[8u] RFR 8233197: Invert JvmtiExport::post_vm_initialized() and Jfr:on_vm_start() start-up order for correct option parsing

[Andrew Dinn]

On 10/06/2020 13:31, Jaroslav Bachorík wrote:
>> 3505 #if INCLUDE_JFR
>> 3506     // The VM creates & returns objects of this class. Make sure it's initialized.
>> 3507     initialize_class(vmSymbols::java_lang_Class(), CHECK_0);
>> 3508 #endif
>>
>> ISTM that this code is run unconditionally if INCLUDE_JFR is set at compile
>> time. Am I misunderstanding something?
> 
> Ok. I misunderstood the original request. I thought it was supposed to
> be in-line with other JFR specific changes which are guarded with
> compile-time conditional blocks.
> 
>>
>> If this is not true, and the initialize_class only runs if JFR is
>> enabled, how does that work?
> 
> This thing you ask (runtime check for whether JFR is enabled) is
> impossible at that place, AFAIK. The affected code is in 'create_vm'
> block so JVM is not really alive. Checking the JFR enabled status
> requires parsing JFR configs which happens to be done in Java and that
> requires java.lang.Class to be already linked and initialized - making
> it impossible to check the JFR enablement status before initializing
> java.lang.Class.
How is that so? I believe command line arguments will have been
processed at this stage. If so then you can easily detect whether or not
JFR has been enabled on the command line. The simplest thing is to check
global boolean FlightRecorder. This global is set from the command line
using -X:+FlightRecorder and cleared using -X:-FlightRecorder. If not
specified it defaults to false.

So, in the case where JFR is conditionally compiled in you can call
initialize_class() early if FlightRecorder is true and call it at the
normal point of FlightRecorder is false.

In the case where JFR is conditionally compiled out then the call should
happen late as happened before this patch.

Is there any reason why that won't work?

regards,


Andrew Dinn
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