Re: [v8-users] How does V8 compiles and executes JavaScript events?

[Jakob Kummerow]

On Mon, Sep 4, 2017 at 4:07 PM, Hanyun Tao  wrote:

> Hi Jakob!
>
> I'm reading the Invoke(..) function you mentioned, and I believe the
> following part of the code plays an important role in executing the
> JavaScript but I cannot fully understand it.
>
>   typedef Object* (*JSEntryFunction)(Object* new_target, Object* target,
>  Object* receiver, int argc,
>  Object*** args);
>
>
>   Handle code = is_construct
>   ? isolate->factory()->js_construct_entry_code()
>   : isolate->factory()->js_entry_code();
>
>
>   {
> // Save and restore context around invocation and block the
> // allocation of handles without explicit handle scopes.
> SaveContext save(isolate);
> SealHandleScope shs(isolate);
> JSEntryFunction stub_entry = FUNCTION_CAST(code->
> entry());
>
>
> if (FLAG_clear_exceptions_on_js_entry) isolate->clear_pending_
> exception();
>
>
> // Call the function through the right JS entry stub.
> Object* orig_func = *new_target;
> Object* func = *target;
> Object* recv = *receiver;
> Object*** argv = reinterpret_cast(args);
> if (FLAG_profile_deserialization && target->IsJSFunction()) {
>   PrintDeserializedCodeInfo(Handle::cast(target));
> }
> RuntimeCallTimerScope timer(isolate, ::JS_Execution);
> value = CALL_GENERATED_CODE(isolate, stub_entry, orig_func, func, recv
> ,
> argc, argv);
>   }
>
>
> I could not find some good document to help me understand those data
> structures especially JSEntryFunction ,and Handle.  It would be
> really helpful if you can give me some document that talks about the
> definition of V8's basic data structures like Code.
>

Code:
https://cs.chromium.org/chromium/src/v8/src/objects.h?type=cs=class+code=package:chromium=3646
(as the name suggests, it's the kind of object V8 uses to store generated
code)
Handle:
https://cs.chromium.org/chromium/src/v8/src/handles.h?type=cs=class+Handle=package:chromium=92
(a GC safe pointer to a HeapObject)

Look around the code for more! :-)

For the functionality of the above code, here is my understanding (guess).
>
> The first part of the code defines a special type of object named
> JSEntryFunction.
>

That's a C++ typedef defining a particular (C++) function signature.

The next part of the code tries to obtain the executable code by calling
> isolate->factory()->js_entry_code();  , which might be the part that
> triggers the (re-)compilation or finds the existing compiled code.
>

No, it's the "stub" that handles calling into generated code. The function
being called is target, and as you can see, it is passed as an argument to
the call.
"Invoke" does not deal with compilation. As I wrote before:

> a v8::internal::JSFunction has a code() property. That's either the
> existing compiled code, or a stub that will trigger (re-)compilation based
> on the script() in the JSFunction's shared_function_info(). It will be
> retrieved and called by the JSEntryStub.

You've identified the place where the JSEntryStub is called. That stub will
look up the function's stored code and call it. This "code" could be
another stub that triggers compilation if needed.

The reset of the code executes the generated code.
>
> Could you please verify if my understanding is correct?
>
> Thanks a lot!
>
> On Thursday, August 31, 2017 at 3:36:53 PM UTC-4, Jakob Kummerow wrote:
>>
>> A v8::Function maps to a v8::internal::JSFunction, which has a code()
>> property. That's either the existing compiled code, or a stub that will
>> trigger (re-)compilation based on the script() in the JSFunction's
>> shared_function_info(). It will be retrieved and called by the
>> JSEntryStub.
>>
>> You might want to put your instrumentation into Invoke(...) in
>> execution.cc.
>>
>> On Thu, Aug 31, 2017 at 11:56 AM, Hanyun Tao  wrote:
>>
>>> Hi Jakob,
>>>
>>> Thank you for answering my questions!
>>>
>>> I would like to modify V8's internals to support such functionality. But
>>> before I start I would like to learn a little more about how chromium
>>> interact with V8 to process JavaScript event.
>>>
>>> By reading the source code, I believe that the WebKit rendering engine
>>> will call *v8::Function::Call* in api.cc, and after that the V8 engine
>>> will execute the Javascript code. Am I correct?
>>>
>>> If this is how things going to work, could you please point out where in
>>> this process, V8 compiles the code, or "read" the compiled code correspond
>>> to the JavaScript?
>>>
>>> Thank you!
>>>
>>> On Wednesday, August 30, 2017 at 8:39:00 PM UTC-4, Jakob Kummerow wrote:

 Object addresses are not exposed either. You would have to build such
 instrumentation into V8's internals.

