RFR: 8330181: Move PcDesc cache from nmethod header

[Vladimir Kozlov]

On Tue, 23 Apr 2024 01:16:40 GMT, Dean Long <dlong at openjdk.org> wrote:

>> Currently PcDescCache (32 bytes in 64-bit VM: PcDesc* _pc_descs[4]) is allocated in `nmethod` header.
>> 
>> Moved PcDescContainer (which includes cache) to C heap similar to ExceptionCache to reduce size of `nmethod` header and to remove WXWrite transition when we update the cache in `PcDescCache::add_pc_desc()`.
>> 
>> Removed `PcDescSearch` class which was leftover from `CompiledMethod` days.
>> 
>> Tested tier1-4,stress,xcomp and performance.
>
> src/hotspot/share/code/nmethod.cpp line 2738:
> 
>> 2736:       // which is typically called in a signal handler
>> 2737:       _pc_desc_cache.add_pc_desc(upper);
>> 2738:     }
> 
> The special case for ASGCT, along with ThreadWXEnable changes here:
> https://github.com/openjdk/jdk/blob/master/src/hotspot/share/runtime/sharedRuntime.cpp#L484
> https://github.com/openjdk/jdk/blob/master/src/hotspot/share/runtime/sharedRuntime.cpp#L1755
> https://github.com/openjdk/jdk/blob/master/src/hotspot/os/posix/signals_posix.cpp#L617
> look like they will no longer be needed.  I suggest filing a follow-up RFE for an ASGCT expert to take.

Thank you, @dean-long, for pointing this. 

[JDK-8302736](https://bugs.openjdk.org/browse/JDK-8302736) added 2 ThreadWXEnable in sharedRuntime.cpp
And later [JDK-8316392](https://bugs.openjdk.org/browse/JDK-8316392) added it to `PcDescCache::add_pc_desc()`
What a mess ...

We can now safely remove (after testing) all ThreadWXEnable which guards calls to `add_pc_desc()`
And file separate RFE for runtime to look on `!Thread::current_in_asgct()`

Is this what you are suggesting? I will do it.

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PR Review Comment: https://git.openjdk.org/jdk/pull/18895#discussion_r1575607477