RFR: 8188055: (ref) Add Reference.refersTo predicate

Kim Barrett kim.barrett at oracle.com
Wed Apr 8 00:25:44 UTC 2020


[Note review on both core-libs and hotspot-gc-dev lists; try not to lose
either when replying.]

Please review a new function: java.lang.ref.Reference.refersTo.

This function is needed to test the referent of a Reference object
without artificially extending the lifetime of the referent object, as
may happen when calling Reference.get.  Some garbage collectors
require extending the lifetime of a weak referent when accessed, in
order to maintain collector invariants.  Lifetime extension may occur
with any collector when the Reference is a SoftReference, as calling
get indicates recent access.  This new function also allows testing
the referent of a PhantomReference, which can't be accessed by calling
get.

The new function uses a native method whose implementation is in the
VM so it can use the Access API.  It is the intent that this function
will be intrinsified by optimizing compilers like C2 or graal, but
that hasn't been implemented yet.  Bear that in mind before rushing
off to change existing uses of Reference.get.

CR:
https://bugs.openjdk.java.net/browse/JDK-8188055
https://bugs.openjdk.java.net/browse/JDK-8241029 (CSR)

Webrev:
https://cr.openjdk.java.net/~kbarrett/8188055/open.04/

Testing:
mach5 tier1

Locally (linux-x64) verified the new test passes with various garbage
collectors. 



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