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 | }