RFR: 8353115: GenShen: mixed evacuation candidate regions need accurate live_data [v2]

[Kelvin Nilsen]

On Wed, 9 Apr 2025 18:01:03 GMT, Kelvin Nilsen <kdnilsen at openjdk.org> wrote:

>> It does look simpler. Do you have an example of one of the failing asserts? 
>> 
>> One thing I hadn't considered is how "hot" `ShenandoahHeapRegion::get_live_data_words` is. Is there going to be a significant performance hit if we make this method do more work? It does look like this method is called frequently.
>
> Examples:
>   FullGC worker:
>    void ShenandoahMCResestCompleteBitmapTask::work(uint worker_id) {
>     ShenandoahParallelWorkerSession worker_session(worker_id);
>     ShenandoahHeapRegion* region = _regions.next();
>     ShenandoahHeap* heap = ShenandoahHeap::heap();
>     ShenandoahMarkingContext* const ctx = heap->complete_marking_context();
>     while (region != nullptr) {
>       if (heap->is_bitmap_slice_committed(region) && !region->is_pinned() && region->has_marked()) {
>         //  kelvin replacing has_live() with new method has_marked() because has_live() calls get_live_data_words()
>         //  and pointer_delta() asserts out because TAMS is not less than top().  has_marked() does what has_live()
>         //  used to do...
>         ctx->clear_bitmap(region);
>       }
>       region = _regions.next();
>     }
>   }
> 
> ShenandoahInitMarkUpdateRegionStateClosure::heap_region_do() {
> -    assert(!r->has_live(), "Region %zu should have no live data", r->index());
> +    assert(!r->has_marked(), "Region %zu should have no marked data", r->index());

Not sure about performance impact, other than implementing and testing...

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PR Review Comment: https://git.openjdk.org/jdk/pull/24319#discussion_r2035888970