RFR: 8341099: GenShen: assert(HAS_FWD == _heap->has_forwarded_objects()) failed: Forwarded object status is sane

[Kelvin Nilsen]

On Thu, 3 Oct 2024 21:35:29 GMT, William Kemper <wkemper at openjdk.org> wrote:

> Evacuations may occur during young or mixed collection cycles. When it is a young cycle during old marking, this barrier may be called to both evacuate cset objects and again to enqueue the targets of the copy (if they are in the old generation). If it is a mixed collection, then old marking will _not_ be in progress and this barrier will not be called to enqueue anything.

Marked as reviewed by kdnilsen (Committer).

src/hotspot/share/gc/shenandoah/shenandoahBarrierSet.inline.hpp line 406:

> 404:         shenandoah_assert_forwarded_except(elem_ptr, obj, _heap->cancelled_gc());
> 405:         ShenandoahHeap::atomic_update_oop(fwd, elem_ptr, o);
> 406:       }

I think the reason we are removing this assignment here is because we will never be at this line of code if (ENQUEUE && !ctx->is_marked_strong_or_old(obj)).  In other words, if we are evacuating this object, then this object is not in old or this object is in old and we are not marking old.  I'd feel a little more comfortable with this code change if there were at minimum a comment here to explain this, or perhaps even better an assert.

assert(!ENQUEUE || !is_old(obj), "Do not evacuate old while old marking is active")

something like this...

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PR Review: https://git.openjdk.org/shenandoah/pull/507#pullrequestreview-2346831096
PR Review Comment: https://git.openjdk.org/shenandoah/pull/507#discussion_r1786933650