RFR: 8340363: Tag-specific default decorators for UnifiedLogging [v4]

[Antón Seoane]

On Tue, 24 Sep 2024 16:43:57 GMT, Antón Seoane <duke at openjdk.org> wrote:

>> Hi all,
>> 
>> Currently, the Unified Logging framework defaults to three decorators (uptime, level, tags) whenever the user does not specify otherwise through `-Xlog`. This can result in cumbersome input whenever a specific user that relies on a particular tag(s) has some predefined needs. For example, C2 developers rarely need decorations, and having to manually specify this every time results inconvenient.
>> 
>> To address this, this PR enables the possibility of adding tag-specific default decorators to UL. These defaults are in no way overriding user input -- they will only act whenever `-Xlog` has no decorators supplied and there is a positive match with the pre-specified defaults. Such a match is based on the following:
>> 
>> - Inclusion: if `-Xlog:jit+compilation` is provided, a default for `jit` may be applied.
>> - Specificity: if, for the above line, there is a more specific default for `jit+compilation` the latter shall be applied. Upon equal specificity cases, both defaults will be applied.
>> - Additionally, defaults may target a specific log level.
>> 
>> Decorators are also associated with an output file, so an output device may only have one set of decorators. For this reason, if different `LogSelection`s trigger defaults, none is to be applied.
>> 
>> In summary, these defaults may be seen as a "tailored" or "flavoured" version of the existing "uptime-level-tags" current defaults.
>> 
>> Please consider this PR, and thanks!
>
> Antón Seoane has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Removed whitespace

Upon further comments and consideration, I am keeping this PR on hold and opening up to further discussion via the hotspot-dev mailing list: https://mail.openjdk.org/pipermail/hotspot-dev/2024-September/094810.html

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PR Comment: https://git.openjdk.org/jdk/pull/20988#issuecomment-2379285507