On Wed, 21 Aug 2024 13:18:03 GMT, Roman Kennke <rken...@openjdk.org> wrote:
>> This is the main body of the JEP 450: Compact Object Headers (Experimental).
>>
>> Main changes:
>> - Introduction of the (experimental) flag UseCompactObjectHeaders. All
>> changes in this PR are protected by this flag. The purpose of the flag is to
>> provide a fallback, in case that users unexpectedly observe problems with
>> the new implementation. The intention is that this flag will remain
>> experimental and opt-in for at least one release, then make it on-by-default
>> and diagnostic (?), and eventually deprecate and obsolete it. However, there
>> are a few unknowns in that plan, specifically, we may want to further
>> improve compact headers to 4 bytes, we are planning to enhance the Klass*
>> encoding to support virtually unlimited number of Klasses, at which point we
>> could also obsolete UseCompressedClassPointers.
>> - The compressed Klass* can now be stored in the mark-word of objects. In
>> order to be able to do this, we are building on #20603 and #20605 to protect
>> the relevant (upper 32) bits of the mark-word. Significant parts of this PR
>> deal with loading the compressed Klass* from the mark-word. This PR also
>> changes some code paths (mostly in GCs) to be more careful when accessing
>> Klass* (or mark-word or size) to be able to fetch it from the forwardee in
>> case the object is forwarded.
>> - The identity hash-code is temporarily narrowed to 25 bits. As soon as we
>> get Tiny Class-Pointers (planned before the JEP can be integrated, and to be
>> opened for review soon), we will widen the hash-bits back to 31 bits.
>> - Instances can now have their base-offset (the offset where the field
>> layouter starts to place fields) at offset 8 (instead of 12 or 16).
>> - Arrays will can now store their length at offset 8.
>> - CDS can now write and read archives with the compressed header. However,
>> it is not possible to read an archive that has been written with an opposite
>> setting of UseCompactObjectHeaders. Some build machinery is added so that
>> _coh variants of CDS archives are generated, next to the _nocoops variant.
>> - Note that oopDesc::klass_offset_in_bytes() is not used by +UCOH paths
>> anymore. The only exception is C2, which uses it as a placeholder/identifier
>> of the special memory slice that only LoadNKlass uses. The backend then
>> extracts the original oop and loads its mark-word and extracts the
>> narrow-Klass* from that. I played with other approaches to implement
>> LoadNKlass. Expanding it as a macro did not easily work, because C2 is
>> missing a way to cast a word-sized integral t...
>
> src/hotspot/cpu/aarch64/c2_MacroAssembler_aarch64.cpp line 2575:
>
>> 2573: } else {
>> 2574: lea(dst, Address(obj, index, Address::lsl(scale)));
>> 2575: ldr(dst, Address(dst, offset));
>
> It ignores the offset, right? Or are you saying that offset must be 0 on that
> path?
Sorry, brain fart.
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PR Review Comment: https://git.openjdk.org/jdk/pull/20640#discussion_r1725176978
