[lilliput-jdk21u:lilliput] RFR: 8316126: [Lilliput/JDK21] Cherry-pick: 8305895: Implementation: JEP 450: Compact Object Headers (Experimental) [v3]

[Roman Kennke]

On Tue, 12 Sep 2023 16:52:46 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

>> Roman Kennke has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   Remove misplaced assert
>
> src/hotspot/share/opto/compile.cpp line 1353:
> 
>> 1351:         // range is OK as-is.
>> 1352:         tj = ta = TypeAryPtr::RANGE;
>> 1353:       } else if( offset == TypeOopPtr::klass_offset_in_bytes() ) {
> 
> Here and everywhere. Not sure why this is here. Why not in `oopDesc::`?

The reason I did it this way is so that I can assert(!UCOH) in oopDesc::klass_offset_in_bytes(). The offset is used in C2 only as a sort of marker (to distinguish memory slice for the klass-word from other memory slices). It is not actually used as offset to load the Klass* - we load the whole mark-word and shift/mask/expand the Klass* from that, including the check for monitor.
In the long run, as soon as the monitor-mapping in the header goes away, we could indeed use the oopDesc::klass_offset_in_bytes() everywhere - unless when we squeeze the narrowKlass in the header at a non-byte boundery, which is bound to happen once we go to 4-byte headers.

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PR Review Comment: https://git.openjdk.org/lilliput-jdk21u/pull/5#discussion_r1324367952