RFR: 8341622: Tag-specific disabled default decorators for UnifiedLogging

[Johan Sjölen]

On Mon, 7 Oct 2024 08:56:40 GMT, Antón Seoane <duke at openjdk.org> wrote:

> Currently, the Unified Logging framework defaults to three decorators (uptime, level, tags) whenever the user does not specify otherwise through -Xlog. However, some specific logging cases do not need decorations, and manually having to disable them results in cumbersome extra input and loss of ergonomics. 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 disabling of default decorators to UL. These disables 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 an inclusion rule: e.g. if -Xlog:jit+compilation is provided, a default for jit may be applied. Additionally, defaults may target a specific log level.
> 
> The original use case for this is related to C2 logging migration to UnifiedLogging, as currently no decorators are found in compiler logs and it would be expected to stay the same without the extra explicit removal every time via -Xlog. However, this would ease the migration of other logging that was initially deterred by this, such as -XX:+PrintInterpreter.
> 
> This PR is a simplification of the [8340363](https://bugs.openjdk.org/browse/JDK-8340363) (closed) ticket.

src/hotspot/share/logging/logDecorators.cpp line 110:

> 108: bool LogDecorators::has_disabled_default_decorators(const LogSelection& selection, const DefaultUndecoratedSelection* defaults, size_t defaults_count) {
> 109:   for (size_t i = 0; i < defaults_count; ++i) {
> 110:     auto current_default = defaults[i];

Please expand with deduced type.

src/hotspot/share/logging/logSelectionList.cpp line 62:

> 60:     }
> 61:   }
> 62:   return LogDecorators(0);

Here I'd like to see either an explanation of 0 as `LogDecorators(0 /* comment */)` or a meaningful name.

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PR Review Comment: https://git.openjdk.org/jdk/pull/21383#discussion_r1790018720
PR Review Comment: https://git.openjdk.org/jdk/pull/21383#discussion_r1790025195