Setting JVMTI Capabilities when VM is in "Live Phase"

[Leela Mohan]

Hi Daniel,

Actually,  I was thinking about the case where compiler choose not to have
complete "de-opt" state. For ex: Local pruning. I can also think of other
cases which need callback events like, posting exceptions to the agent.

JVMTI requests for examining/changing the stack frame would conservatively
de-optimize the compile methods but not all de-optimizable locations can
restore the java state user expect.

What are the expectations for VM for these cases ?

Thanks,
Leela

On Fri, Aug 12, 2016 at 4:04 PM, Daniel D. Daugherty <
daniel.daugherty at oracle.com> wrote:

> On 8/12/16 4:21 PM, Leela Mohan wrote:
>
>> Hi experts,
>>
>> It looks like, we don't disallow setting capabilities when VM is in
>> "JVMTI_PHASE_LIVE". And, I notice, for every new compilation of method,
>> ciEnv caches the JVMTI state and expects those assumptions to be true
>> during the compilation. Otherwise, dump the compiled method.
>>
>> However, we don't seem to do anything with the methods which were compiled
>> before setting the capability.
>>
>> What is the understanding?
>>
>> Thanks,
>> Leela
>>
>
> Hi Leela,
>
> I'm guessing that you are talking about this capability:
>
> http://docs.oracle.com/javase/8/docs/platform/jvmti/jvmti.ht
> ml#jvmtiCapabilities.can_generate_compiled_method_load_events
>
> and this event:
>
> http://docs.oracle.com/javase/8/docs/platform/jvmti/jvmti.ht
> ml#CompiledMethodLoad
>
>
> The can_generate_compiled_method_load_events capability needs to be added
> in order to generate CompiledMethodLoad events. Capabilities are added via
> http://docs.oracle.com/javase/8/docs/platform/jvmti/jvmti.ht
> ml#AddCapabilities
> which can be called from different JVM/TI phases. Different VM
> implementations
> can require that certain capabilities can only be added in certain JVM/TI
> phases.
> However, if AddCapabilities() does not return a JVM/TI error when a
> capability
> is added in a phase, e.g., the live phase, then you can safely assume that
> the capability has been added.
>
>
> In your example, it sounds like the capability is added in the live phase
> because you are seeing events generated for newly compiled methods. In
> order
> to see synthetic events for methods that were compiled before you added the
> capability, your agent needs to use a different function:
>
> http://docs.oracle.com/javase/8/docs/platform/jvmti/jvmti.ht
> ml#GenerateEvents
>
> The documentation for Compiled Method Load has this line:
>
> > These events can be sent after their initial occurrence with
> GenerateEvents.
>
> and that sounds just like your situation.
>
> Hope this helps.
>
> Dan
>
>
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