ART世界探险(20) - Android N上的编译流程

就在我们分析Android M版本的ART还只走出了一小段路的时候，Android N的新ART就问世了。

Android N上的ART还是有不小的改进的。不过做为一个关注细节的系列文章，我们还是从Compile的过程说起。

流程概述

在安装的时候，默认情况下，Android N只做interpret-only的编译，如下命令行所示：

/system/bin/dex2oat --zip-fd=7 --zip-location=base.apk --oat-fd=8 --oat-location=/data/app/vmdl692968727.tmp/oat/arm64/base.odex --instruction-set=arm64 --instruction-set-variant=kryo --instruction-set-features=default --runtime-arg -Xms64m --runtime-arg -Xmx512m --compiler-filter=interpret-only --swap-fd=10 --debuggable, priority: 10, policy: 5:freezer:/,4:name=systemd:/,3:cpuset:/,2:cpu:/bg_non_interactive,1:cpuacct:/,

所以我们更新一下编译时候的时序图：

在安装的时候不编译了，那么就要在运行时通过JIT来编译，这个流程如下：

CompilerDriver::CompileAll

比起M上我们分析过的版本，N上的新版本的CompileAll的注释更详细了，结构也更清晰了一点。

我们先来看看Android M版的：

void CompilerDriver::Compile(jobject class_loader, const std::vector
    
     & dex_files,                             ThreadPool* thread_pool, TimingLogger* timings) {  for (size_t i = 0; i != dex_files.size(); ++i) {    const DexFile* dex_file = dex_files[i];    CHECK(dex_file != nullptr);    CompileDexFile(class_loader, *dex_file, dex_files, thread_pool, timings);  }  VLOG(compiler) << "Compile: " << GetMemoryUsageString(false);}
    

再看看对应的Android N的CompileAll，是不是看起来正规了许多呢？

void CompilerDriver::CompileAll(jobject class_loader,                                const std::vector
    
     & dex_files,                                TimingLogger* timings) {  DCHECK(!Runtime::Current()->IsStarted());  InitializeThreadPools();  VLOG(compiler) << "Before precompile " << GetMemoryUsageString(false);  // Precompile:  // 1) Load image classes  // 2) Resolve all classes  // 3) Attempt to verify all classes  // 4) Attempt to initialize image classes, and trivially initialized classes  PreCompile(class_loader, dex_files, timings);  // Compile:  // 1) Compile all classes and methods enabled for compilation. May fall back to dex-to-dex  //    compilation.  if (!GetCompilerOptions().VerifyAtRuntime()) {    Compile(class_loader, dex_files, timings);  }  if (dump_stats_) {    stats_->Dump();  }  FreeThreadPools();}
    

CompilerDriver::Compile

CompilerDriver的Compile方法，从Android M时代的不足10行，到了Android N上也变成40多行了。

Android M上的CompilerDriver::Compile:

void CompilerDriver::Compile(jobject class_loader, const std::vector
    
     & dex_files,                             ThreadPool* thread_pool, TimingLogger* timings) {  for (size_t i = 0; i != dex_files.size(); ++i) {    const DexFile* dex_file = dex_files[i];    CHECK(dex_file != nullptr);    CompileDexFile(class_loader, *dex_file, dex_files, thread_pool, timings);  }  VLOG(compiler) << "Compile: " << GetMemoryUsageString(false);}
    

Android N对应的Compile方法：

void CompilerDriver::Compile(jobject class_loader,                             const std::vector
    
     & dex_files,                             TimingLogger* timings) {  if (kDebugProfileGuidedCompilation) {    LOG(INFO) << "[ProfileGuidedCompilation] " <<        ((profile_compilation_info_ == nullptr)            ? "null"            : profile_compilation_info_->DumpInfo(&dex_files));  }  DCHECK(current_dex_to_dex_methods_ == nullptr);  for (const DexFile* dex_file : dex_files) {    CHECK(dex_file != nullptr);    CompileDexFile(class_loader,                   *dex_file,                   dex_files,                   parallel_thread_pool_.get(),                   parallel_thread_count_,                   timings);    const ArenaPool* const arena_pool = Runtime::Current()->GetArenaPool();    const size_t arena_alloc = arena_pool->GetBytesAllocated();    max_arena_alloc_ = std::max(arena_alloc, max_arena_alloc_);    Runtime::Current()->ReclaimArenaPoolMemory();  }  ArrayRef
     
       dex_to_dex_references;  {    // From this point on, we shall not modify dex_to_dex_references_, so    // just grab a reference to it that we use without holding the mutex.    MutexLock lock(Thread::Current(), dex_to_dex_references_lock_);    dex_to_dex_references = ArrayRef
      
       (dex_to_dex_references_);  }  for (const auto& method_set : dex_to_dex_references) {    current_dex_to_dex_methods_ = &method_set.GetMethodIndexes();    CompileDexFile(class_loader,                   method_set.GetDexFile(),                   dex_files,                   parallel_thread_pool_.get(),                   parallel_thread_count_,                   timings);  }  current_dex_to_dex_methods_ = nullptr;  VLOG(compiler) << "Compile: " << GetMemoryUsageString(false);}
      
     
    

CompilerDriver::CompileDexFile

Android M版的CompileDexFile:

void CompilerDriver::CompileDexFile(jobject class_loader, const DexFile& dex_file,                                    const std::vector
    
