llvm/lib/Transforms/ObjCARC/ObjCARCAPElim.cpp

   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 }