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

[Stefan Karlsson]

On Tue, 11 Mar 2025 17:10:10 GMT, Kim Barrett <kbarrett at openjdk.org> wrote:

>> Stefan Karlsson has updated the pull request incrementally with two additional commits since the last revision:
>> 
>>  - Make ZPageAge ZForwarding member fileds constant
>>  - Review comments
>
> src/hotspot/share/gc/z/zStringDedup.hpp line 31:
> 
>> 29: #include "oops/oopsHierarchy.hpp"
>> 30: 
>> 31: class ZStringDedupContext {
> 
> The file name and the class that constitutes all of the content of the file don't match.  That seems
> kind of strange.  Also contrary to the style guide:
> https://github.com/openjdk/jdk/blame/da2b4f0749dffc99fa42c7311fbc74231af273bd/doc/hotspot-style.md#L85-L87

This was intentional. I named them zStringDedup.* to show that this is the file that contains ZGC's support to interface with StringDedup. We do that for other sub-systems that ZGC interfaces with.

The crux is that the string dedup API requires us to maintain the lifecycle of a `StringDedup::Requests` instance, so we can't simply have a function like `ZStringDedup::request(obj)`. Instead we need to add a ZStringDedupContext class, just so that we maintain the Requests object. (I choose suffix `Context` instead of `Requests`, because that naming fits better with the rest of the ZGC code).

About the style _guide_. I see that section more as a helpful guide, but not as a complete mandate to how to name files in HotSpot.

Note, that even the StringDedup::Request class is placed in a file named stringDedup.hpp and not stringDedupRequest.hpp! I could easily also create a structure that simulates the structure used in stringDedup.hpp:


class ZStringDedup {
public:
  class Context {
    ...
  };
};


However, I don't find that particularly appealing and it somewhat goes against the informal style guide that Per and I used when we first started to write ZGC.

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

PR Review Comment: https://git.openjdk.org/jdk/pull/23965#discussion_r1989934945