RFR: 8256516: Simplify clearing References

[Aleksey Shipilev]

On Wed, 18 Nov 2020 09:53:11 GMT, Kim Barrett <kbarrett 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

Looks good, with minor nit.

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`?

src/hotspot/share/gc/shenandoah/shenandoahReferenceProcessor.cpp line 319:

> 317:   } else {
> 318:     // Clear referent
> 319:     reference_clear_referent(reference);

Now I am looking at this code and wonder if we could just inline `reference_clear_referent` and `reference_set_next` both in Shenandoah and ZGC code. Probably something for a followup.

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Marked as reviewed by shade (Reviewer).

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