RFR: 8305895: Implement JEP 450: Compact Object Headers (Experimental) [v9]

[Coleen Phillimore]

On Mon, 9 Sep 2024 20:04:22 GMT, Stefan Karlsson <stefank at openjdk.org> wrote:

>> src/hotspot/share/oops/typeArrayKlass.cpp line 175:
>> 
>>> 173: size_t TypeArrayKlass::oop_size(oop obj) const {
>>> 174:   // In this assert, we cannot safely access the Klass* with compact headers.
>>> 175:   assert(UseCompactObjectHeaders || obj->is_typeArray(),"must be a type array");
>> 
>> Why not?  I think I'm missing something.  Klass should be in the markWord and that should be ok (?)
>
> I tracked this down to only (at least in my testing) happen from `size_given_klass` when called from the GC when it is about to copy an object. While that happens another thread can racingly succeed to copy the object and install a forwarding pointer over the old copy. When that happens the klass pointer is broken and the call to oopDesc::is_typeArray() crashes.

I did miss something.  I thought the markWord was never overwritten by the forwarding pointer.

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PR Review Comment: https://git.openjdk.org/jdk/pull/20677#discussion_r1750882259