RFR: 8374445: Fix -Wzero-as-null-pointer-constant warnings in JfrSet [v2]

[Markus Grönlund]

On Thu, 15 Jan 2026 16:50:14 GMT, Kim Barrett <kbarrett at openjdk.org> wrote:

>> Please review this change to fix JfrSet to avoid triggering
>> -Wzero-as-null-pointer-constant warnings when that warning is enabled.
>> 
>> The old code uses an entry value with representation 0 to indicate the entry
>> doesn't have a value. It compares an entry value against literal 0 to check
>> for that. If the key type is a pointer type, this involves an implicit 0 =>
>> null pointer constant conversion, so we get a warning when that warning is
>> enabled.
>> 
>> Instead we initialize entry values to a value-initialized key, and compare
>> against a value-initialized key. This changes the (currently undocumented)
>> requirements on the key type. The key type is no longer required to be
>> trivially constructible (to permit memset-based initialization), but is now
>> required to be value-initializable. That's currently a wash, since all of the
>> in-use key types are fundamental types (traceid (u8) and Klass*).
>> 
>> Testing: mach5 tier1-3 (tier3 is where most jfr tests are run)
>
> Kim Barrett has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains two additional commits since the last revision:
> 
>  - Merge branch 'master' into jfrset-zero-as-null-pointer-warnings
>  - fix -Wzero-as-null-poniter-constant warnings in jfrSet.hpp

Look good, thanks Kim.

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

Marked as reviewed by mgronlun (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/29022#pullrequestreview-3666675208