RFR: 8324241: Always record evol_method deps to avoid excessive method flushing [v2]

[Volker Simonis]

On Tue, 23 Jan 2024 23:36:37 GMT, Dean Long <dlong at openjdk.org> wrote:

>> A good question, but after deep digging (it took me quite some time to figure this out myself :) I don't think that `can_hotswap_or_post_breakpoint`/`all_dependencies_are_recorded` can ever be reset. Here's how it works:
>> 
>> - there's a global set of `always_capabilities` which is initialized when the first JVMTI environment is created:
>> 
>> // capabilities which are always potentially available
>> jvmtiCapabilities JvmtiManageCapabilities::always_capabilities;
>> 
>> void JvmtiManageCapabilities::initialize() {
>>   _capabilities_lock = new Mutex(Mutex::nosafepoint, "Capabilities_lock");
>>   always_capabilities = init_always_capabilities();
>> 
>> 
>> - as the name implies, this set of capabilities contains all generally available capabilities (except for the `onload` and `solo` capabilites, which are maintained in separate sets).
>> - `JvmtiManageCapabilities::update()` only operates on this global sets of capabilites (and *not* on the concrete capabilities of a specific JVMTI environment):
>> 
>> void JvmtiManageCapabilities::update() {
>>   jvmtiCapabilities avail;
>>   // all capabilities
>>   either(&always_capabilities, &always_solo_capabilities, &avail);
>> ...
>>   // If can_redefine_classes is enabled in the onload phase then we know that the
>>   // dependency information recorded by the compiler is complete.
>>   if ((avail.can_redefine_classes || avail.can_retransform_classes) &&
>>       JvmtiEnv::get_phase() == JVMTI_PHASE_ONLOAD) {
>>     JvmtiExport::set_all_dependencies_are_recorded(true);
>>   }
>> ...
>>   JvmtiExport::set_can_hotswap_or_post_breakpoint(
>>     avail.can_generate_breakpoint_events ||
>>     avail.can_redefine_classes ||
>>     avail.can_retransform_classes);
>> 
>> - This means that `JvmtiManageCapabilities::update()` is always conservative regarding its exports to `JvmtiExport` as it always exports *all* potentially available capabilites once a JVMTI environment is created and not only the specific capabilities requested by concrete JVMTI environment. This means that even if we create a JVMTI agent with an empty set of capabilities, `can_hotswap_or_post_breakpoint`/`all_dependencies_are_recorded` will still be set in `JvmtiExport`.
>> - There's no code path (at least I couldn't find one) which takes capabilities away from the `always_capabilities` set. Only if an environment requests `on_load` capabilities, they will be transferred from the global `onload_capabilities` to the `always_capabilities` set:
>> 
>> jvmtiError JvmtiManageCapabilities::...
>
> An assert works for me.  Thanks.

Added assert in `JvmtiExport::set_can_hotswap_or_post_breakpoint()`. Thanks.

-------------

PR Review Comment: https://git.openjdk.org/jdk/pull/17509#discussion_r1464670162