On Sun, 4 Oct 2020 03:59:59 GMT, Kim Barrett <kbarr...@openjdk.org> wrote:
> Finally returning to this review that was started in April 2020. I've > recast it as a github PR. I think the security concern raised by Gil > has been adequately answered. > https://mail.openjdk.java.net/pipermail/hotspot-gc-dev/2020-April/029203.html > https://mail.openjdk.java.net/pipermail/hotspot-gc-dev/2020-July/030401.html > https://mail.openjdk.java.net/pipermail/hotspot-gc-dev/2020-August/030677.html > https://mail.openjdk.java.net/pipermail/hotspot-gc-dev/2020-September/030793.html > > 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 native methods whose implementations are in the VM so > they can use the Access API. It is the intent that these methods 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. > > There are two native methods involved, one in Reference and an override in > PhantomReference, both package private in java.lang.ref. The reason for this > split is to simplify the intrinsification. This is a change from the version > from April 2020; that version had a single native method in Reference, > implemented using the ON_UNKNOWN_OOP_REF Access reference strength category. > However, adding support for that category in the compilers adds significant > implementation effort and complexity. Splitting avoids that complexity. > > Testing: > mach5 tier1 > Locally (linux-x64) verified the new test passes with various garbage > collectors. This pull request has now been integrated. Changeset: 6023f6b1 Author: Kim Barrett <kbarr...@openjdk.org> URL: https://git.openjdk.java.net/jdk/commit/6023f6b1 Stats: 501 lines in 13 files changed: 488 ins; 0 del; 13 mod 8188055: (ref) Add Reference::refersTo predicate Reviewed-by: mchung, pliden, rriggs, dholmes, ihse, smarks, alanb ------------- PR: https://git.openjdk.java.net/jdk/pull/498