     & dex_files,                                    ThreadPool* thread_pool, TimingLogger* timings) {  TimingLogger::ScopedTiming t("Compile Dex File", timings);  ParallelCompilationManager context(Runtime::Current()->GetClassLinker(), class_loader, this,                                     &dex_file, dex_files, thread_pool);  context.ForAll(0, dex_file.NumClassDefs(), CompilerDriver::CompileClass, thread_count_);}
    

Android N版本增加了一个CompileClassVisitor，访问者模式啊，更加上档次了。

void CompilerDriver::CompileDexFile(jobject class_loader,                                    const DexFile& dex_file,                                    const std::vector
    
     & dex_files,                                    ThreadPool* thread_pool,                                    size_t thread_count,                                    TimingLogger* timings) {  TimingLogger::ScopedTiming t("Compile Dex File", timings);  ParallelCompilationManager context(Runtime::Current()->GetClassLinker(), class_loader, this,                                     &dex_file, dex_files, thread_pool);  CompileClassVisitor visitor(&context);  context.ForAll(0, dex_file.NumClassDefs(), &visitor, thread_count);}
    

CompileClassVisitor

虽然在Android N上改用了CompileClassVisitor，但是本质上还是跟Android M上的CompilerDriver::CompileClass是一回事情。最终还是要落实到CompileMethod方法上去。

细节就略过不分析了。我们先过一下CompileClassVisitor的逻辑：

virtual void Visit(size_t class_def_index) REQUIRES(!Locks::mutator_lock_) OVERRIDE {    ATRACE_CALL();    const DexFile& dex_file = *manager_->GetDexFile();    const DexFile::ClassDef& class_def = dex_file.GetClassDef(class_def_index);    ClassLinker* class_linker = manager_->GetClassLinker();    jobject jclass_loader = manager_->GetClassLoader();    ClassReference ref(&dex_file, class_def_index);    // Skip compiling classes with generic verifier failures since they will still fail at runtime    if (manager_->GetCompiler()->verification_results_->IsClassRejected(ref)) {      return;    }    // Use a scoped object access to perform to the quick SkipClass check.    const char* descriptor = dex_file.GetClassDescriptor(class_def);    ScopedObjectAccess soa(Thread::Current());    StackHandleScope<3> hs(soa.Self());    Handle
    
      class_loader(        hs.NewHandle(soa.Decode
     
      (jclass_loader)));    Handle
      
        klass(        hs.NewHandle(class_linker->FindClass(soa.Self(), descriptor, class_loader)));    Handle
       
         dex_cache;    if (klass.Get() == nullptr) {      soa.Self()->AssertPendingException();      soa.Self()->ClearException();      dex_cache = hs.NewHandle(class_linker->FindDexCache(soa.Self(), dex_file));    } else if (SkipClass(jclass_loader, dex_file, klass.Get())) {      return;    } else {      dex_cache = hs.NewHandle(klass->GetDexCache());    }    const uint8_t* class_data = dex_file.GetClassData(class_def);    if (class_data == nullptr) {      // empty class, probably a marker interface      return;    }    // Go to native so that we don't block GC during compilation.    ScopedThreadSuspension sts(soa.Self(), kNative);    CompilerDriver* const driver = manager_->GetCompiler();    // Can we run DEX-to-DEX compiler on this class ?    optimizer::DexToDexCompilationLevel dex_to_dex_compilation_level =        GetDexToDexCompilationLevel(soa.Self(), *driver, jclass_loader, dex_file, class_def);    ClassDataItemIterator it(dex_file, class_data);    // Skip fields    while (it.HasNextStaticField()) {      it.Next();    }    while (it.HasNextInstanceField()) {      it.Next();    }    bool compilation_enabled = driver->IsClassToCompile(        dex_file.StringByTypeIdx(class_def.class_idx_));    // Compile direct methods    int64_t previous_direct_method_idx = -1;    while (it.HasNextDirectMethod()) {      uint32_t method_idx = it.GetMemberIndex();      if (method_idx == previous_direct_method_idx) {        // smali can create dex files with two encoded_methods sharing the same method_idx        // http://code.google.com/p/smali/issues/detail?id=119        it.Next();        continue;      }      previous_direct_method_idx = method_idx;      CompileMethod(soa.Self(), driver, it.GetMethodCodeItem(), it.GetMethodAccessFlags(),                    it.GetMethodInvokeType(class_def), class_def_index,                    method_idx, jclass_loader, dex_file, dex_to_dex_compilation_level,                    compilation_enabled, dex_cache);      it.Next();    }    // Compile virtual methods    int64_t previous_virtual_method_idx = -1;    while (it.HasNextVirtualMethod()) {      uint32_t method_idx = it.GetMemberIndex();      if (method_idx == previous_virtual_method_idx) {        // smali can create dex files with two encoded_methods sharing the same method_idx        // http://code.google.com/p/smali/issues/detail?id=119        it.Next();        continue;      }      previous_virtual_method_idx = method_idx;      CompileMethod(soa.Self(), driver, it.GetMethodCodeItem(), it.GetMethodAccessFlags(),                    it.GetMethodInvokeType(class_def), class_def_index,                    method_idx, jclass_loader, dex_file, dex_to_dex_compilation_level,                    compilation_enabled, dex_cache);      it.Next();    }    DCHECK(!it.HasNext());  } private:  const ParallelCompilationManager* const manager_;};
       
      
     
    

然后会调到CompilerDriver中的static函数CompileMethod。