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 36 using namespace llvm; 37 using namespace llvm::objcarc; 38 39 #define DEBUG_TYPE "objc-arc-ap-elim" 40 41 namespace { 42 43 /// Interprocedurally determine if calls made by the given call site can 44 /// possibly produce autoreleases. 45 bool 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>(&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(*JCB, Depth + 1)) 56 return true; 57 } 58 return false; 59 } 60 61 return true; 62 } 63 64 bool OptimizeBB(BasicBlock *BB) { 65 bool Changed = false; 66 67 Instruction *Push = nullptr; 68 for (Instruction &Inst : llvm::make_early_inc_range(*BB)) { 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 } // namespace 149 150 PreservedAnalyses 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