RFR: 8305895: Implementation: JEP 450: Compact Object Headers (Experimental)

[Coleen Phillimore]

On Fri, 12 May 2023 17:27:25 GMT, Roman Kennke <rkennke at openjdk.org> wrote:

> This is the main body of the JEP 450: Compact Object Headers (Experimental).
> 
> Main changes:
>  - Introduction of the (experimental) flag UseCompactObjectHeaders. All changes in this PR are protected by this flag.
>  - The compressed Klass* can now be stored in the mark-word of objects. In order to be able to do this, we are building on #10907, #13582 and #13779 to protect the relevant (upper 32) bits of the mark-word. Significant parts of this PR deal with loading the compressed Klass* from the mark-word, and dealing with (monitor-)locked objects. When the object is monitor-locked, we load the displaced mark-word from the monitor, and load the compressed Klass* from there. 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, and/or reach through to the monitor when the object is locked by a monitor.
>  - The identity hash-code is narrowed to 25 bits.
>  - 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 can now store their length at offset 8. Due to alignment restrictions, array elements will still start at offset 16. #11044 will resolve that restriction and allow array elements to start at offset 12 (except for long, double and uncompressed oops, which are still required to start at an element-aligned offset).
>  - 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.
> 
> Testing:
>  (+UseCompactObjectHeaders tests are run with the flag hard-patched into the build, to also catch @flagless tests, and to avoid mismatches with CDS - see above.)
>  - [x] tier1 (x86_64)
>  - [x] tier2 (x86_64)
>  - [x] tier3 (x86_64)
>  - [ ] tier4 (x86_64)
>  - [x] tier1 (aarch64)
>  - [x] tier2 (aarch64)
>  - [x] tier3 (aarch64)
>  - [ ] tier4 (aarch64)
>  - [ ] tier1 (x86_64) +UseCompactObjectHeaders
>  - [ ] tier2 (x86_64) +UseCompactObjectHeaders
>  - [ ] tier3 (x86_64) +UseCompactObjectHeaders
>  - [ ] tier4 (x86_64) +UseCompactObjectHeaders
>  - [ ] tier1 (aarch64) +UseCompactObjectHeaders
>  - [ ] tier2 (aarch64) +UseCompactObjectHeaders
>  - [ ] tier3 (aarch64) +UseCompactObjectHeaders
>  - [ ] tier4 (aarch64) +UseCompactObjectHeaders

These changes are an improvement.

src/hotspot/cpu/x86/c1_MacroAssembler_x86.cpp line 193:

> 191:       movl(Address(obj, arrayOopDesc::length_offset_in_bytes() + sizeof(jint)), t1);
> 192:     }
> 193: #endif

This endif should go after UseCompressedClassPointers conditional, and consolidate to one set of #ifdef _LP64.

src/hotspot/cpu/x86/macroAssembler_x86.cpp line 5126:

> 5124:   assert(UseCompactObjectHeaders, "expect compact object headers");
> 5125: 
> 5126:   if (!UseCompactObjectHeaders) {

Now this isn't needed, right?

src/hotspot/share/cds/archiveBuilder.cpp line 726:

> 724:       k->set_prototype_header(markWord::prototype().set_narrow_klass(nk));
> 725:     }
> 726: #endif //_LP64

If CDS is turned off for UseCompactObjectHeaders, I don't understand this change or the one to archiveHeapWriter.  -Xshare:dump objects would be the wrong size.  If CDS is not supported, then there should be something in arguments.cpp that gives an error for that.  And write a test for that error of mixing and matching.

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PR Review: https://git.openjdk.org/jdk/pull/13961#pullrequestreview-1424925882
PR Review Comment: https://git.openjdk.org/jdk/pull/13961#discussion_r1192648245
PR Review Comment: https://git.openjdk.org/jdk/pull/13961#discussion_r1192646637
PR Review Comment: https://git.openjdk.org/jdk/pull/13961#discussion_r1192661859