[lilliput-jdk17u:lilliput] RFR: 8310944: [Lilliput/JDK17] Streamline and cleanup GC code [v2]

[Aleksey Shipilev]

On Thu, 29 Jun 2023 11:02:31 GMT, Roman Kennke <rkennke at openjdk.org> wrote:

>> We have a bunch of ugly hacks in GC code paths where we do stuff like:
>> if (UseCompactObjectHeaders) {
>>  // fetch object Klass* or size in a safe way...
>> } else {
>>   // fetch object Klass* or size in the traditional way
>> }
>> 
>> This happens in various places in similar ways. I propose to add relevant methods to oopDesc, and make all GC code use those new methods instead.
>> 
>> This helps streamline and clean up a bunch of GC code, and bring it closer to the JDK22 upstreaming PR.
>> 
>> Testing:
>>  - [x] hotspot_gc
>>  - [x] hotspot_gc (+UCOH)
>>  - [x] tier1
>>  - [x] tier1 (+ShenandoahGC)
>>  - [x] tier1 (+ParallelGC)
>>  - [x] tier1 (+SerialGC)
>>  - [x] tier1 (+UCOH)
>>  - [x] tier1 (+UCOH +ShenandoahGC)
>>  - [x] tier1 (+UCOH +ParallelGC)
>>  - [x] tier1 (+UCOH +SerialGC)
>>  - [ ] tier2
>>  - [ ] tier2 (+UCOH)
>
> Roman Kennke has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Remove unused method args

I have questions :)

src/hotspot/share/oops/oop.inline.hpp line 97:

> 95: markWord oopDesc::prototype_mark() const {
> 96:   if (UseCompactObjectHeaders) {
> 97:     return klass()->prototype_header();

This is called from `MarkSweep::mark_object`. Shouldn't it then use `forward_safe_klass`?

src/hotspot/share/oops/oop.inline.hpp line 303:

> 301:     set_mark(forward_safe_klass()->prototype_header());
> 302:   } else {
> 303:     set_mark(markWord::prototype());

This thing is used from `PreservedMarks::init_forwarded_mark`, which in current 17u upstream calls `oopDesc::init_mark`, which then calls: `  set_mark(markWord::prototype_for_klass(klass()));`.

So, does this miss the +BiasedLocking mark prototype?

-------------

PR Review: https://git.openjdk.org/lilliput-jdk17u/pull/50#pullrequestreview-1505343296
PR Review Comment: https://git.openjdk.org/lilliput-jdk17u/pull/50#discussion_r1246657851
PR Review Comment: https://git.openjdk.org/lilliput-jdk17u/pull/50#discussion_r1246650743