RFR: 8305896: Alternative full GC forwarding [v32]

[Roman Kennke]

On Fri, 5 May 2023 16:19:01 GMT, Thomas Schatzl <tschatzl at openjdk.org> wrote:

> > I have been working aggressively strength-reducing operations and use biased arrays (yes, you can, not sure why not?) (performance seems better if you separate the ´_bases_table` into two logical ones too), reducing the overhead from baseline to +UseAltGCForwarding from a bit more than 10% to ~7.8% you gave on an Ice Lake machine. Same code also shows improvements on some Zen3 machine (from ~9% to 5.3% - but the result is noisy and the machine isn't "clean"), both on that benchmark you gave. tada
> > Let me clean up the code a bit and provide to you on Monday so that you can test too if you are interested.
> 
> Actually, current code is at https://github.com/openjdk/jdk/compare/master...tschatzl:jdk:alt-fullgc-forwarding?expand=1 if you want to play with it ; the [strength reduction](https://github.com/openjdk/jdk/commit/10e1f43a736b28e18dc951cdd23309a71dc83694) commit does strength reduction and biasing, keeping current code intact, the other two split the `bases_table` into two separate tables and clean up a bit.
> 
> Hopefully I wasn't just dreaming or something when testing it (fwiw, I only kind of guarantee that it runs that `Retain` test. Not really tested otherwise). But it tended to crash if I didn't get things right for that test.

Nice! Thank you for doing that! I'll give it a test as soon as I get to it (perhaps only on Monday). Would you be ok if I integrate it into the main PR once my testing is good? Or do you have more things you want to do on that branch?

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PR Comment: https://git.openjdk.org/jdk/pull/13582#issuecomment-1536510796