RFR: 8305895: Implement JEP 450: Compact Object Headers (Experimental) [v8]

[Stefan Karlsson]

On Mon, 9 Sep 2024 12:21:19 GMT, Thomas Schatzl <tschatzl at openjdk.org> wrote:

>> Roman Kennke has updated the pull request incrementally with two additional commits since the last revision:
>> 
>>  - Try to avoid lea in loadNklass (aarch64)
>>  - Fix release build error
>
> src/hotspot/share/oops/oop.hpp line 134:
> 
>> 132:   inline Klass*   forward_safe_klass(markWord m) const;
>> 133:   inline size_t   forward_safe_size();
>> 134:   inline void     forward_safe_init_mark();
> 
> Given the comment these methods do not seem "safe" to me. Maybe use "raw" or something to better indicate that care must be taken to use them.
> 
> Maybe the "safe" refers to use them only in "safe" contexts, but in Hotspot code iirc we use something like "raw" or "unsafe".

Restating my earlier comment about this: These functions are mainly used by the GCs. In one of the patches I've cleaned away all usages except for those in Shenandoah. I would prefer to see these completely removed from the oops/ directory and let the GCs decide when and how to perform "safe" reads of these values.

> src/hotspot/share/oops/oop.hpp line 356:
> 
>> 354:       return mark_offset_in_bytes() + sizeof(markWord) / 2;
>> 355:     } else
>> 356: #endif
> 
> Maybe instead of trying to calculate some random, meaningless value just use some "random" value directly?
> I am fine with the existing code, but first stating directly that "any value" works here, this additional code seems to confuse the message. (Fwiw, the method is also used during Universe initialization).

Just to be clear, the second part of the quoted sentence is important:
> could be any value *that is not a valid field offset*

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PR Review Comment: https://git.openjdk.org/jdk/pull/20677#discussion_r1750428581
PR Review Comment: https://git.openjdk.org/jdk/pull/20677#discussion_r1750432186