 On Tue, Aug 29, 2017 at 12:32 PM, Hanyun Tao 
 wrote:

> Thank you Jakob!
>
> I can understand why it is 

Re: [v8-users] How does V8 compiles and executes JavaScript events?

[Jakob Kummerow]

A v8::Function maps to a v8::internal::JSFunction, which has a code()
property. That's either the existing compiled code, or a stub that will
trigger (re-)compilation based on the script() in the JSFunction's
shared_function_info(). It will be retrieved and called by the JSEntryStub.

You might want to put your instrumentation into Invoke(...) in execution.cc.

On Thu, Aug 31, 2017 at 11:56 AM, Hanyun Tao  wrote:

> Hi Jakob,
>
> Thank you for answering my questions!
>
> I would like to modify V8's internals to support such functionality. But
> before I start I would like to learn a little more about how chromium
> interact with V8 to process JavaScript event.
>
> By reading the source code, I believe that the WebKit rendering engine
> will call *v8::Function::Call* in api.cc, and after that the V8 engine
> will execute the Javascript code. Am I correct?
>
> If this is how things going to work, could you please point out where in
> this process, V8 compiles the code, or "read" the compiled code correspond
> to the JavaScript?
>
> Thank you!
>
> On Wednesday, August 30, 2017 at 8:39:00 PM UTC-4, Jakob Kummerow wrote:
>>
>> Object addresses are not exposed either. You would have to build such
>> instrumentation into V8's internals.
>>
>> On Tue, Aug 29, 2017 at 12:32 PM, Hanyun Tao  wrote:
>>
>>> Thank you Jakob!
>>>
>>> I can understand why it is hidden from external access. But would it be
>>> possible for the user to get access to the instruction addresses of the
>>> compiled code?
>>>
>>> We are studying the cache performance of JavaScript execution and we
>>> want to know if it is possible to prefetch the instructions in the next
>>> event handlers into the cache before it is executed based on the
>>> information collected in the v8 engine.
>>>
>>> Best regards,
>>>
>>> On Tuesday, August 29, 2017 at 12:29:33 PM UTC-4, Jakob Kummerow wrote:

 No, compiled code is an internal implementation detail and as such is
 hidden from JavaScript and other external access. If there ever is a way
 for users to get to compiled code, then it's a (probably severe security)
 bug and we would like to hear about it! :-)

 On Mon, Aug 28, 2017 at 9:03 PM, Hanyun Tao  wrote:

> Hi Jakob,
>
> Thanks again!
>
> According to what you have said, v8 compiles the event handlers are
> installed(compiled?) before it is executed. If it is true, then I believe
> v8 will store the compiled code somewhere in the system.
> Would it be possible for the user to get access to the compiled code?
>
> Best regards,
>
> On Monday, August 28, 2017 at 1:29:29 PM UTC-4, Jakob Kummerow wrote:
>>
>> The main API entry point for compilation is
>> v8::ScriptCompiler::Compile().
>>
>> I don't think event handling itself triggers compilation; but I'm not
>> an expert on that part of the system. AFAIK event handlers are installed
>> during page load (or more precisely: DOM element creation); they may 
>> still
>> be compiled on-demand on first use but that's not controlled via the V8 
>> API.
>>
>> On Mon, Aug 28, 2017 at 9:06 AM, Hanyun Tao 
>> wrote:
>>
>>> Hi Jakob,
>>>
>>> Thank you for replying!
>>>
>>> To be more specific, I'm looking for the point (function) that
>>> initiate the compilation process.
>>>
>>> In my understanding, when handling an "event", the renderer process
>>> in the browser will figure out the JavaScript related to the event, and 
>>> ask
>>> the V8 engine to execute it by calling some api function.
>>>
>>> Inside those api function, there should be a point where V8 initiate
>>> the compilation process, and that is what I'm looking for.
>>>
>>> Best regards,
>>>
>>>
>>>
>>> On Sunday, August 27, 2017 at 7:58:37 PM UTC-4, Jakob Kummerow wrote:

 Hi Hanyun,

 V8 is fairly complicated, and pretty much all it does is to compile
 and execute JavaScript. Do you have a more specific question?

 If you just want a starting point for reading code: maybe
 "CompileTopLevel" in src/compiler.cc would be a reasonable choice.

 On Sun, Aug 27, 2017 at 1:06 PM, Hanyun Tao 
 wrote:

> Hi all,
>
> I'm trying to understand how V8 engine compiles and execute
> JavaScript events. I used GDB to track the function call path when 
> handling
> an event and it look like this.
>
>
>
>  #0  v8::internal::Logger::TimerEvent (this=,
> se=, name=) at ../../v8/src/log.cc:866
>
>
>  #1  0x7f0cfcb23b39 in TimerEventScope (this=,
> isolate=) at ../../v8/src/log.h:354
>
>
>  #2  v8::Function::Call (this=, 

Re: [v8-users] How does V8 compiles and executes JavaScript events?

