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 "llvm/ADT/STLExtras.h"
27#include "llvm/Analysis/ObjCARCAnalysisUtils.h"
28#include "llvm/Analysis/ObjCARCInstKind.h"
29#include "llvm/IR/Constants.h"
30#include "llvm/IR/InstrTypes.h"
31#include "llvm/IR/PassManager.h"
32#include "llvm/Support/Debug.h"
33#include "llvm/Support/raw_ostream.h"
34#include "llvm/Transforms/ObjCARC.h"
35
36using namespace llvm;
37using namespace llvm::objcarc;
38
39#define DEBUG_TYPE "objc-arc-ap-elim"
40
41namespace {
42
43/// Interprocedurally determine if calls made by the given call site can
44/// possibly produce autoreleases.
45bool MayAutorelease(const CallBase &CB, unsigned Depth = 0) {
46 if (const Function *Callee = CB.getCalledFunction()) {
47 if (!Callee->hasExactDefinition())
48 return true;
49 for (const BasicBlock &BB : *Callee) {
50 for (const Instruction &I : BB)
51 if (const CallBase *JCB = dyn_cast<CallBase>(Val: &I))
52 // This recursion depth limit is arbitrary. It's just great
53 // enough to cover known interesting testcases.
54 if (Depth < 3 && !JCB->onlyReadsMemory() &&
55 MayAutorelease(CB: *JCB, Depth: Depth + 1))
56 return true;
57 }
58 return false;
59 }
60
61 return true;
62}
63
64bool OptimizeBB(BasicBlock *BB) {
65 bool Changed = false;
66
67 Instruction *Push = nullptr;
68 for (Instruction &Inst : llvm::make_early_inc_range(Range&: *BB)) {
69 switch (GetBasicARCInstKind(V: &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>(Val: &Inst)->getArgOperand(i: 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(CB: cast<CallBase>(Val&: Inst)))
91 Push = nullptr;
92 break;
93 default:
94 break;
95 }
96 }
97
98 return Changed;
99}
100
101bool 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(Name: "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>(Val: 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>(Val: cast<ConstantStruct>(Val: Op)->getOperand(i_nocapture: 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(x: F->begin()) != F->end())
140 continue;
141 // Ok, a single-block constructor function definition. Try to optimize it.
142 Changed |= OptimizeBB(BB: &F->front());
143 }
144
145 return Changed;
146}
147
148} // namespace
149
150PreservedAnalyses ObjCARCAPElimPass::run(Module &M, ModuleAnalysisManager &AM) {
151 if (!runImpl(M))
152 return PreservedAnalyses::all();
153 PreservedAnalyses PA;
154 PA.preserveSet<CFGAnalyses>();
155 return PA;
156}
157