Roman Kennke

@tschatzl are you good with this PR now?

> > 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...

> The https://github.com/openjdk/jdk/compare/master...tschatzl:jdk:alt-fullgc-forwarding?expand=1 branch now contains the promised cleanup. Thanks, Thomas! This looks useful. I've merged your branch into this PR.

/contributor add @tschatzl

> > it can forward objects from one region to a maximum of two regions > > Those two regions must be adjacent, right? Something like: `_biased_bases[1][from_reg_idx] - _biased_bases[0][from_reg_idx] ==...

> > Parallel full GCs divide up ... > > Why is Parallel relevant here? > > The description mentions only "the full-GC modes of Serial, Shenandoah and G1 GCs...

@tschatzl or anybody else: any concerns remaining with this PR? If not, I would integrate it later today (if the GHA are green).

@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...

Using the same Retain.java program that Aleksey posted earlier, I now get the following numbers: Baseline: 286.9ms -AltGCForwarding: 286.3ms (-0.2%) +AltGCForwarding: 309.1ms (+7.7%)

I get the following results on a Graviton2, the columns are different vector-length multipliers. There doesn't seem to be a clear winner. 1 or 2 may be preferable.   | 1...