RFR: JDK-8211270: GC abstraction to get real object and headers size

[Aleksey Shipilev]

On 10/05/2018 06:20 PM, Roman Kennke wrote:
>> Webrev:
>> http://cr.openjdk.java.net/~rkennke/JDK-8211270/webrev.00/

Do we really need the abstractions for header sizes? I think all usages need the object size, and
header sizes can be just picked up from the usual places. No GC, including Shenandoah is changing
the header size?

Trying to understand the use case, looking at Shenandoah's diff here:

https://builds.shipilev.net/patch-openjdk-shenandoah-jdk/latest/src/hotspot/share/gc/shared/plab.cpp.sdiff.html

...it seems what we really want is to mix in oop_extra_words into plab.cpp accessors. Because doing
ShenandoahHeap::obj_header_size() { return obj_header_size + oop_extra_words; } seems wrong down the
line -- it would _not_ be the header size.

Cheers,
-Aleksey

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