RFR: 8305895: Implement JEP 450: Compact Object Headers (Experimental) [v6]

[Thomas Schatzl]

On Thu, 22 Aug 2024 20:08:43 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 incrementally with one additional commit since the last revision:
> 
>   Fix bit counts in GCForwarding

src/hotspot/share/gc/shared/collectedHeap.cpp line 232:

> 230:   }
> 231: 
> 232:   // With compact headers, we can't safely access the class, due

Suggestion:

  // With compact headers, we can't safely access the klass, due


This is the case why? Because we might not have copied the header yet? Is this method actually ever used while the forwarded object is unstable?
Given this is used for verification only afaik, we should make an effort to provide that check.

src/hotspot/share/gc/shared/gcForwarding.hpp line 34:

> 32: 
> 33: /*
> 34:  * Implements forwarding for the full-GCs of Serial, Parallel, G1 and Shenandoah in

Suggestion:

 * Implements forwarding for the Full GCs of Serial, Parallel, G1 and Shenandoah in

src/hotspot/share/gc/shared/gcForwarding.hpp line 41:

> 39:  * bits (to indicate 'forwarded' state as usual).
> 40:  */
> 41: class GCForwarding : public AllStatic {

Since this class is only used for Full GCs, it may be useful to include that information, i.e. something like `FullGCForwarding` to avoid confusion why it is not used for other GCs too.
(Unless this has been discussed and even rejected by me before).

src/hotspot/share/oops/compressedKlass.hpp line 43:

> 41: 
> 42:   // Tiny-class-pointer mode
> 43:   static int _tiny_cp; // -1, 0=true, 1=false

Suggestion:

  static int _tiny_cp; // -1 = uninitialized, 0 = true, 1 = false

In addition to that, I am not sure if introducing a new term ("tiny") for compact class header related changes (and just here) makes the code more clear; I would have expected a "_compact_" prefix. Also all other members use "k"-klass and spell out "klass pointer", so I would prefer to keep that style.

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PR Review Comment: https://git.openjdk.org/jdk/pull/20677#discussion_r1749995275
PR Review Comment: https://git.openjdk.org/jdk/pull/20677#discussion_r1749980748
PR Review Comment: https://git.openjdk.org/jdk/pull/20677#discussion_r1749987945
PR Review Comment: https://git.openjdk.org/jdk/pull/20677#discussion_r1749969456