Hi, Please review:
http://cr.openjdk.java.net/~psandoz/jdk9/JDK-8154755-weak-cas-volatile/webrev/ This webrev adds a new VarHandle access mode method, weakCompareAndSetVolatile [*]. (I also fixed some doc errors regarding AccessMode enum names that i forgot to update in a previous issue). Aleksey noticed a discrepancy with the current set of weak CAS methods when comparing with C++ atomics. On hardware that supports LL/SC (e.g. ARM) the weak volatile variant may be more performant than the strong volatile variant (compareAndSet) when the CAS is performed in a loop. See the thread on the jmm-dev list for more details: http://mail.openjdk.java.net/pipermail/jmm-dev/2016-April/000239.html Implementations currently defer to the strong volatile Unsafe CAS. We will follow up with changes to Unsafe, hotspot, and updating the VH implementations as separate issues. Thanks, Paul. [*] /** * Possibly atomically sets the value of a variable to the {@code newValue} * with the memory semantics of {@link #setVolatile} if the variable's * current value, referred to as the <em>witness value</em>, {@code ==} the * {@code expectedValue}, as accessed with the memory semantics of * {@link #getVolatile}. * * <p>This operation may fail spuriously (typically, due to memory * contention) even if the witness value does match the expected value. * ... */ public final native @MethodHandle.PolymorphicSignature @HotSpotIntrinsicCandidate boolean weakCompareAndSetVolatile(Object... args); Paul.
