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      1 //===- ObjCARCExpand.cpp - ObjC ARC Optimization --------------------------===//
      2 //
      3 //                     The LLVM Compiler Infrastructure
      4 //
      5 // This file is distributed under the University of Illinois Open Source
      6 // License. See LICENSE.TXT for details.
      7 //
      8 //===----------------------------------------------------------------------===//
      9 /// \file
     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 deals with early optimizations which perform certain
     15 /// cleanup operations.
     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/StringRef.h"
     28 #include "llvm/IR/Function.h"
     29 #include "llvm/IR/InstIterator.h"
     30 #include "llvm/IR/Instruction.h"
     31 #include "llvm/IR/Instructions.h"
     32 #include "llvm/IR/Value.h"
     33 #include "llvm/Pass.h"
     34 #include "llvm/PassAnalysisSupport.h"
     35 #include "llvm/PassRegistry.h"
     36 #include "llvm/PassSupport.h"
     37 #include "llvm/Support/Casting.h"
     38 #include "llvm/Support/Debug.h"
     39 #include "llvm/Support/raw_ostream.h"
     40 
     41 #define DEBUG_TYPE "objc-arc-expand"
     42 
     43 namespace llvm {
     44   class Module;
     45 }
     46 
     47 using namespace llvm;
     48 using namespace llvm::objcarc;
     49 
     50 namespace {
     51   /// \brief Early ARC transformations.
     52   class ObjCARCExpand : public FunctionPass {
     53     void getAnalysisUsage(AnalysisUsage &AU) const override;
     54     bool doInitialization(Module &M) override;
     55     bool runOnFunction(Function &F) override;
     56 
     57     /// A flag indicating whether this optimization pass should run.
     58     bool Run;
     59 
     60   public:
     61     static char ID;
     62     ObjCARCExpand() : FunctionPass(ID) {
     63       initializeObjCARCExpandPass(*PassRegistry::getPassRegistry());
     64     }
     65   };
     66 }
     67 
     68 char ObjCARCExpand::ID = 0;
     69 INITIALIZE_PASS(ObjCARCExpand,
     70                 "objc-arc-expand", "ObjC ARC expansion", false, false)
     71 
     72 Pass *llvm::createObjCARCExpandPass() {
     73   return new ObjCARCExpand();
     74 }
     75 
     76 void ObjCARCExpand::getAnalysisUsage(AnalysisUsage &AU) const {
     77   AU.setPreservesCFG();
     78 }
     79 
     80 bool ObjCARCExpand::doInitialization(Module &M) {
     81   Run = ModuleHasARC(M);
     82   return false;
     83 }
     84 
     85 bool ObjCARCExpand::runOnFunction(Function &F) {
     86   if (!EnableARCOpts)
     87     return false;
     88 
     89   // If nothing in the Module uses ARC, don't do anything.
     90   if (!Run)
     91     return false;
     92 
     93   bool Changed = false;
     94 
     95   DEBUG(dbgs() << "ObjCARCExpand: Visiting Function: " << F.getName() << "\n");
     96 
     97   for (inst_iterator I = inst_begin(&F), E = inst_end(&F); I != E; ++I) {
     98     Instruction *Inst = &*I;
     99 
    100     DEBUG(dbgs() << "ObjCARCExpand: Visiting: " << *Inst << "\n");
    101 
    102     switch (GetBasicInstructionClass(Inst)) {
    103     case IC_Retain:
    104     case IC_RetainRV:
    105     case IC_Autorelease:
    106     case IC_AutoreleaseRV:
    107     case IC_FusedRetainAutorelease:
    108     case IC_FusedRetainAutoreleaseRV: {
    109       // These calls return their argument verbatim, as a low-level
    110       // optimization. However, this makes high-level optimizations
    111       // harder. Undo any uses of this optimization that the front-end
    112       // emitted here. We'll redo them in the contract pass.
    113       Changed = true;
    114       Value *Value = cast<CallInst>(Inst)->getArgOperand(0);
    115       DEBUG(dbgs() << "ObjCARCExpand: Old = " << *Inst << "\n"
    116                       "               New = " << *Value << "\n");
    117       Inst->replaceAllUsesWith(Value);
    118       break;
    119     }
    120     default:
    121       break;
    122     }
    123   }
    124 
    125   DEBUG(dbgs() << "ObjCARCExpand: Finished List.\n\n");
    126 
    127   return Changed;
    128 }
    129