xref: /freebsd/contrib/llvm-project/llvm/lib/Transforms/ObjCARC/ObjCARCAPElim.cpp (revision 38a52bd3b5cac3da6f7f6eef3dd050e6aa08ebb3)
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 
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 (Instruction &Inst : llvm::make_early_inc_range(*BB)) {
68     switch (GetBasicARCInstKind(&Inst)) {
69     case ARCInstKind::AutoreleasepoolPush:
70       Push = &Inst;
71       break;
72     case ARCInstKind::AutoreleasepoolPop:
73       // If this pop matches a push and nothing in between can autorelease,
74       // zap the pair.
75       if (Push && cast<CallInst>(&Inst)->getArgOperand(0) == Push) {
76         Changed = true;
77         LLVM_DEBUG(dbgs() << "ObjCARCAPElim::OptimizeBB: Zapping push pop "
78                              "autorelease pair:\n"
79                              "                           Pop: "
80                           << Inst << "\n"
81                           << "                           Push: " << *Push
82                           << "\n");
83         Inst.eraseFromParent();
84         Push->eraseFromParent();
85       }
86       Push = nullptr;
87       break;
88     case ARCInstKind::CallOrUser:
89       if (MayAutorelease(cast<CallBase>(Inst)))
90         Push = nullptr;
91       break;
92     default:
93       break;
94     }
95   }
96 
97   return Changed;
98 }
99 
100 bool runImpl(Module &M) {
101   if (!EnableARCOpts)
102     return false;
103 
104   // If nothing in the Module uses ARC, don't do anything.
105   if (!ModuleHasARC(M))
106     return false;
107   // Find the llvm.global_ctors variable, as the first step in
108   // identifying the global constructors. In theory, unnecessary autorelease
109   // pools could occur anywhere, but in practice it's pretty rare. Global
110   // ctors are a place where autorelease pools get inserted automatically,
111   // so it's pretty common for them to be unnecessary, and it's pretty
112   // profitable to eliminate them.
113   GlobalVariable *GV = M.getGlobalVariable("llvm.global_ctors");
114   if (!GV)
115     return false;
116 
117   assert(GV->hasDefinitiveInitializer() &&
118          "llvm.global_ctors is uncooperative!");
119 
120   bool Changed = false;
121 
122   // Dig the constructor functions out of GV's initializer.
123   ConstantArray *Init = cast<ConstantArray>(GV->getInitializer());
124   for (User::op_iterator OI = Init->op_begin(), OE = Init->op_end();
125        OI != OE; ++OI) {
126     Value *Op = *OI;
127     // llvm.global_ctors is an array of three-field structs where the second
128     // members are constructor functions.
129     Function *F = dyn_cast<Function>(cast<ConstantStruct>(Op)->getOperand(1));
130     // If the user used a constructor function with the wrong signature and
131     // it got bitcasted or whatever, look the other way.
132     if (!F)
133       continue;
134     // Only look at function definitions.
135     if (F->isDeclaration())
136       continue;
137     // Only look at functions with one basic block.
138     if (std::next(F->begin()) != F->end())
139       continue;
140     // Ok, a single-block constructor function definition. Try to optimize it.
141     Changed |= OptimizeBB(&F->front());
142   }
143 
144   return Changed;
145 }
146 
147 /// Autorelease pool elimination.
148 class ObjCARCAPElim : public ModulePass {
149   void getAnalysisUsage(AnalysisUsage &AU) const override;
150   bool runOnModule(Module &M) override;
151 
152 public:
153   static char ID;
154   ObjCARCAPElim() : ModulePass(ID) {
155     initializeObjCARCAPElimPass(*PassRegistry::getPassRegistry());
156   }
157 };
158 } // namespace
159 
160 char ObjCARCAPElim::ID = 0;
161 INITIALIZE_PASS(ObjCARCAPElim, "objc-arc-apelim",
162                 "ObjC ARC autorelease pool elimination", false, false)
163 
164 Pass *llvm::createObjCARCAPElimPass() { return new ObjCARCAPElim(); }
165 
166 void ObjCARCAPElim::getAnalysisUsage(AnalysisUsage &AU) const {
167   AU.setPreservesCFG();
168 }
169 
170 bool ObjCARCAPElim::runOnModule(Module &M) {
171   if (skipModule(M))
172     return false;
173   return runImpl(M);
174 }
175 
176 PreservedAnalyses ObjCARCAPElimPass::run(Module &M, ModuleAnalysisManager &AM) {
177   if (!runImpl(M))
178     return PreservedAnalyses::all();
179   PreservedAnalyses PA;
180   PA.preserveSet<CFGAnalyses>();
181   return PA;
182 }
183