RFR: JDK-8269650: Optimize gc-locker in [Get|Release]StringCritical for latin string

[David Holmes]

On 1/07/2021 9:30 am, Kim Barrett wrote:
> On Wed, 30 Jun 2021 11:55:49 GMT, Hamlin Li <mli at openjdk.org> wrote:
> 
>> Currently, JNI GetStringCritical locks gc locker for all strings including latin and non-latin until ReleaseStringCritical.
>> But for latin, it's not necessary to still lock gc locker after GetStringCritical, as it's copied anyway whether obj pining is supported or not, so it's fine to unlock gc locker after GetStringCritical.
> 
>> _Mailing list message from [David Holmes](mailto:david.holmes at oracle.com) on [hotspot-dev](mailto:hotspot-dev at mail.openjdk.java.net):_
>>
>> On 30/06/2021 10:35 pm, Thomas Schatzl wrote:
>>
>>> On Wed, 30 Jun 2021 11:55:49 GMT, Hamlin Li <mli at openjdk.org> wrote:
>>>> Currently, JNI GetStringCritical locks gc locker for all strings including latin and non-latin until ReleaseStringCritical.
>>>> But for latin, it's not necessary to still lock gc locker after GetStringCritical, as it's copied anyway whether obj pining is supported or not, so it's fine to unlock gc locker after GetStringCritical.
>>>
>>>
>>> Actually I think the *String* object can be unlocked regardless of `is_latin1` or not. The code returns the *char array* that the native code is going to process after all - which is not locked *at all* but probably should be. I filed [JDK-8269661](https://bugs.openjdk.java.net/browse/JDK-8269661) for this.
>>
>> I admit I do not know how String objects are laid out since compact
>> strings came along but IIRC before that the char* pointed to an actual
>> array embedded in the String's value, and so the String (and its value
>> array) had to be pinned/locked.
>>
>> David
> 
> I don’t see how that embedding of the char array could work (without bespoke GC support or something like Valhalla).
> I also don’t see any sign of that in the code history.

I was unclear in what I was trying to describe.

The String object contains a reference to its byte[] 'value'. The byte[] 
object embeds the actual array of bytes (C char) that we expose via 
GetStringCritical. So the byte[] has to be pinned, but not the String.

David

> -------------
> 
> PR: https://git.openjdk.java.net/jdk/pull/4637
>