RFR: 8372444: Genshen: Optimize evacuation function

[William Kemper]

On Wed, 26 Nov 2025 02:13:39 GMT, Y. Srinivas Ramakrishna <ysr at openjdk.org> wrote:

>> This is a hot code path. Many of the branches can be eliminated at compile time by introducing template parameters. This change shows a 5% reduction in concurrent evacuation times at the trimmed-10 average on the extremem benchmark:
>> 
>> 
>> gen/control/extremem
>>                                 Category |  Count |         Total |      GeoMean |      Average |     Trim 0.1 |       StdDev |      Minimum |      Maximum
>>         concurrent_evacuation_young_data |     65 |   9625198.000 |   118747.249 |   148079.969 |   145182.189 |    76534.845 |     7216.000 |   317261.000
>> 
>> gen/template/extremem
>>                                 Category |  Count |         Total |      GeoMean |      Average |     Trim 0.1 |       StdDev |      Minimum |      Maximum
>>         concurrent_evacuation_young_data |     65 |   9095084.000 |   113036.539 |   139924.369 |   137661.226 |    71091.273 |     7523.000 |   294442.000
>
> LGTM. Impressed that templatization led to such a substantial improvement.
> Should the 2-case switch in `ShenandoahGenerationalHeap::evacuate_object()` be converted to an `if-else` ?

@ysramakrishna , we have had reviewers in the past ask us to use a switch with this enumeration. I agree an `if/else` would be more compact here, but it would probably belong on a different pull request. Thanks for the review.

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

PR Comment: https://git.openjdk.org/jdk/pull/28496#issuecomment-3597245231