How Date.now() function value is returned (Is it from kernel of v8 engine has its own implementation)

[Abhishek Kanike]

Hi,

I want to know how the date.now() function is called in javascript (or how it returns the value). I believe that javascript uses date.now() by system call. I want to in chrome source code how it is being set. 

This is useful for one of the performance benchmark that I am working on.

Can someone please guide me to know how this happens in v8 engine.

Thanks in advance.

Regards,

K Abhishek

[Jakob Kummerow]

The time always has to be retrieved from the kernel. V8's implementation is in base::OS::TimeCurrentMillis, implemented in src/base/platform/platform-{win32,posix}.cc.

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[Abhishek Kanike]

Cool.. I see it. Thanks a lot Jacob.

[Abhishek Kanike]

Sorry JaKob

[Abhishek Kanike]

​Hi,

For confirmation i am using a simple Date.now() call on button click [Attached]. 

I have added a print log in Time::NOW() at https://cs.chromium.org/chromium/src/v8/src/base/platform/time.cc?type=cs&sq=package:chromium&g=0&l=402 as printf("v8:DEBUG::TIME:NOW called").

I am not getting this log in adb logcat. So is this the correct place from where Date.now() is getting called?

Regards, 

K Abhishek

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Cheers, 

K Abhishek

[Jakob Kummerow]

Yes, it is.

Have you tried using a debugger? You can set a breakpoint in the BUILTIN(DateNow) function here:

https://cs.chromium.org/chromium/src/v8/src/builtins/builtins-date.cc?q=builtins-date.cc&sq=package:chromium&dr&l=282

and step through the code from there.

[Abhishek Kanike]

Jakob,

Thanks for quick reply. 

That's a good point, should have tried with gdb. Will check with that.

Is there a way to add logs in v8 source code, it will be lot faster to debug.

In cc (compositor), i used LOG(INFO)<<"print"; from base/logging.h (of main chromium src)

[Jakob Kummerow]

Sure, just use printf or std::cout << "print\n". I don't know whether adb logcat has any special requirements.

[Abhishek Kanike]

Hi Jakob and v8-users,​​

I have following observations on printing logs in v8 and date.now call trace.

1. Logs in v8:
   

   printf and std::cout were not outputting any logs in adb logcat. I ​tried redirecting stdio logs to logcat by following commands - 

   $ adb shell stop
   $ adb shell setprop log.redirect-stdio true
   $ adb shell start

   But still logs were not getting displayed in adb logcat. So I used android log library. Following is the procedure

   • Import header file:
     

     #if defined(V8_OS_ANDROID)
       #include <android/log.h>
     #endif

   • Print log:
     

     #if defined(V8_OS_ANDROID)
       __android_log_print(ANDROID_LOG_INFO, "<TAG>", "<MESSAGE>");
     #endif
     

   
   
2. Date.now() final system call:
   Date.now() calls V8Platform::CurrentClockTimeMillis() from src/gin/v8_platform.cc ---> Time::Now() from src/base/time/time.cc ---> getTimeofDay() rather than base::OS::TimeCurrentMillis. Can anyone explain this behaviour?

Regards,

K Abhishek

[Jakob Kummerow]

gin provides its own v8::Platform implementation, overriding the default platform.

[Abhishek Kanike]

​Yes gin has its own implementation of time apart from default-platform.

I have one more concern.
How can i implement my own javascript API?

Basically I am thinking to create two javascript API which calls two respective calls from my shared library (external)​.

Instead of creating two new api's is there any other method from JavaScript which calls my external library functions. I dont want to fork processes from javascript to execute my library functions since I am planning to profile chrome browser.

Regards,

K Abhishek