RFR (S) 8155965: Unsafe.weakCompareAndSetVolatile entry points and intrinsics

[Aleksey Shipilev]

Hi,

Please review this addition to Unsafe and C2 intrinsics, that adds
Unsafe.weakCompareAndSetVolatile family of entry points. This provides
the VM support for additional VarHandles access modes (see JDK-8154755).

Bug:
 https://bugs.openjdk.java.net/browse/JDK-8155965

Webrevs:
 http://cr.openjdk.java.net/~shade/8155965/webrev.jdk.00/
 http://cr.openjdk.java.net/~shade/8155965/webrev.hs.00/

The bulk of changes is in tests. The actual change in compiler code is
just a few lines declaring a new weakCAS shape.

Testing: Linux x86_64 tests: hotspot/compiler/unsafe; RBT (hs-tier0-comp).

Thanks,
-Aleksey

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