> 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.
Kim Barrett has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains 11 additional commits since the last revision: - Merge branch 'master' into refersto - More explicit refersTo0 comment. - simplify test - cleanup nits from Mandy - use Object instead of TestObject - improve refersTo0 descriptions - basic functional test - change referent access - expand test - remove CMS comment - ... and 1 more: https://git.openjdk.java.net/jdk/compare/1c92ac39...ab4e519b ------------- Changes: - all: https://git.openjdk.java.net/jdk/pull/498/files - new: https://git.openjdk.java.net/jdk/pull/498/files/c1062ec6..ab4e519b Webrevs: - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=498&range=04 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=498&range=03-04 Stats: 331969 lines in 1120 files changed: 314757 ins; 10957 del; 6255 mod Patch: https://git.openjdk.java.net/jdk/pull/498.diff Fetch: git fetch https://git.openjdk.java.net/jdk pull/498/head:pull/498 PR: https://git.openjdk.java.net/jdk/pull/498
