RFR: 8261492: Shenandoah: reconsider forwardee accesses memory ordering

[Zhengyu Gu]

On Wed, 10 Feb 2021 08:55:39 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

> Shenandoah carries forwardee information in object's mark word. Installing the new mark word is effectively "releasing" the object copy, and reading from the new mark word is "acquiring" that object copy.
> 
> For the forwardee update side, Hotspot's default for atomic operations is memory_order_conservative, which emits two-way memory fences around the CASes at least on AArch64 and PPC64. This seems to be excessive for Shenandoah forwardee updates, and "release" is enough.
> 
> For the forwardee load side, we need to guarantee "acquire". We do not do it now, reading the markword without memory semantics. It does not seem to pose a practical problem today, because GC does not access the object contents in the new copy, and mutators get this from the JRT-called stub that separates the fwdptr access and object contents access by a lot. It still should be cleaner to "acquire" the mark on load to avoid surprises.
> 
> Additional testing:
>  - [x] Linux x86_64 `hotspot_gc_shenandoah`
>  - [x] Linux AArch64 `hotspot_gc_shenandoah`
>  - [x] Linux AArch64 `tier1` with Shenandoah

Changes requested by zgu (Reviewer).

src/hotspot/share/gc/shenandoah/shenandoahForwarding.inline.hpp line 89:

> 87:   // (would be paired with acquire in forwardee accessors). Acquire on failed update
> 88:   // would get the updated object after the forwardee load.
> 89:   markWord prev_mark = obj->cas_set_mark(new_mark, old_mark, memory_order_acq_rel);

You have obj->mark_acquire() above, I don't think you need leading acquire here.

src/hotspot/share/gc/shenandoah/shenandoahForwarding.inline.hpp line 43:

> 41:   // fwdptr. That object is still not forwarded, and we need to return
> 42:   // the object itself.
> 43:   markWord mark = obj->mark_acquire();

We also need the acquire barrier in fast path in generated code, right?

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PR: https://git.openjdk.java.net/jdk/pull/2496