS, unsigned Depth) {
72 if (const Function *Callee = CS.getCalledFunction()) {
73 if (Callee->isDeclaration() || Callee->mayBeOverridden())
74 return true;
75 for (const BasicBlock &BB : *Callee) {
76 for (const Instruction &I : BB)
77 if (ImmutableCallSite JCS = ImmutableCallSite(&I))
78 // This recursion depth limit is arbitrary. It's just great
79 // enough to cover known interesting testcases.
80 if (Depth < 3 &&
81 !JCS.onlyReadsMemory() &&
82 MayAutorelease(JCS, Depth + 1))
83 return true;
84 }
85 return false;
86 }
87
88 return true;
89 }
90
OptimizeBB(BasicBlock * BB)91 bool ObjCARCAPElim::OptimizeBB(BasicBlock *BB) {
92 bool Changed = false;
93
94 Instruction *Push = nullptr;
95 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ) {
96 Instruction *Inst = &*I++;
97 switch (GetBasicARCInstKind(Inst)) {
98 case ARCInstKind::AutoreleasepoolPush:
99 Push = Inst;
100 break;
101 case ARCInstKind::AutoreleasepoolPop:
102 // If this pop matches a push and nothing in between can autorelease,
103 // zap the pair.
104 if (Push && cast<CallInst>(Inst)->getArgOperand(0) == Push) {
105 Changed = true;
106 DEBUG(dbgs() << "ObjCARCAPElim::OptimizeBB: Zapping push pop "
107 "autorelease pair:\n"
108 " Pop: " << *Inst << "\n"
109 << " Push: " << *Push << "\n");
110 Inst->eraseFromParent();
111 Push->eraseFromParent();
112 }
113 Push = nullptr;
114 break;
115 case ARCInstKind::CallOrUser:
116 if (MayAutorelease(ImmutableCallSite(Inst)))
117 Push = nullptr;
118 break;
119 default:
120 break;
121 }
122 }
123
124 return Changed;
125 }
126
runOnModule(Module & M)127 bool ObjCARCAPElim::runOnModule(Module &M) {
128 if (!EnableARCOpts)
129 return false;
130
131 // If nothing in the Module uses ARC, don't do anything.
132 if (!ModuleHasARC(M))
133 return false;
134
135 // Find the llvm.global_ctors variable, as the first step in
136 // identifying the global constructors. In theory, unnecessary autorelease
137 // pools could occur anywhere, but in practice it's pretty rare. Global
138 // ctors are a place where autorelease pools get inserted automatically,
139 // so it's pretty common for them to be unnecessary, and it's pretty
140 // profitable to eliminate them.
141 GlobalVariable *GV = M.getGlobalVariable("llvm.global_ctors");
142 if (!GV)
143 return false;
144
145 assert(GV->hasDefinitiveInitializer() &&
146 "llvm.global_ctors is uncooperative!");
147
148 bool Changed = false;
149
150 // Dig the constructor functions out of GV's initializer.
151 ConstantArray *Init = cast<ConstantArray>(GV->getInitializer());
152 for (User::op_iterator OI = Init->op_begin(), OE = Init->op_end();
153 OI != OE; ++OI) {
154 Value *Op = *OI;
155 // llvm.global_ctors is an array of three-field structs where the second
156 // members are constructor functions.
157 Function *F = dyn_cast<Function>(cast<ConstantStruct>(Op)->getOperand(1));
158 // If the user used a constructor function with the wrong signature and
159 // it got bitcasted or whatever, look the other way.
160 if (!F)
161 continue;
162 // Only look at function definitions.
163 if (F->isDeclaration())
164 continue;
165 // Only look at functions with one basic block.
166 if (std::next(F->begin()) != F->end())
167 continue;
168 // Ok, a single-block constructor function definition. Try to optimize it.
169 Changed |= OptimizeBB(&F->front());
170 }
171
172 return Changed;
173 }
174