On Wed, 3 Dec 2025 13:23:18 GMT, Jorn Vernee <[email protected]> wrote:
>>> any captured adaptation is valid/compatible
>>
>> Yes, if `vh` is a constant, any captured adaptation from
>> `vh.getMethodHandle(mode).asType(symbolicMethodTypeInvoker)` is
>> valid/compatible.
>>
>> For thread safety, MethodHandle supports safe publication, so I think we are
>> fine publishing this way.
>
> Looking at this, I'm not sure we can assume that we only see one mode and
> type when the VH is constant. There seems to be a lot of non-local reasoning
> involved.
>
> For example, you could have a var handle invoker created with
> `MethodHandless::varHandleInvoker`, which is cached, so the
> `AccessDescriptor` can be shared among many different use sites. For an
> individual use-site, the receiver VH may well be a constant, but that doesn't
> mean that the cache isn't polluted by the var handle from another use site,
> as far as I can tell.
>
> The thread safety issue comes from a C2 thread racing to read the
> `lastAdaption` cache vs another Java thread writing to the cache. AFAICS,
> this race is still possible even when `vh` is a compile time constant.
I think even without using an invoker, you could end up in a similar situation
if you have something like:
static Object m(VarHandle vh) {
return vh.get();
}
Which is called by several different threads. At some point this method may be
inlined into one of its callees, where `vh` then becomes a constant. But at the
same time, other threads are still writing to the cache.
-------------
PR Review Comment: https://git.openjdk.org/jdk/pull/28585#discussion_r2585142665
