On Wed, 25 Sep 2024 12:53:17 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: > > Allow LM_MONITOR on 32-bit platforms src/hotspot/cpu/aarch64/c2_MacroAssembler_aarch64.cpp line 2570: > 2568: // we get the heapBase in obj, and the > narrowOop+klass_offset_in_bytes/sizeof(narrowOop) in index. > 2569: // When that happens, we need to lea the address into a single > register, and subtract the > 2570: // klass_offset_in_bytes, to get the address of the mark-word. Parts of this comment are obsolete after commit 2c4a7877, please update the comment. src/hotspot/cpu/aarch64/macroAssembler_aarch64.hpp line 882: > 880: void store_klass(Register dst, Register src); > 881: void cmp_klass(Register oop, Register trial_klass, Register tmp); > 882: void cmp_klass(Register src, Register dst, Register tmp1, Register > tmp2); Same suggestion as for the analogous x86 functions: consider renaming the four-argument `cmp_klass` function to `cmp_klasses_from_objects` or similar, and the `src` and `dst` parameters to `oop1` and `oop2` or similar if there is no notion of "source" and "destination". ------------- PR Review Comment: https://git.openjdk.org/jdk/pull/20677#discussion_r1776927247 PR Review Comment: https://git.openjdk.org/jdk/pull/20677#discussion_r1776942226
