RFR: 8305895: Implement JEP 450: Compact Object Headers (Experimental) [v17]

[Axel Boldt-Christmas]

On Mon, 16 Sep 2024 13:28:00 GMT, Roman Kennke <rkennke at openjdk.org> wrote:

>> This is the main body of the JEP 450: Compact Object Headers (Experimental).
>> 
>> It is also a follow-up to #20640, which now also includes (and supersedes) #20603 and #20605, plus the Tiny Class-Pointers parts that have been previously missing.
>> 
>> 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 add some changes to GC forwarding (see below) to protect the relevant (upper 22) 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.
>>  - Self-forwarding in GCs (which is used to deal with promotion failure) now uses a bit to indicate 'self-forwarding'. This is needed to preserve the crucial Klass* bits in the header. This also allows to get rid of preserved-header machinery in SerialGC and G1 (Parallel GC abuses preserved-marks to also find all other relevant oops).
>>  - Full GC forwarding now uses an encoding similar to compressed-oops. We have 40 bits for that, and can encode up to 8TB of heap. When exceeding 8TB, we turn off UseCompressedClassPointers (except in ZGC, which doesn't use the GC forwarding at all).
>>  - 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 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 _co...
>
> Roman Kennke has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 54 commits:
> 
>  - Merge remote-tracking branch 'origin/master' into JDK-8305895-v4
>  - Fix  test/hotspot/jtreg/runtime/CompressedOops/CompressedClassPointersEncodingScheme.java
>  - Fix loop on aarch64
>  - clarify obscure assert in metasapce setup
>  - Rework compressedklass encoding
>  - remove stray debug output
>  - Fixes post 8338526
>  - Merge commit '597788850041e7272a23714c05ba546ee6080856' into JDK-8305895-v4
>  - Various touch-ups
>  - Hide log timestamps in test to prevent false failures
>  - ... and 44 more: https://git.openjdk.org/jdk/compare/54595188...2125cd81

src/hotspot/cpu/aarch64/aarch64.ad line 6459:

> 6457:   format %{ "ldrw  $dst, $mem\t# compressed class ptr" %}
> 6458:   ins_encode %{
> 6459:     __ load_nklass_compact_c2($dst$$Register, $mem$$base$$Register, $mem$$index$$Register, $mem$$scale, $mem$$disp);

I wonder if something along the line of this is required here.
Suggestion:

    Address addr = mem2address($mem->opcode(), $mem$$base$$Register, $mem$$index, $mem$$scale, $mem$$disp);
    __ load_nklass_compact_c2($dst$$Register, __ adjust_compact_object_header_address_c2(addr, rscratch1));

With `adjust_compact_object_header_address_c2` being:
```C++
Address C2_MacroAssembler::adjust_compact_object_header_address_c2(Address addr, Register tmp) {
  // The incoming address is pointing into obj-start + klass_offset_in_bytes. We need to extract
  // obj-start, so that we can load from the object's mark-word instead. Usually the address
  // comes as obj-start in addr.base() and klass_offset_in_bytes in addr.offset().
  if (addr.getMode() != Address::base_plus_offset) {
    lea(tmp, addr);
    addr = Address(tmp, -oopDesc::klass_offset_in_bytes());
  } else {
    addr = Address(addr.base(), addr.offset() - oopDesc::klass_offset_in_bytes());
  }
  return legitimize_address(addr, 8, tmp);
}


Maybe it is the case that we never get the case where `$mem->opcode()` is not `lsl` variant, nor that the offset is to far away for an immediate fixed by `legitimize_address`. But it seems like this would at least make those cases correct, while avoiding the `lea` in the common case. 

Maybe someone with better experience in aarch64 macroassembler+ad files and C2 can give an opinion.

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PR Review Comment: https://git.openjdk.org/jdk/pull/20677#discussion_r1761455581