ART世界探险(20) - Android N上的编译流程

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<const DexFile*>& 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<const DexFile*>& 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<const DexFile*>& 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<const DexFile*>& 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<DexFileMethodSet> 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<DexFileMethodSet>(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<const DexFile*>& 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<const DexFile*>& 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<mirror::ClassLoader> class_loader(
hs.NewHandle(soa.Decode<mirror::ClassLoader*>(jclass_loader)));
Handle<mirror::Class> klass(
hs.NewHandle(class_linker->FindClass(soa.Self(), descriptor, class_loader)));
Handle<mirror::DexCache> 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。