JEP 193: Enhanced Volatiles
fweimer at redhat.com
Thu Mar 6 15:25:16 UTC 2014
On 03/05/2014 01:05 AM, Doug Lea wrote:
> On 03/04/2014 05:12 PM, Florian Weimer wrote:
>> On 03/04/2014 01:05 PM, Doug Lea wrote:
>>> On 03/04/2014 02:41 AM, Jeroen Frijters wrote:
>>>> I understand pass-by-reference is an expensive feature, but IMNSHO
>>>> Java with this proposal will prove to be more expensive in the long
>>>> run. It's
>>>> like erased generics all over again.
>>> The expensive version of pass-by-reference is already supported
>>> using java.lang.reflect.Field.
>> And per the statistics posted in
>> the slightly faster pointer-to-field-member support is one of the
>> prevalent use
>> cases for sun.misc.Unsafe. That's why I share Jeroen's puzzlement.
> Sorry, I'm not sure what usages you have in mind, or
> what constructions and implementable JVM mechanics
> could be used to deal with them?
If we had lightweight pointer-to-fields, things like compare-and-swap
could be implemented as regular intrinsics. It would require less magic
syntax, and make it clear that the operations only work with non-static
fields and not on local variables etc. It would be possible to write
functions that apply a complex sequence of updates to a field, which is
impossible under the current proposal. (One aspect I like about Java is
that is fairly uniform, e.g., that you can take a subexpression and turn
it into a function, or that you can store intermediate results,
something that is often impossible in Ada or C++).
Pointer-to-fields could be as lightweight as a single integer (they are
in C++ and with sun.misc.Unsafe), assuming that the VM enforces type
safety. Whether it is a good idea to add another generic type at the VM
level before the arrival of reified generics, is a different matter.
Florian Weimer / Red Hat Product Security Team
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