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

[Thomas Stuefe]

On Mon, 9 Sep 2024 15:50:50 GMT, Coleen Phillimore <coleenp 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/compressedKlass.cpp line 222:
> 
>> 220:     return;
>> 221:   }
>> 222: #endif
> 
> Why not add null pd_initialize to zero to remove this conditional code?

I can do that. Added to backlist (https://wiki.openjdk.org/display/lilliput/JEP-450+Review+Todo)

> src/hotspot/share/oops/compressedKlass.cpp line 224:
> 
>> 222: #endif
>> 223: 
>> 224:   if (tiny_classpointer_mode()) {
> 
> I kind of agree with Thomas Schatzl for this.  Maybe it should be compact_classpointer_mode(). It's nice to have a new string for grep, but they're not really that tiny.

Yes, makes sense. Added to backlist. 

This coding was developed somewhat independently from +COH at the beginning, but now the two parts (tinycp and the rest of COH) depend on each other anyway. I should just use UseCompactObjectHeaders or a flag directly derived from it.

> src/hotspot/share/oops/compressedKlass.cpp line 234:
> 
>> 232:     _range = len;
>> 233: 
>> 234:     constexpr int log_cacheline = 6;
> 
> Is 6 the log of DEFAULT_CACHE_LINE_SIZE?

64, yes

> src/hotspot/share/oops/compressedKlass.cpp line 243:
> 
>> 241:   } else {
>> 242: 
>> 243:     // In legacy mode, we try, in order of preference:
> 
> Can you not use the word 'legacy' here?  Maybe in "non-compact object header mode"...

okay.

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PR Review Comment: https://git.openjdk.org/jdk/pull/20677#discussion_r1751828214
PR Review Comment: https://git.openjdk.org/jdk/pull/20677#discussion_r1751831035
PR Review Comment: https://git.openjdk.org/jdk/pull/20677#discussion_r1751831994
PR Review Comment: https://git.openjdk.org/jdk/pull/20677#discussion_r1751833034