llvm/lib/Transforms/ObjCARC/ObjCARCExpand.cpp

//===- ObjCARCExpand.cpp - ObjC ARC Optimization --------------------------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
/// \file
/// This file defines ObjC ARC optimizations. ARC stands for Automatic
/// Reference Counting and is a system for managing reference counts for objects
/// in Objective C.
///
/// This specific file deals with early optimizations which perform certain
/// cleanup operations.
///
/// WARNING: This file knows about certain library functions. It recognizes them
/// by name, and hardwires knowledge of their semantics.
///
/// WARNING: This file knows about how certain Objective-C library functions are
/// used. Naive LLVM IR transformations which would otherwise be
/// behavior-preserving may break these assumptions.
///
//===----------------------------------------------------------------------===//

#include "ObjCARC.h"
#include "llvm/IR/Function.h"
#include "llvm/IR/InstIterator.h"
#include "llvm/IR/Instruction.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/Value.h"
#include "llvm/InitializePasses.h"
#include "llvm/Pass.h"
#include "llvm/PassAnalysisSupport.h"
#include "llvm/PassRegistry.h"
#include "llvm/PassSupport.h"
#include "llvm/Support/Casting.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"

#define DEBUG_TYPE "objc-arc-expand"

namespace llvm {
  class Module;
}

using namespace llvm;
using namespace llvm::objcarc;

namespace {
  /// Early ARC transformations.
  class ObjCARCExpand : public FunctionPass {
    void getAnalysisUsage(AnalysisUsage &AU) const override;
    bool doInitialization(Module &M) override;
    bool runOnFunction(Function &F) override;

    /// A flag indicating whether this optimization pass should run.
    bool Run;

  public:
    static char ID;
    ObjCARCExpand() : FunctionPass(ID) {
      initializeObjCARCExpandPass(*PassRegistry::getPassRegistry());
    }
  };
}

char ObjCARCExpand::ID = 0;
INITIALIZE_PASS(ObjCARCExpand,
                "objc-arc-expand", "ObjC ARC expansion", false, false)

Pass *llvm::createObjCARCExpandPass() {
  return new ObjCARCExpand();
}

void ObjCARCExpand::getAnalysisUsage(AnalysisUsage &AU) const {
  AU.setPreservesCFG();
}

bool ObjCARCExpand::doInitialization(Module &M) {
  Run = ModuleHasARC(M);
  return false;
}

bool ObjCARCExpand::runOnFunction(Function &F) {
  if (!EnableARCOpts)
    return false;

  // If nothing in the Module uses ARC, don't do anything.
  if (!Run)
    return false;

  bool Changed = false;

  LLVM_DEBUG(dbgs() << "ObjCARCExpand: Visiting Function: " << F.getName()
                    << "\n");

  for (inst_iterator I = inst_begin(&F), E = inst_end(&F); I != E; ++I) {
    Instruction *Inst = &*I;

    LLVM_DEBUG(dbgs() << "ObjCARCExpand: Visiting: " << *Inst << "\n");

    switch (GetBasicARCInstKind(Inst)) {
    case ARCInstKind::Retain:
    case ARCInstKind::RetainRV:
    case ARCInstKind::Autorelease:
    case ARCInstKind::AutoreleaseRV:
    case ARCInstKind::FusedRetainAutorelease:
    case ARCInstKind::FusedRetainAutoreleaseRV: {
      // These calls return their argument verbatim, as a low-level
      // optimization. However, this makes high-level optimizations
      // harder. Undo any uses of this optimization that the front-end
      // emitted here. We'll redo them in the contract pass.
      Changed = true;
      Value *Value = cast<CallInst>(Inst)->getArgOperand(0);
      LLVM_DEBUG(dbgs() << "ObjCARCExpand: Old = " << *Inst
                        << "\n"
                           "               New = "
                        << *Value << "\n");
      Inst->replaceAllUsesWith(Value);
      break;
    }
    default:
      break;
    }
  }

  LLVM_DEBUG(dbgs() << "ObjCARCExpand: Finished List.\n\n");

  return Changed;
}