RFR: 8305896: Alternative full GC forwarding [v29]

[David Holmes]

On Thu, 4 May 2023 13:46:18 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

>> Roman Kennke has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   Add usual header include guards
>
> src/hotspot/share/gc/shared/gc_globals.hpp line 698:
> 
>> 696:           constraint(GCCardSizeInBytesConstraintFunc,AtParse)               \
>> 697:                                                                             \
>> 698:   develop(bool, UseAltGCForwarding, false,                                  \
> 
> I don't think you can opt-in into `true` in release bits, if this flag is `develop` when the rest of Lilliput arrives. In release bits, all checks involving this flag would fold with `false`. Maybe that's the intent here, as it keeps the release performance at the baseline level, but this makes performance overhead estimations for this PR a bit hard :) [I'll hack the flag back to experimental for tests]

If you need this for compact object headers then shouldn't this flag be experimental?

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PR Review Comment: https://git.openjdk.org/jdk/pull/13582#discussion_r1187028649