RFR: 8256516: Simplify clearing References

[Kim Barrett]

On Wed, 18 Nov 2020 10:07:25 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

>> Please review this simplification of jlr.Reference clearing by VM code.
>> 
>> The function java_lang_ref_Reference::set_referent_raw was being used to
>> clear the referent of Reference objects, and only for that purpose.  This
>> change replaces that function with java_lang_ref_Reference::clear_referent,
>> which is much more obvious in intent.  That change is then percolated up
>> through callers in the obvious way.
>> 
>> Testing:
>> mach5 tier1
>
> src/hotspot/share/gc/shared/referenceProcessor.hpp line 123:
> 
>> 121: 
>> 122:   // Apply the keep_alive function to the referent address.
>> 123:   void make_referent_alive();
> 
> I wonder if moving this from the `.hpp` to `.cpp` has performance implications for callers. Maybe move to `.inline.hpp`?

The reason for moving it out of the .hpp was of course that the
change to call java_lang_ref_Reference::referent_addr_raw needs to #include
javaClasses.inline.hpp.

I don't see this move having any measureable performance difference, and not
even sure what the sign of any change might be.  A better refactoring might
be to package up the common remove/make_referent_alive/move_to_next sequence.

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PR: https://git.openjdk.java.net/jdk/pull/1286