[8u76] Request for approval: 8131782: C1 Class.cast optimization breaks when Class is loaded from static final

Aleksey Shipilev aleksey.shipilev at oracle.com
Thu Dec 3 17:33:54 UTC 2015

Thank you, Rob!

Any Hotspot+8u sponsors around?


On 12/03/2015 08:22 PM, Rob McKenna wrote:
> Approved. Please work with your colleagues on the hotspot team to find a
> sponsor & please add an appropriate noreg label to the bug.
>     -Rob
> On 03/12/15 17:18, Aleksey Shipilev wrote:
>> Hi,
>> We'd like to backport a simple Hotspot compiler change that serves as
>> the important prerequisite for another upcoming JDK backport for
>> Atomic*FieldUpdaters (A*FU) performance.
>> Bug:
>>    https://bugs.openjdk.java.net/browse/JDK-8131782
>> The dependency is simple: C1 intrinsics for Class methods distrust the
>> classes loaded from final fields due to an oddity in constant parsing.
>> When A*FU implementations do the typechecks/casts against the target
>> classes loaded from their internal final fields, those operations are
>> not folded, but really should be. It is not visible now, since current
>> 8u code does the direct class check, but it will be important when we
>> backport that other A*FU change
>> (https://bugs.openjdk.java.net/browse/JDK-8140587).
>> As other A*FU-related patches, thus provides a migration path with the
>> eventual demise of sun.misc.Unsafe. Therefore, I'd like us to consider
>> this in 8u76 to expose users to this improvement earlier than JDK 9
>> release.
>> I am not a 8u Committer, so if the change is approved, I'd need a
>> sponsor.
>> Webrev:
>>    http://cr.openjdk.java.net/~shade/8131782/webrev.02/
>> JDK 9 Changeset:
>>    http://hg.openjdk.java.net/jdk9/jdk9/hotspot/rev/df802f98b828
>> JDK 9 Review:
>> http://mail.openjdk.java.net/pipermail/hotspot-compiler-dev/2015-July/018473.html
>> The change was in jdk9 since August 2015. The patch applies without
>> fuzz. jdk8u builds and runs fine at least on Linux x86_64, the
>> performance improvement is the same as in JDK 9.
>> Thanks,
>> -Aleksey

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