RFR: 8347337: ZGC: String dedups short-lived strings [v2]

[Kim Barrett]

On Tue, 11 Mar 2025 08:56:19 GMT, Stefan Karlsson <stefank at openjdk.org> wrote:

>> src/hotspot/share/gc/z/zStringDedup.inline.hpp line 41:
>> 
>>> 39:     // Not a String object
>>> 40:     return;
>>> 41:   }
>> 
>> Consider using `StringDedup::is_enabled_string(obj)`, which combines `is_enabled` and `is_instance`
>> into a single test.
>
> From what I can see, the `is_enable_string` takes a 'Klass*' and not an `oop`. So, that would require us to fetch the Klass bits, read the global `_klass_mode`, and decode the Klass*. I think I prefer the version we currently have because it optimize for the default (and most common run mode) that users are not using string dedup with ZGC.
> 
> Also, I don't think it is that important to optimize these checks now that the code is only run for promoted objects, which should be a significant lower number of objects compared to the number of objects that are visited by the marking code path.

Good points.  Places where `is_enabled_string` are used already have the `Klass*` in hand for other reasons.

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

PR Review Comment: https://git.openjdk.org/jdk/pull/23965#discussion_r1989783345