[Hanyun Tao]

Hi Jakob,

Thank you for answering my questions! 

I would like to modify V8's internals to support such functionality. But 
before I start I would like to learn a little more about how chromium 
interact with V8 to process JavaScript event.

By reading the source code, I believe that the WebKit rendering engine will 
call *v8::Function::Call* in api.cc, and after that the V8 engine will 
execute the Javascript code. Am I correct?

If this is how things going to work, could you please point out where in 
this process, V8 compiles the code, or "read" the compiled code correspond 
to the JavaScript?

Thank you!

On Wednesday, August 30, 2017 at 8:39:00 PM UTC-4, Jakob Kummerow wrote:
>
> Object addresses are not exposed either. You would have to build such 
> instrumentation into V8's internals.
>
> On Tue, Aug 29, 2017 at 12:32 PM, Hanyun Tao  > wrote:
>
>> Thank you Jakob!
>>
>> I can understand why it is hidden from external access. But would it be 
>> possible for the user to get access to the instruction addresses of the 
>> compiled code? 
>>
>> We are studying the cache performance of JavaScript execution and we want 
>> to know if it is possible to prefetch the instructions in the next event 
>> handlers into the cache before it is executed based on the information 
>> collected in the v8 engine.
>>
>> Best regards,
>>
>> On Tuesday, August 29, 2017 at 12:29:33 PM UTC-4, Jakob Kummerow wrote:
>>>
>>> No, compiled code is an internal implementation detail and as such is 
>>> hidden from JavaScript and other external access. If there ever is a way 
>>> for users to get to compiled code, then it's a (probably severe security) 
>>> bug and we would like to hear about it! :-)
>>>
>>> On Mon, Aug 28, 2017 at 9:03 PM, Hanyun Tao  wrote:
>>>
 Hi Jakob,

 Thanks again!

 According to what you have said, v8 compiles the event handlers are 
 installed(compiled?) before it is executed. If it is true, then I believe 
 v8 will store the compiled code somewhere in the system.
 Would it be possible for the user to get access to the compiled code? 

 Best regards,

 On Monday, August 28, 2017 at 1:29:29 PM UTC-4, Jakob Kummerow wrote:
>
> The main API entry point for compilation is 
> v8::ScriptCompiler::Compile().
>
> I don't think event handling itself triggers compilation; but I'm not 
> an expert on that part of the system. AFAIK event handlers are installed 
> during page load (or more precisely: DOM element creation); they may 
> still 
> be compiled on-demand on first use but that's not controlled via the V8 
> API.
>
> On Mon, Aug 28, 2017 at 9:06 AM, Hanyun Tao  
> wrote:
>
>> Hi Jakob,
>>
>> Thank you for replying!
>>
>> To be more specific, I'm looking for the point (function) that 
>> initiate the compilation process.
>>
>> In my understanding, when handling an "event", the renderer process 
>> in the browser will figure out the JavaScript related to the event, and 
>> ask 
>> the V8 engine to execute it by calling some api function.
>>
>> Inside those api function, there should be a point where V8 initiate 
>> the compilation process, and that is what I'm looking for.
>>
>> Best regards,
>>
>>
>>
>> On Sunday, August 27, 2017 at 7:58:37 PM UTC-4, Jakob Kummerow wrote:
>>>
>>> Hi Hanyun,
>>>
>>> V8 is fairly complicated, and pretty much all it does is to compile 
>>> and execute JavaScript. Do you have a more specific question? 
>>>
>>> If you just want a starting point for reading code: maybe 
>>> "CompileTopLevel" in src/compiler.cc would be a reasonable choice.
>>>
>>> On Sun, Aug 27, 2017 at 1:06 PM, Hanyun Tao  
>>> wrote:
>>>
 Hi all,

 I'm trying to understand how V8 engine compiles and execute 
 JavaScript events. I used GDB to track the function call path when 
 handling 
 an event and it look like this.



  #0  v8::internal::Logger::TimerEvent (this=, 
 se=, name=) at ../../v8/src/log.cc:866


  #1  0x7f0cfcb23b39 in TimerEventScope (this=, 
 isolate=) at ../../v8/src/log.h:354


  #2  v8::Function::Call (this=, context=..., 
 recv=..., argc=, argv=) at 
 ../../v8/src/api.cc:5094


  #3  0x7f0cf18daa0b in blink::V8ScriptRunner::callFunction 
 (function=..., context=0x201b099829d0, receiver=..., argc=1, 
 args=0x7fffe9e1cea0, 
 isolate=0x2def318c6020) at ../../third_party/WebKit/Source/
 bindings/core/v8/V8ScriptRunner.cpp:658


  #4  0x7f0cf18a1c26 in 
 blink::V8EventListener::callListenerFunction (this=0xe9381bfbea0,