A Detailed View inside V8 Javascript Engine
In this tutorial we will find out how exactly V8 executes the js code 'hello' + 'world', by analyzing and examining every line in hello-world.cc file in v8/samples directory.
Note : The version of v8 that we will used here is v8-7.9.2.
What are we waiting for, lets get started!
Initializing The Platform
If we look at the line 16 of the v8/samples/hello-world.cc we will see the following code:
16 | std::unique_ptr<v8::Platform> platform = v8::platform::NewDefaultPlatform();You will notice that we create a v8::Platform object that wraps in a std::unique_ptr object by using NewDefaultPlatform() method of the namespace v8::platform, the definition of this method is in the line 34 of v8/src/libplatform/default-platform.cc, while the declaration is in the v8/include/libplatform/libplatform.h at line 37.
// src/libplatform/libplatform.h
37 | V8_PLATFORM_EXPORT std::unique_ptr<v8::Platform> NewDefaultPlatform(
38 | /* 1 */ int thread_pool_size = 0,
39 | /* 2 */ IdleTaskSupport idle_task_support = IdleTaskSupport::kDisabled,
40 | /* 3 */ InProcessStackDumping in_process_stack_dumping =
41 | InProcessStackDumping::kDisabled,
42 | /* 4 */ std::unique_ptr<v8::TracingController> tracing_controller = {});The first parameters is thread_pool_size which is an integer with an initial value of zero. We will discussed it later.
The second is idle_task_support which is a key of enum class IdleTaskSupport with an initial value IdleTaskSupportkDisabled::kDisabled.
// src/libplatform/libplatform.h
16 | enum class IdleTaskSupport { kDisabled, kEnabled };The third is in_process_stack_dumping which is a key of enum class InProcessStackDumpingInProcessStackDumping::kDisabled with an initial value kDisabled.
// src/libplatform/libplatform.h
17 | enum class InProcessStackDumping { kDisabled, kEnabled };And the last is v8::TracingController that wraps in an empty std::unique_ptr object.
// include/v8-platform.h
137 | class TracingController {
138 | ...
199 | }Inside this method there is a checking on our parameter.
// src/libplatform/default-platform.cc
38 | if (in_process_stack_dumping == InProcessStackDumping::kEnabled) {
39 | v8::base::debug::EnableInProcessStackDumping();
40 | }In the code above it checked if the in_process_stack_dumping argument is kEnabled so we can call EnableInProcessStackDumping method, but in our case our in_process_stack_dumping is kDisabled so we will proceed on creation of platform.
// src/libplatform/default-platform.cc
41 | std::unique_ptr<DefaultPlatform> platform(
42 | new DefaultPlatform(idle_task_support, std::move(tracing_controller)));In this line of code we allocate DefaultPlatform object instance on the heap by using new operator and wraps it in std::unique_ptr object. DefaultPlatform is a child class of v8::Platform and we used it as our default platform.
Declaration:
// src/libplatform/default-platform.h
32 | class V8_PLATFORM_EXPORT DefaultPlatform : public NON_EXPORTED_BASE(Platform) {
...
90 | } Implementation:
// src/libplatform/default-platform.cc
69 | DefaultPlatform::DefaultPlatform(
70 | IdleTaskSupport idle_task_support,
71 | std::unique_ptr<v8::TracingController> tracing_controller)
72 | : thread_pool_size_(0),
73 | idle_task_support_(idle_task_support),
74 | tracing_controller_(std::move(tracing_controller)),
75 | page_allocator_(new v8::base::PageAllocator()),
76 | time_function_for_testing_(nullptr) {
77 | if (!tracing_controller_) {
78 | tracing::TracingController* controller = new tracing::TracingController();
79 | controller->Initialize(nullptr);
80 | tracing_controller_.reset(controller);
81 | }
82 | }Inside this constructor it only check if tracing_controller_ is empty and then it will create a new v8::platform::tracing::TracingController object instance and it will Initialize as you've seen on the code above. TracingController extends the public base class v8::TracingController.
Declaration
// include/libplatform/v8-tracing.h
229 | class V8_PLATFORM_EXPORT TracingController
230 | : public V8_PLATFORM_NON_EXPORTED_BASE(v8::TracingController) {
231 | public:
| ...
249 | void Initialize(TraceBuffer* trace_buffer);
| ...
310 | }; Implementation
// src/libplatform/tracing/tracing-controller.cc
64 | TracingController::TracingController() = default;
| ...
81 | void TracingController::Initialize(TraceBuffer* trace_buffer) {
82 | trace_buffer_.reset(trace_buffer);
83 | mutex_.reset(new base::Mutex());
84 | }