LLVM API Documentation

ObjCARCExpand.cpp
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00001 //===- ObjCARCExpand.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 /// This file defines ObjC ARC optimizations. ARC stands for Automatic
00011 /// Reference Counting and is a system for managing reference counts for objects
00012 /// in Objective C.
00013 ///
00014 /// This specific file deals with early optimizations which perform certain
00015 /// cleanup operations.
00016 ///
00017 /// WARNING: This file knows about certain library functions. It recognizes them
00018 /// by name, and hardwires knowledge of their semantics.
00019 ///
00020 /// WARNING: This file knows about how certain Objective-C library functions are
00021 /// used. Naive LLVM IR transformations which would otherwise be
00022 /// behavior-preserving may break these assumptions.
00023 ///
00024 //===----------------------------------------------------------------------===//
00025 
00026 #define DEBUG_TYPE "objc-arc-expand"
00027 
00028 #include "ObjCARC.h"
00029 #include "llvm/ADT/StringRef.h"
00030 #include "llvm/IR/Function.h"
00031 #include "llvm/IR/InstIterator.h"
00032 #include "llvm/IR/Instruction.h"
00033 #include "llvm/IR/Instructions.h"
00034 #include "llvm/IR/Value.h"
00035 #include "llvm/Pass.h"
00036 #include "llvm/PassAnalysisSupport.h"
00037 #include "llvm/PassRegistry.h"
00038 #include "llvm/PassSupport.h"
00039 #include "llvm/Support/Casting.h"
00040 #include "llvm/Support/Debug.h"
00041 #include "llvm/Support/raw_ostream.h"
00042 
00043 namespace llvm {
00044   class Module;
00045 }
00046 
00047 using namespace llvm;
00048 using namespace llvm::objcarc;
00049 
00050 namespace {
00051   /// \brief Early ARC transformations.
00052   class ObjCARCExpand : public FunctionPass {
00053     void getAnalysisUsage(AnalysisUsage &AU) const override;
00054     bool doInitialization(Module &M) override;
00055     bool runOnFunction(Function &F) override;
00056 
00057     /// A flag indicating whether this optimization pass should run.
00058     bool Run;
00059 
00060   public:
00061     static char ID;
00062     ObjCARCExpand() : FunctionPass(ID) {
00063       initializeObjCARCExpandPass(*PassRegistry::getPassRegistry());
00064     }
00065   };
00066 }
00067 
00068 char ObjCARCExpand::ID = 0;
00069 INITIALIZE_PASS(ObjCARCExpand,
00070                 "objc-arc-expand", "ObjC ARC expansion", false, false)
00071 
00072 Pass *llvm::createObjCARCExpandPass() {
00073   return new ObjCARCExpand();
00074 }
00075 
00076 void ObjCARCExpand::getAnalysisUsage(AnalysisUsage &AU) const {
00077   AU.setPreservesCFG();
00078 }
00079 
00080 bool ObjCARCExpand::doInitialization(Module &M) {
00081   Run = ModuleHasARC(M);
00082   return false;
00083 }
00084 
00085 bool ObjCARCExpand::runOnFunction(Function &F) {
00086   if (!EnableARCOpts)
00087     return false;
00088 
00089   // If nothing in the Module uses ARC, don't do anything.
00090   if (!Run)
00091     return false;
00092 
00093   bool Changed = false;
00094 
00095   DEBUG(dbgs() << "ObjCARCExpand: Visiting Function: " << F.getName() << "\n");
00096 
00097   for (inst_iterator I = inst_begin(&F), E = inst_end(&F); I != E; ++I) {
00098     Instruction *Inst = &*I;
00099 
00100     DEBUG(dbgs() << "ObjCARCExpand: Visiting: " << *Inst << "\n");
00101 
00102     switch (GetBasicInstructionClass(Inst)) {
00103     case IC_Retain:
00104     case IC_RetainRV:
00105     case IC_Autorelease:
00106     case IC_AutoreleaseRV:
00107     case IC_FusedRetainAutorelease:
00108     case IC_FusedRetainAutoreleaseRV: {
00109       // These calls return their argument verbatim, as a low-level
00110       // optimization. However, this makes high-level optimizations
00111       // harder. Undo any uses of this optimization that the front-end
00112       // emitted here. We'll redo them in the contract pass.
00113       Changed = true;
00114       Value *Value = cast<CallInst>(Inst)->getArgOperand(0);
00115       DEBUG(dbgs() << "ObjCARCExpand: Old = " << *Inst << "\n"
00116                       "               New = " << *Value << "\n");
00117       Inst->replaceAllUsesWith(Value);
00118       break;
00119     }
00120     default:
00121       break;
00122     }
00123   }
00124 
00125   DEBUG(dbgs() << "ObjCARCExpand: Finished List.\n\n");
00126 
00127   return Changed;
00128 }