On Thu, 22 Aug 2024 18:18:01 GMT, Roman Kennke <[email protected]> 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.
-------------
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
