Can @Stable (or something similar) be made accessible?

Vitaly Davidovich vitalyd at
Fri Jan 12 13:41:06 UTC 2018

On Fri, Jan 12, 2018 at 7:50 AM, Vladimir Ivanov <
vladimir.x.ivanov at> wrote:

> Would you be so kind to explain how this breakage occurs? I can
>>> understand that improper use of @Stable annotation may break the
>>> intended semantics of a program, but to break the integrity of VM? I'm
>>> trying to imagine the scenario, but can't.
>> One example might be a @Stable array field used with a constant index to
>> write a value into it.  If the JIT trusts the field to be final it could
>> elide a range check when storing into the slot.  If the field is actually
>> modified to be a smaller length array, you’d end up with a write to an out
>> of bounds memory area.
>> I suppose something similar can be done with a non-array field - make the
>> field type Object, store a Foo initially.  The JIT can assume the type is
>> always Foo and generate read/writes using its layout.  Then change the
>> field to another type.
> No, it doesn't work that way. @Stable enables JIT to constant fold loads
> from fields/arrays (holding non-default values) whenever possible. After
> that JIT has a constant value at its hands. All other optimizations follows
> from that: range check before array store can be elided since array
> instance is known and its length is known as well, type check can be elided
> since constant type is known. But the value won't be reloaded from memory,
> all optimizations happen on a constant which was loaded from memory once
> during compilation.
> So, the worst case scenario is: a value written into @Stable field/array
> is never visible in some code no matter what you do. It can lead to nasty
> bugs when different parts of program don't agree on observed value. It can
> happen when user doesn't obey @Stable contract and performs multiple writes
> into a @Stable field/array. Current implementation in HotSpot doesn't
> forbid that.
Thanks for clarifying Vladimir - that makes sense.  So back to the
integrity of the VM question - what can be compromised then? I suppose if
one was to use Unsafe access in combination with relying on a @Stable field
to feed it info (e.g. index into some memory accessed via Unsafe), then
things can go south.  But Unsafe is already, well, unsafe.  What extra VM
integrity issues would a misbehaving @Stable cause that Andrew was alluding

> Best regards,
> Vladimir Ivanov

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