LLVM API Documentation

ObjCARCAPElim.cpp
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00001 //===- ObjCARCAPElim.cpp - ObjC ARC Optimization --------------------------===//
00002 //
00003 //                     The LLVM Compiler Infrastructure
00004 //
00005 // This file is distributed under the University of Illinois Open Source
00006 // License. See LICENSE.TXT for details.
00007 //
00008 //===----------------------------------------------------------------------===//
00009 /// \file
00010 ///
00011 /// This file defines ObjC ARC optimizations. ARC stands for Automatic
00012 /// Reference Counting and is a system for managing reference counts for objects
00013 /// in Objective C.
00014 ///
00015 /// This specific file implements optimizations which remove extraneous
00016 /// autorelease pools.
00017 ///
00018 /// WARNING: This file knows about certain library functions. It recognizes them
00019 /// by name, and hardwires knowledge of their semantics.
00020 ///
00021 /// WARNING: This file knows about how certain Objective-C library functions are
00022 /// used. Naive LLVM IR transformations which would otherwise be
00023 /// behavior-preserving may break these assumptions.
00024 ///
00025 //===----------------------------------------------------------------------===//
00026 
00027 #include "ObjCARC.h"
00028 #include "llvm/ADT/STLExtras.h"
00029 #include "llvm/IR/Constants.h"
00030 #include "llvm/Support/Debug.h"
00031 #include "llvm/Support/raw_ostream.h"
00032 
00033 using namespace llvm;
00034 using namespace llvm::objcarc;
00035 
00036 #define DEBUG_TYPE "objc-arc-ap-elim"
00037 
00038 namespace {
00039   /// \brief Autorelease pool elimination.
00040   class ObjCARCAPElim : public ModulePass {
00041     void getAnalysisUsage(AnalysisUsage &AU) const override;
00042     bool runOnModule(Module &M) override;
00043 
00044     static bool MayAutorelease(ImmutableCallSite CS, unsigned Depth = 0);
00045     static bool OptimizeBB(BasicBlock *BB);
00046 
00047   public:
00048     static char ID;
00049     ObjCARCAPElim() : ModulePass(ID) {
00050       initializeObjCARCAPElimPass(*PassRegistry::getPassRegistry());
00051     }
00052   };
00053 }
00054 
00055 char ObjCARCAPElim::ID = 0;
00056 INITIALIZE_PASS(ObjCARCAPElim,
00057                 "objc-arc-apelim",
00058                 "ObjC ARC autorelease pool elimination",
00059                 false, false)
00060 
00061 Pass *llvm::createObjCARCAPElimPass() {
00062   return new ObjCARCAPElim();
00063 }
00064 
00065 void ObjCARCAPElim::getAnalysisUsage(AnalysisUsage &AU) const {
00066   AU.setPreservesCFG();
00067 }
00068 
00069 /// Interprocedurally determine if calls made by the given call site can
00070 /// possibly produce autoreleases.
00071 bool ObjCARCAPElim::MayAutorelease(ImmutableCallSite CS, unsigned Depth) {
00072   if (const Function *Callee = CS.getCalledFunction()) {
00073     if (Callee->isDeclaration() || Callee->mayBeOverridden())
00074       return true;
00075     for (Function::const_iterator I = Callee->begin(), E = Callee->end();
00076          I != E; ++I) {
00077       const BasicBlock *BB = I;
00078       for (BasicBlock::const_iterator J = BB->begin(), F = BB->end();
00079            J != F; ++J)
00080         if (ImmutableCallSite JCS = ImmutableCallSite(J))
00081           // This recursion depth limit is arbitrary. It's just great
00082           // enough to cover known interesting testcases.
00083           if (Depth < 3 &&
00084               !JCS.onlyReadsMemory() &&
00085               MayAutorelease(JCS, Depth + 1))
00086             return true;
00087     }
00088     return false;
00089   }
00090 
00091   return true;
00092 }
00093 
00094 bool ObjCARCAPElim::OptimizeBB(BasicBlock *BB) {
00095   bool Changed = false;
00096 
00097   Instruction *Push = 0;
00098   for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ) {
00099     Instruction *Inst = I++;
00100     switch (GetBasicInstructionClass(Inst)) {
00101     case IC_AutoreleasepoolPush:
00102       Push = Inst;
00103       break;
00104     case IC_AutoreleasepoolPop:
00105       // If this pop matches a push and nothing in between can autorelease,
00106       // zap the pair.
00107       if (Push && cast<CallInst>(Inst)->getArgOperand(0) == Push) {
00108         Changed = true;
00109         DEBUG(dbgs() << "ObjCARCAPElim::OptimizeBB: Zapping push pop "
00110                         "autorelease pair:\n"
00111                         "                           Pop: " << *Inst << "\n"
00112                      << "                           Push: " << *Push << "\n");
00113         Inst->eraseFromParent();
00114         Push->eraseFromParent();
00115       }
00116       Push = 0;
00117       break;
00118     case IC_CallOrUser:
00119       if (MayAutorelease(ImmutableCallSite(Inst)))
00120         Push = 0;
00121       break;
00122     default:
00123       break;
00124     }
00125   }
00126 
00127   return Changed;
00128 }
00129 
00130 bool ObjCARCAPElim::runOnModule(Module &M) {
00131   if (!EnableARCOpts)
00132     return false;
00133 
00134   // If nothing in the Module uses ARC, don't do anything.
00135   if (!ModuleHasARC(M))
00136     return false;
00137 
00138   // Find the llvm.global_ctors variable, as the first step in
00139   // identifying the global constructors. In theory, unnecessary autorelease
00140   // pools could occur anywhere, but in practice it's pretty rare. Global
00141   // ctors are a place where autorelease pools get inserted automatically,
00142   // so it's pretty common for them to be unnecessary, and it's pretty
00143   // profitable to eliminate them.
00144   GlobalVariable *GV = M.getGlobalVariable("llvm.global_ctors");
00145   if (!GV)
00146     return false;
00147 
00148   assert(GV->hasDefinitiveInitializer() &&
00149          "llvm.global_ctors is uncooperative!");
00150 
00151   bool Changed = false;
00152 
00153   // Dig the constructor functions out of GV's initializer.
00154   ConstantArray *Init = cast<ConstantArray>(GV->getInitializer());
00155   for (User::op_iterator OI = Init->op_begin(), OE = Init->op_end();
00156        OI != OE; ++OI) {
00157     Value *Op = *OI;
00158     // llvm.global_ctors is an array of pairs where the second members
00159     // are constructor functions.
00160     Function *F = dyn_cast<Function>(cast<ConstantStruct>(Op)->getOperand(1));
00161     // If the user used a constructor function with the wrong signature and
00162     // it got bitcasted or whatever, look the other way.
00163     if (!F)
00164       continue;
00165     // Only look at function definitions.
00166     if (F->isDeclaration())
00167       continue;
00168     // Only look at functions with one basic block.
00169     if (std::next(F->begin()) != F->end())
00170       continue;
00171     // Ok, a single-block constructor function definition. Try to optimize it.
00172     Changed |= OptimizeBB(F->begin());
00173   }
00174 
00175   return Changed;
00176 }