RFR: 8305896: Alternative full GC forwarding [v33]

[Roman Kennke]

On Mon, 8 May 2023 10:12:41 GMT, Thomas Schatzl <tschatzl at openjdk.org> wrote:

>> Roman Kennke has updated the pull request incrementally with two additional commits since the last revision:
>> 
>>  - Fix typos
>>  - Make FallbackTable an inner class of SlidingForwarding
>
> The https://github.com/openjdk/jdk/compare/master...tschatzl:jdk:alt-fullgc-forwarding?expand=1 branch now contains the promised cleanup.

@tschatzl @fisk @coleenp @shipilev I've pushed a change that specializes all affected full-GC loops to get the flag-check out of the hot loops. This should get performance in the -UseAltGCForwarding paths back to normal, and show minimal effect (if any) in the +UseAltGCForwarding path. It is a little uglier though. If you prefer the clean and slightly slower version, let me know and I would back it out. Consider that once (after a few release cycles) the Lilliput stuff is stable and default, all that specialized stuff will disappear..
I guess the alternative would be to not put it under flag to begin with.

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PR Comment: https://git.openjdk.org/jdk/pull/13582#issuecomment-1557338343