RFR: 8336685: Shenandoah: Remove experimental incremental update mode [v2]

[Aleksey Shipilev]

On Wed, 24 Jul 2024 19:25:24 GMT, William Kemper <wkemper at openjdk.org> wrote:

>> src/hotspot/cpu/ppc/gc/shenandoah/shenandoahBarrierSetAssembler_ppc.cpp line 571:
>> 
>>> 569:   /* ==== Apply keep-alive barrier, if required (e.g., to inhibit weak reference resurrection) ==== */
>>> 570:   if (ShenandoahBarrierSet::need_keep_alive_barrier(decorators, type)) {
>>> 571:     if (ShenandoahSATBBarrier) {
>> 
>> A bit weird to replace IU with SATB barrier here.
>
> @shipilev [The original code](https://github.com/openjdk/jdk/pull/20316/files#diff-cb01b36a8c7017c9e21645a0ff9075897e5bfa67ae37d4f0d69ccc582656ec31L71) used the function `iu_barrier` to emit the pre-write barrier for both SATB and IU modes. This only happened in the `ppc` port. Other platforms just invoke the function to emit the `satb_write_barrier` directly here (see [x86](https://github.com/openjdk/jdk/blob/master/src/hotspot/cpu/x86/gc/shenandoah/shenandoahBarrierSetAssembler_x86.cpp#L580), for example).
> 
> @kdnilsen No, `need_keep_alive_barrier` may be true in SATB mode. The use of the barrier here is to make sure weak references that get loaded during mark are added to the SATB buffer.

Oh, okay. So that weirdness is pre-existing, fine.

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PR Review Comment: https://git.openjdk.org/jdk/pull/20316#discussion_r1691057726