VarHandles on non-int-sized fields and atomic operations

[Aleksey Shipilev]

On 05/23/2016 11:16 PM, Martin Buchholz wrote:
> With VarHandles, the declared fields are Ordinary Java Fields, so they
> are likely to be layed out as with normal fields.
> As a result, it looks like you can't do a cas with a VarHandle to a
> short field.
> If that's true, then the hardware is intruding into the API.
> What happens when a platform with only a 64-bit CAS comes along?

Then you will get an exception trying to do the CAS.

> Since atomic fields need different field layout from regular fields,
> it seems to make sense to require that fields which will be accessed
> via a VarHandle are clearly marked as being "atomic" in some way.

Mark that as "volatile int" :) Because if marking boolean field with
some "atomic" quantifier would blow up its storage to at least int,
that's what you get in the end anyhow.

> With the Unsafe API, we sort-of got this by requiring that fields be
> volatile (although some users surely cheated here), and because there
> was no Unsafe.compareAndSwapShort (but there is with VarHandles!)

Notice there is still no magic: we still can do CASes only on ints,
longs, references. (This is still a common denominator among all
supported platforms).

> My original motivation was to be able to replace an AtomicBoolean with
> a VarHandle to a boolean field.

No can do. AtomicBoolean, alas, should still keep "volatile int".

See the relevant discussion about this here:
 http://mail.openjdk.java.net/pipermail/jmm-dev/2015-August/000193.html

Thanks,
-Aleksey