RFR: 8353115: GenShen: mixed evacuation candidate regions need accurate live_data [v11]

[William Kemper]

On Fri, 3 Oct 2025 20:31:10 GMT, Kelvin Nilsen <kdnilsen at openjdk.org> wrote:

>> The existing implementation of get_live_data_bytes() and git_live_data_words() does not always behave as might be expected.  In particular, the value returned ignores any allocations that occur subsequent to the most recent mark effort that identified live data within the region.  This is typically ok for young regions, where the amount of live data determines whether a region should be added to the collection set during the final-mark safepoint.
>> 
>> However, old-gen regions that are placed into the set of candidates for mixed evacuation are more complicated.  In particular, by the time the old-gen region is added to a mixed evacuation, its live data may be much larger than at the time concurrent old marking ended.
>> 
>> This PR provides comments to clarify the shortcomings of the existing functions, and adds new functions that provide more accurate accountings of live data for mixed-evacuation candidate regions.
>
> Kelvin Nilsen has updated the pull request incrementally with one additional commit since the last revision:
> 
>   remove _mixed_candidate_garbage_words from ShenandoahHeapRegion

Code looks good to me. Let's double check performance results (especially for non-generational mode).

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Marked as reviewed by wkemper (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/24319#pullrequestreview-3301312685