On Wed, 11 Sep 2024 17:38:57 GMT, Roman Kennke <rken...@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: > > Revert accidental change of UCOH default I was starting to understand the concerns with having prototype_header in Klass. It seems like it would simplify encoding the klass for object allocation. My recent change https://bugs.openjdk.org/browse/JDK-8338526 breaks this. You need to pass a parameter to Klass() to tell whether to encode the klass pointer or not, and pass this to Klass() constructor. diff --git a/src/hotspot/share/oops/instanceKlass.cpp b/src/hotspot/share/oops/instanceKlass.cpp index fd198f54fc9..7aa4bd24948 100644 --- a/src/hotspot/share/oops/instanceKlass.cpp +++ b/src/hotspot/share/oops/instanceKlass.cpp @@ -511,7 +511,7 @@ InstanceKlass::InstanceKlass() { } InstanceKlass::InstanceKlass(const ClassFileParser& parser, KlassKind kind, ReferenceType reference_type) : - Klass(kind), + Klass(kind, (!parser.is_interface() && !parser.is_abstract())), _nest_members(nullptr), _nest_host(nullptr), _permitted_subclasses(nullptr), ------------- PR Comment: https://git.openjdk.org/jdk/pull/20677#issuecomment-2344715540
