RFR: 8305895: Implement JEP 450: Compact Object Headers (Experimental) [v8]

[Coleen Phillimore]

On Mon, 9 Sep 2024 11:55:52 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 two additional commits since the last revision:
> 
>  - Try to avoid lea in loadNklass (aarch64)
>  - Fix release build error

src/hotspot/cpu/aarch64/compressedKlass_aarch64.cpp line 147:

> 145: #endif
> 146: 
> 147:   return true;

This should only be in the compressedKlass.cpp file.

src/hotspot/share/oops/compressedKlass.cpp line 214:

> 212:     ss.print("Class space size (%zu) exceeds the maximum possible size (%zu)",
> 213:               len, max_encoding_range_size());
> 214:     vm_exit_during_initialization(ss.base());

Why does this exit and not turn off compressed klass pointers and compact object headers?

src/hotspot/share/oops/compressedKlass.cpp line 222:

> 220:     return;
> 221:   }
> 222: #endif

Why not add null pd_initialize to zero to remove this conditional code?

src/hotspot/share/oops/compressedKlass.cpp line 224:

> 222: #endif
> 223: 
> 224:   if (tiny_classpointer_mode()) {

I kind of agree with Thomas Schatzl for this.  Maybe it should be compact_classpointer_mode(). It's nice to have a new string for grep, but they're not really that tiny.

src/hotspot/share/oops/compressedKlass.cpp line 234:

> 232:     _range = len;
> 233: 
> 234:     constexpr int log_cacheline = 6;

Is 6 the log of DEFAULT_CACHE_LINE_SIZE?

src/hotspot/share/oops/compressedKlass.cpp line 243:

> 241:   } else {
> 242: 
> 243:     // In legacy mode, we try, in order of preference:

Can you not use the word 'legacy' here?  Maybe in "non-compact object header mode"...

src/hotspot/share/oops/compressedKlass.inline.hpp line 100:

> 98:   check_valid_klass(k, base(), shift());
> 99:   // Also assert that k falls into what we know is the valid Klass range. This is usually smaller
> 100:   // than the encoding range (e.g. encoding range covers 4G, but we only have 1G class space and a

1G is the default CompressedClassSpaceSize but can be larger, right?  So the comment isn't quite accurate.  Or with tiny class pointers can it only be 1G?

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PR Review Comment: https://git.openjdk.org/jdk/pull/20677#discussion_r1750527537
PR Review Comment: https://git.openjdk.org/jdk/pull/20677#discussion_r1750511912
PR Review Comment: https://git.openjdk.org/jdk/pull/20677#discussion_r1750513660
PR Review Comment: https://git.openjdk.org/jdk/pull/20677#discussion_r1750515923
PR Review Comment: https://git.openjdk.org/jdk/pull/20677#discussion_r1750520712
PR Review Comment: https://git.openjdk.org/jdk/pull/20677#discussion_r1750524690
PR Review Comment: https://git.openjdk.org/jdk/pull/20677#discussion_r1750662637