On Thu, 22 Aug 2024 16:23:48 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:
>
> Remove hashcode leftovers from SA
src/hotspot/share/gc/parallel/mutableSpace.cpp line 232:
> 230: p += obj->forwardee()->size();
> 231: } else {
> 232: p += obj->size();
I feel it's more correct to go through the forwardee for forwarded objs even
for the non-COMPACT_HEADERS case. (This method is meant to cover all objs, so
should not be perf-critical.) IOW, the `false` case should just be dropped.
src/hotspot/share/gc/serial/defNewGeneration.cpp line 707:
> 705: } else if (obj->is_forwarded()) {
> 706: // To restore the klass-bits in the header.
> 707: obj->forward_safe_init_mark();
I wonder if not modifying successful-forwarded objs is cleaner. Sth like:
reset_self_forwarded_in_space(space) {
cur = space->bottom();
top = space->top();
while (cur < top) {
obj = cast_to_oop(cur);
if (obj->is_self_forwarded()) {
obj->unset_self_forwarded();
obj_size = obj->size();
} else {
assert(obj->is_forwarded(), "inv");
obj_size = obj->forwardee()->size();
}
cur += obj_size;
}
}
reset_self_forwarded_in_space(eden());
reset_self_forwarded_in_space(from());
src/hotspot/share/gc/serial/serialArguments.cpp line 33:
> 31: void SerialArguments::initialize_heap_flags_and_sizes() {
> 32: GenArguments::initialize_heap_flags_and_sizes();
> 33: GCForwarding::initialize_flags(MaxNewSize + MaxOldSize);
Can one use `MaxHeapSize` here?
-------------
PR Review Comment: https://git.openjdk.org/jdk/pull/20677#discussion_r1727547638
PR Review Comment: https://git.openjdk.org/jdk/pull/20677#discussion_r1727524479
PR Review Comment: https://git.openjdk.org/jdk/pull/20677#discussion_r1727548413
