RFR: 8254598: StringDedupTable should use OopStorage

[Kim Barrett]

On Sun, 25 Apr 2021 17:32:51 GMT, Zhengyu Gu <zgu at openjdk.org> wrote:

>> Please review this change to the String Deduplication facility.  It is being
>> changed to use OopStorage to hold weak references to relevant objects,
>> rather than bespoke data structures requiring dedicated processing phases by
>> supporting GCs.
>> 
>> (The Shenandoah update was contributed by Zhengyu Gu.)
>> 
>> This change significantly simplifies the interface between a given GC and
>> the String Deduplication facility, which should make it much easier for
>> other GCs to opt in.  However, this change does not alter the set of GCs
>> providing support; currently only G1 and Shenandoah support string
>> deduplication.  Adding support by other GCs will be in followup RFEs.
>> 
>> Reviewing via the diffs might not be all that useful for some parts, as
>> several files have been essentially completely replaced, and a number of
>> files have been added or eliminated.  The full webrev might be a better
>> place to look.
>> 
>> The major changes are in gc/shared/stringdedup/* and in the supporting
>> collectors, but there are also some smaller changes in other places, most
>> notably classfile/{stringTable,javaClasses}.
>> 
>> This change is additionally labeled for review by core-libs, although there
>> are no source changes there.  This change injects a byte field of bits into
>> java.lang.String, using one of the previously unused padding bytes in that
>> class.  (There were two unused bytes, now only one.)
>> 
>> Testing:
>> mach5 tier1-2 with and without -XX:+UseStringDeduplication
>> 
>> Locally (linux-x64) ran all of the existing tests that use string
>> deduplication with both G1 and Shenandoah.  Note that
>> TestStringDeduplicationInterned.java is disabled for shenandoah, as it
>> currently fails, for reasons I haven't figured out but suspect are test
>> related.
>> 
>> - gc/stringdedup/   -- these used to be in gc/g1
>> - runtime/cds/SharedStringsDedup.java
>> - runtime/cds/appcds/cacheObject/DifferentHeapSizes.java
>> - runtime/cds/appcds/sharedStrings/*
>> 
>> shenandoah-only:
>> - gc/shenandoah/jvmti/TestHeapDump.java
>> - gc/shenandoah/TestStringDedup.java
>> - gc/shenandoah/TestStringDedupStress.java
>> 
>> Performance tested baseline, baseline + stringdedup, new with stringdedup,
>> finding no significant differences.
>
> src/hotspot/share/classfile/stringTable.cpp line 355:
> 
>> 353:   // Notify deduplication support that the string is being interned.  A string
>> 354:   // must never be deduplicated after it has been interned.  Doing so interferes
>> 355:   // with compiler optimizations don on e.g. interned string literals.
> 
> typo: don -> done

Fixed locally.

-------------

PR: https://git.openjdk.java.net/jdk/pull/3662