RFR: 8305895: Implement JEP 450: Compact Object Headers (Experimental) [v5]

[Stefan Karlsson]

On Thu, 22 Aug 2024 19:36:00 GMT, Albert Mingkun Yang <ayang at openjdk.org> wrote:

>> Roman Kennke has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   Fix hash shift for 32 bit builds
>
> src/hotspot/share/gc/shared/gcForwarding.cpp line 37:
> 
>> 35:   size_t max_narrow_heap_size = right_n_bits(NumLowBitsNarrow - Shift);
>> 36:   if (UseCompactObjectHeaders && max_heap_size > max_narrow_heap_size * HeapWordSize) {
>> 37:     FLAG_SET_DEFAULT(UseCompactObjectHeaders, false);
> 
> Maybe a log-info/warning would be nice.

Yes. This silent setting of UseCompactObjectHeaders ended up hiding why we got CDS failures. I would also suggest that we change this to FLAG_SET_ERGO.

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PR Review Comment: https://git.openjdk.org/jdk/pull/20677#discussion_r1738104783