Questions about CompareAndSwap

[Aleksey Shipilev]

Hi Roman,

On 11/20/2015 06:31 PM, Roman Kennke wrote:
> 2. Would it be possible to also get the actual old-value out of
> CompareAndSwapP, in addition to the flags? My idea is that
> CompareAndSwapP would be a MultiNode producing two outputs. Would that
> be possible? Would there be any disadvantages (i.e. loss of performance
> due to messed up optimizations, or such?)

Actually, VarHandles add new compareAndExchange methods, that are
intrinsified with the new C1/C2 code. Those methods return old value on
success, and expected value on failure (actually, they leave accumulator
untouched on failure) -- pretty much what you expect from x86-style lock
cmpxchg.

We already did prototypal x86_*.ad changes for them. See:
 https://bugs.openjdk.java.net/browse/JDK-8136761

We are planning to sweep the implementation, and hit the review soon.

Thanks,
-Aleksey

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