1 //===- ObjCARCAPElim.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 ///
10 /// This file defines ObjC ARC optimizations. ARC stands for Automatic
11 /// Reference Counting and is a system for managing reference counts for objects
12 /// in Objective C.
13 ///
14 /// This specific file implements optimizations which remove extraneous
15 /// autorelease pools.
16 ///
17 /// WARNING: This file knows about certain library functions. It recognizes them
18 /// by name, and hardwires knowledge of their semantics.
19 ///
20 /// WARNING: This file knows about how certain Objective-C library functions are
21 /// used. Naive LLVM IR transformations which would otherwise be
22 /// behavior-preserving may break these assumptions.
23 ///
24 //===----------------------------------------------------------------------===//
25
26 #include "ObjCARC.h"
27 #include "llvm/ADT/STLExtras.h"
28 #include "llvm/IR/Constants.h"
29 #include "llvm/IR/PassManager.h"
30 #include "llvm/InitializePasses.h"
31 #include "llvm/Support/Debug.h"
32 #include "llvm/Support/raw_ostream.h"
33 #include "llvm/Transforms/ObjCARC.h"
34
35 using namespace llvm;
36 using namespace llvm::objcarc;
37
38 #define DEBUG_TYPE "objc-arc-ap-elim"
39
EraseInstruction(Instruction * CI)40 namespace {
41
42 /// Interprocedurally determine if calls made by the given call site can
43 /// possibly produce autoreleases.
44 bool MayAutorelease(const CallBase &CB, unsigned Depth = 0) {
45 if (const Function *Callee = CB.getCalledFunction()) {
46 if (!Callee->hasExactDefinition())
47 return true;
48 for (const BasicBlock &BB : *Callee) {
49 for (const Instruction &I : BB)
50 if (const CallBase *JCB = dyn_cast<CallBase>(&I))
51 // This recursion depth limit is arbitrary. It's just great
52 // enough to cover known interesting testcases.
53 if (Depth < 3 && !JCB->onlyReadsMemory() &&
54 MayAutorelease(*JCB, Depth + 1))
55 return true;
56 }
57 return false;
58 }
59
60 return true;
61 }
62
63 bool OptimizeBB(BasicBlock *BB) {
64 bool Changed = false;
65
66 Instruction *Push = nullptr;
67 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ) {
68 Instruction *Inst = &*I++;
69 switch (GetBasicARCInstKind(Inst)) {
70 case ARCInstKind::AutoreleasepoolPush:
71 Push = Inst;
72 break;
73 case ARCInstKind::AutoreleasepoolPop:
74 // If this pop matches a push and nothing in between can autorelease,
75 // zap the pair.
76 if (Push && cast<CallInst>(Inst)->getArgOperand(0) == Push) {
77 Changed = true;
78 LLVM_DEBUG(dbgs() << "ObjCARCAPElim::OptimizeBB: Zapping push pop "
79 "autorelease pair:\n"
80 " Pop: "
81 << *Inst << "\n"
82 << " Push: " << *Push
83 << "\n");
84 Inst->eraseFromParent();
85 Push->eraseFromParent();
86 }
87 Push = nullptr;
88 break;
89 case ARCInstKind::CallOrUser:
90 if (MayAutorelease(cast<CallBase>(*Inst)))
91 Push = nullptr;
92 break;
93 default:
94 break;
95 }
96 }
97
98 return Changed;
99 }
100
101 bool runImpl(Module &M) {
102 if (!EnableARCOpts)
103 return false;
104
105 // If nothing in the Module uses ARC, don't do anything.
106 if (!ModuleHasARC(M))
107 return false;
108 // Find the llvm.global_ctors variable, as the first step in
109 // identifying the global constructors. In theory, unnecessary autorelease
110 // pools could occur anywhere, but in practice it's pretty rare. Global
111 // ctors are a place where autorelease pools get inserted automatically,
112 // so it's pretty common for them to be unnecessary, and it's pretty
113 // profitable to eliminate them.
114 GlobalVariable *GV = M.getGlobalVariable("llvm.global_ctors");
115 if (!GV)
116 return false;
117
118 assert(GV->hasDefinitiveInitializer() &&
119 "llvm.global_ctors is uncooperative!");
120
121 bool Changed = false;
122
123 // Dig the constructor functions out of GV's initializer.
124 ConstantArray *Init = cast<ConstantArray>(GV->getInitializer());
125 for (User::op_iterator OI = Init->op_begin(), OE = Init->op_end();
126 OI != OE; ++OI) {
127 Value *Op = *OI;
128 // llvm.global_ctors is an array of three-field structs where the second
129 // members are constructor functions.
130 Function *F = dyn_cast<Function>(cast<ConstantStruct>(Op)->getOperand(1));
131 // If the user used a constructor function with the wrong signature and
132 // it got bitcasted or whatever, look the other way.
133 if (!F)
134 continue;
135 // Only look at function definitions.
136 if (F->isDeclaration())
137 continue;
138 // Only look at functions with one basic block.
139 if (std::next(F->begin()) != F->end())
140 continue;
141 // Ok, a single-block constructor function definition. Try to optimize it.
142 Changed |= OptimizeBB(&F->front());
143 }
144
145 return Changed;
146 }
147
148 /// Autorelease pool elimination.
149 class ObjCARCAPElim : public ModulePass {
150 void getAnalysisUsage(AnalysisUsage &AU) const override;
151 bool runOnModule(Module &M) override;
152
153 public:
154 static char ID;
155 ObjCARCAPElim() : ModulePass(ID) {
156 initializeObjCARCAPElimPass(*PassRegistry::getPassRegistry());
157 }
158 };
159 } // namespace
160
161 char ObjCARCAPElim::ID = 0;
162 INITIALIZE_PASS(ObjCARCAPElim, "objc-arc-apelim",
163 "ObjC ARC autorelease pool elimination", false, false)
164
165 Pass *llvm::createObjCARCAPElimPass() { return new ObjCARCAPElim(); }
166
167 void ObjCARCAPElim::getAnalysisUsage(AnalysisUsage &AU) const {
168 AU.setPreservesCFG();
169 }
170
171 bool ObjCARCAPElim::runOnModule(Module &M) {
172 if (skipModule(M))
173 return false;
174 return runImpl(M);
175 }
176
177 PreservedAnalyses ObjCARCAPElimPass::run(Module &M, ModuleAnalysisManager &AM) {
178 if (!runImpl(M))
179 return PreservedAnalyses::all();
180 PreservedAnalyses PA;
181 PA.preserveSet<CFGAnalyses>();
182 return PA;
183 }
184