ObjCARCContract.cpp | Explore in Territory

//===- ObjCARCContract.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 late 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 mainly deals with ``contracting'' multiple lower level
/// operations into singular higher level operations through pattern matching.
///
/// 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.
///
//===----------------------------------------------------------------------===//

// TODO: ObjCARCContract could insert PHI nodes when uses aren't
// dominated by single calls.

#include "ARCRuntimeEntryPoints.h"
#include "DependencyAnalysis.h"
#include "ObjCARC.h"
#include "ProvenanceAnalysis.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Analysis/AliasAnalysis.h"
#include "llvm/Analysis/BasicAliasAnalysis.h"
#include "llvm/Analysis/ObjCARCUtil.h"
#include "llvm/IR/Dominators.h"
#include "llvm/IR/EHPersonalities.h"
#include "llvm/IR/InlineAsm.h"
#include "llvm/IR/InstIterator.h"
#include "llvm/IR/Operator.h"
#include "llvm/IR/PassManager.h"
#include "llvm/InitializePasses.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Transforms/ObjCARC.h"

usingnamespacellvm;
usingnamespacellvm::objcarc;

#define DEBUG_TYPE …

STATISTIC(NumPeeps,       "Number of calls peephole-optimized");
STATISTIC(NumStoreStrongs, "Number objc_storeStrong calls formed");

//===----------------------------------------------------------------------===//
//                                Declarations
//===----------------------------------------------------------------------===//

namespace {
/// Late ARC optimizations
///
/// These change the IR in a way that makes it difficult to be analyzed by
/// ObjCARCOpt, so it's run late.

class ObjCARCContract { … };

class ObjCARCContractLegacyPass : public FunctionPass { … };
}

//===----------------------------------------------------------------------===//
//                               Implementation
//===----------------------------------------------------------------------===//

/// Turn objc_retain into objc_retainAutoreleasedReturnValue if the operand is a
/// return value. We do this late so we do not disrupt the dataflow analysis in
/// ObjCARCOpt.
bool ObjCARCContract::optimizeRetainCall(Function &F, Instruction *Retain) { … }

/// Merge an autorelease with a retain into a fused call.
bool ObjCARCContract::contractAutorelease(Function &F, Instruction *Autorelease,
                                          ARCInstKind Class) { … }

static StoreInst *findSafeStoreForStoreStrongContraction(LoadInst *Load,
                                                         Instruction *Release,
                                                         ProvenanceAnalysis &PA,
                                                         AAResults *AA) { … }

static Instruction *
findRetainForStoreStrongContraction(Value *New, StoreInst *Store,
                                    Instruction *Release,
                                    ProvenanceAnalysis &PA) { … }

/// Attempt to merge an objc_release with a store, load, and objc_retain to form
/// an objc_storeStrong. An objc_storeStrong:
///
///   objc_storeStrong(i8** %old_ptr, i8* new_value)
///
/// is equivalent to the following IR sequence:
///
///   ; Load old value.
///   %old_value = load i8** %old_ptr               (1)
///
///   ; Increment the new value and then release the old value. This must occur
///   ; in order in case old_value releases new_value in its destructor causing
///   ; us to potentially have a dangling ptr.
///   tail call i8* @objc_retain(i8* %new_value)    (2)
///   tail call void @objc_release(i8* %old_value)  (3)
///
///   ; Store the new_value into old_ptr
///   store i8* %new_value, i8** %old_ptr           (4)
///
/// The safety of this optimization is based around the following
/// considerations:
///
///  1. We are forming the store strong at the store. Thus to perform this
///     optimization it must be safe to move the retain, load, and release to
///     (4).
///  2. We need to make sure that any re-orderings of (1), (2), (3), (4) are
///     safe.
void ObjCARCContract::tryToContractReleaseIntoStoreStrong(
    Instruction *Release, inst_iterator &Iter,
    const DenseMap<BasicBlock *, ColorVector> &BlockColors) { … }

bool ObjCARCContract::tryToPeepholeInstruction(
    Function &F, Instruction *Inst, inst_iterator &Iter,
    bool &TailOkForStoreStrongs,
    const DenseMap<BasicBlock *, ColorVector> &BlockColors) { … }

//===----------------------------------------------------------------------===//
//                              Top Level Driver
//===----------------------------------------------------------------------===//

bool ObjCARCContract::init(Module &M) { … }

bool ObjCARCContract::run(Function &F, AAResults *A, DominatorTree *D) { … }

//===----------------------------------------------------------------------===//
//                             Misc Pass Manager
//===----------------------------------------------------------------------===//

char ObjCARCContractLegacyPass::ID = …;
INITIALIZE_PASS_BEGIN(ObjCARCContractLegacyPass, "objc-arc-contract",
                      "ObjC ARC contraction", false, false)
INITIALIZE_PASS_DEPENDENCY(AAResultsWrapperPass)
INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
INITIALIZE_PASS_END(ObjCARCContractLegacyPass, "objc-arc-contract",
                    "ObjC ARC contraction", false, false)

void ObjCARCContractLegacyPass::getAnalysisUsage(AnalysisUsage &AU) const { … }

Pass *llvm::createObjCARCContractPass() { … }

bool ObjCARCContractLegacyPass::runOnFunction(Function &F) { … }

PreservedAnalyses ObjCARCContractPass::run(Function &F,
                                           FunctionAnalysisManager &AM) { … }
llvm/llvm/lib/Transforms/ObjCARC/ObjCARCContract.cpp
