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

[Albert Mingkun Yang]

On Thu, 22 Aug 2024 18:18:01 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 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.

src/hotspot/share/gc/shared/gcForwarding.hpp line 36:

> 34:  * Implements forwarding for the full-GCs of Serial, Parallel, G1 and Shenandoah in
> 35:  * a way that preserves upper N bits of object mark-words, which contain crucial
> 36:  * Klass* information when running with compact headers. The encoding is similar to

This doc suggests this forwarding is only for compact-header so I wonder if we can check `UseCompactObjectHeaders` directly instead of heap-size in `GCForwarding::initialize`.

src/hotspot/share/gc/shared/gcForwarding.hpp line 40:

> 38:  * heap-base, shifts that difference into the right place, and sets the lowest two
> 39:  * bits (to indicate 'forwarded' state as usual).
> 40:  */

> "can use 40 bits for forwardee encoding. That's enough for 8TB of heap."

I feel this 8T-constraint is significant and should be in the doc.

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PR Review Comment: https://git.openjdk.org/jdk/pull/20677#discussion_r1727708193
PR Review Comment: https://git.openjdk.org/jdk/pull/20677#discussion_r1727727638
PR Review Comment: https://git.openjdk.org/jdk/pull/20677#discussion_r1727732496