[11u] AArch64: Support for LSE atomics C++ HotSpot code

[Hohensee, Paul]

Looks fine, except that I see you've included the patch for https://bugs.openjdk.java.net/browse/JDK-8261660 as part of the webrev for 8261027. If at all possible, please backport 8261660 separately and push the two patches (for 8261027 and 8261660) together.

Thanks,
Paul

-----Original Message-----
From: jdk-updates-dev <jdk-updates-dev-retn at openjdk.java.net> on behalf of Andrew Haley <aph at redhat.com>
Date: Thursday, March 18, 2021 at 9:11 AM
To: "jdk-updates-dev at openjdk.java.net" <jdk-updates-dev at openjdk.java.net>
Subject: [11u] AArch64: Support for LSE atomics C++ HotSpot code

This is the first of two patches which allow C++ code in HotSpot
to use Arm's Large System Extensions.

This is necessary because some recent AArch64 hardware performs
very badly, particularly under heavy contention, with the older
way of doing atomic updates, using LDXR and STXR. Java code has
been using these LSE instructions, where available, for several
years.

This first patch is mostly just scaffolding: the actual performance
improvement is https://bugs.openjdk.java.net/browse/JDK-8261649.

The original patch did not apply because the argument order of
class Atomic functions changed with 8234740. Otherwise, it was
fairly straightforward.

http://cr.openjdk.java.net/~aph/8261027-jdk11u/

--
Andrew Haley  (he/him)
Java Platform Lead Engineer
Red Hat UK Ltd. <https://www.redhat.com>
https://keybase.io/andrewhaley
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