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

[Kelvin Nilsen]

On Thu, 3 Oct 2024 23:28:33 GMT, William Kemper <wkemper at openjdk.org> wrote:

>> 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...
>
> Yes, I added an assert at the top of the function to this effect.
> 
>   // This function cannot be called to handle marking and evacuation at the same time (they operate on
>   // different sides of the copy).
>   assert((HAS_FWD || EVAC) != ENQUEUE, "Cannot evacuate and mark both sides of copy.");

Thanks.  I missed the significance of that.

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PR Review Comment: https://git.openjdk.org/shenandoah/pull/507#discussion_r1786955264