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/InitializePasses.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   /// 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;
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 
61 Pass *llvm::createObjCARCAPElimPass() {
62   return new ObjCARCAPElim();
63 }
64 
65 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.
71 bool ObjCARCAPElim::MayAutorelease(ImmutableCallSite CS, unsigned Depth) {
72   if (const Function *Callee = CS.getCalledFunction()) {
73     if (!Callee->hasExactDefinition())
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 
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         LLVM_DEBUG(dbgs() << "ObjCARCAPElim::OptimizeBB: Zapping push pop "
107                              "autorelease pair:\n"
108                              "                           Pop: "
109                           << *Inst << "\n"
110                           << "                           Push: " << *Push
111                           << "\n");
112         Inst->eraseFromParent();
113         Push->eraseFromParent();
114       }
115       Push = nullptr;
116       break;
117     case ARCInstKind::CallOrUser:
118       if (MayAutorelease(ImmutableCallSite(Inst)))
119         Push = nullptr;
120       break;
121     default:
122       break;
123     }
124   }
125 
126   return Changed;
127 }
128 
129 bool ObjCARCAPElim::runOnModule(Module &M) {
130   if (!EnableARCOpts)
131     return false;
132 
133   // If nothing in the Module uses ARC, don't do anything.
134   if (!ModuleHasARC(M))
135     return false;
136 
137   if (skipModule(M))
138     return false;
139 
140   // Find the llvm.global_ctors variable, as the first step in
141   // identifying the global constructors. In theory, unnecessary autorelease
142   // pools could occur anywhere, but in practice it's pretty rare. Global
143   // ctors are a place where autorelease pools get inserted automatically,
144   // so it's pretty common for them to be unnecessary, and it's pretty
145   // profitable to eliminate them.
146   GlobalVariable *GV = M.getGlobalVariable("llvm.global_ctors");
147   if (!GV)
148     return false;
149 
150   assert(GV->hasDefinitiveInitializer() &&
151          "llvm.global_ctors is uncooperative!");
152 
153   bool Changed = false;
154 
155   // Dig the constructor functions out of GV's initializer.
156   ConstantArray *Init = cast<ConstantArray>(GV->getInitializer());
157   for (User::op_iterator OI = Init->op_begin(), OE = Init->op_end();
158        OI != OE; ++OI) {
159     Value *Op = *OI;
160     // llvm.global_ctors is an array of three-field structs where the second
161     // members are constructor functions.
162     Function *F = dyn_cast<Function>(cast<ConstantStruct>(Op)->getOperand(1));
163     // If the user used a constructor function with the wrong signature and
164     // it got bitcasted or whatever, look the other way.
165     if (!F)
166       continue;
167     // Only look at function definitions.
168     if (F->isDeclaration())
169       continue;
170     // Only look at functions with one basic block.
171     if (std::next(F->begin()) != F->end())
172       continue;
173     // Ok, a single-block constructor function definition. Try to optimize it.
174     Changed |= OptimizeBB(&F->front());
175   }
176 
177   return Changed;
178 }
179