[master] RFR: Eliminate Klass* word [v2]

[Aleksey Shipilev]

On Mon, 21 Feb 2022 16:34:07 GMT, Roman Kennke <rkennke at openjdk.org> wrote:

>> This change removes the dedicated Klass* word in the oop structure, at least in 64 bit builds. It has been usused, except for verification, since integration of #29.
>> 
>> The change makes regular objects smaller, the field layouter can now use the extra space and start laying out fields at offset 8 (instead of 12 or 16, depending on class-compression). However, arrays remain unchanged because they still store the array length in offset 8, and current array code expects the array elements to start at 64bit aligned address, even for smaller data types. I will address this in a subsequent change.
>> 
>> Much of the change is reverting the klass_offset -> nklass_offset change by #29, as promised. I also disabled s390x and ppc64le GHA builds, because they are broken by this change.
>> 
>> I problem-listed a number of SA tests. The SA is broken by this change because it needs to load the Klass* from a regular field. I tried to change it, but it looks very difficult: it doesn't have a way to safely access the object mark word in the face of concurrent activity by the locking subsystem. Fixing the SA is not very high priority at this point, it may ultimately not even be necessary when/if the locking subsystem doesn't use the mark word anymore (or at least, does no longer displace the header).
>> 
>> Testing:
>>  - [x] tier1 (x86_64, x86_32, aarch64)
>>  - [x] tier2 (x86_64, x86_32, aarch64)
>>  - [ ] tier3 (x86_64, x86_32, aarch64)
>
> Roman Kennke has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Fix test that failed because of smaller object size

Looks fine to me for the experimental code.

src/hotspot/share/gc/parallel/psOldGen.cpp line 390:

> 388: 
> 389:   virtual void do_object(oop obj) {
> 390:     HeapWord* test_addr = cast_from_oop<HeapWord*>(obj);

A bit confused by this change, why?

test/langtools/ProblemList.txt line 76:

> 74: 
> 75: 
> 76: jdk/jshell/ToolTabSnippetTest.java 1234567 generic-all

This still fails?

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Marked as reviewed by shade (Committer).

PR: https://git.openjdk.java.net/lilliput/pull/40