RFR: 8255614: Shenandoah: Consolidate/streamline runtime LRBs [v2]

[Roman Kennke]

On Fri, 30 Oct 2020 13:05:23 GMT, Zhengyu Gu <zgu at openjdk.org> wrote:

>> Roman Kennke has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   Add null-check
>
> src/hotspot/share/gc/shenandoah/shenandoahCollectionSet.inline.hpp line 49:
> 
>> 47: bool ShenandoahCollectionSet::is_in_loc(void* p) const {
>> 48:   assert(p == NULL || _heap->is_in(p), "Must be in the heap");
>> 49:   uintx index = ((uintx) p) >> _region_size_bytes_shift;
> 
> Is this right? if heap is not zero-based

Yes. The biased cset-map is allocated such that NULL object maps to a special page that always yields false when checking in_cset(NULL). We use that same technique in JIT-compiled code to avoid explicit NULL-checks.

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PR: https://git.openjdk.java.net/jdk/pull/953