Review Request: JDK-8185148: [MVT] Class.java's isValueClass causes all non-MVT enabled JVM executions to have an InstanceKlass created for __Value reference

[Maurizio Cimadamore]

On 28/07/17 15:26, Mandy Chung wrote:
> One thing to mention:
>
> LambdaType.BasicType Q_TYPE:
>     Q_TYPE('Q', MinimalValueTypes_1_0.getValueClass(), Wrapper.OBJECT),
>
> If value type is not enabled, Q_TYPE would have a null basic type Class.
> I expect Q_TYPE should not be used in that case.  Do you see any issue?
I think that should be OK. I already too care of making sure that 
there's no eager initialization of Q-based lambda forms at startup, as 
this was creating issues in case value type support is disabled. So, 
yes, only way to have Q-typed lambda forms is to have MVT enabled (or at 
least that's how it should be). Of course, I can always make a mess, and 
try to do something like this:

MethodHandles.constant(__Value.class)

And that will likely blow up - but I think the core issue here is that 
__Value should NOT be part of java.base (or at least should not be part 
of an exported package), or there will always be escape hatches to get 
into troubles again.

Maurizio
> Mandy
>
>> On Jul 28, 2017, at 5:28 AM, Maurizio Cimadamore <maurizio.cimadamore at oracle.com> wrote:
>>
>> Looks great - thanks!
>>
>> Maurizio
>>
>>
>> On 28/07/17 05:56, Mandy Chung wrote:
>>> A class referencing java.lang.__Value would fail verification when value type is not enabled.  Class::isValueType and MinimalValueTypes_1_0::getValueType both references java.lang.__Value but they may be loaded when value type is not enabled.  VerifyError that Tobias has ran into [1] is related to this issue.
>>>
>>> A simple fix is to refactor the reference to java.lang.__Value in a separate class that will only be loaded when -XX:+EnableMVT and —XX:+EnableValhalla is set.  We may look into a cleaner fix in the future.
>>>
>>> Webrev:
>>>    http://cr.openjdk.java.net/~mchung/valhalla/webrevs/8185148/webrev.00/index.html
>>>
>>> Mandy
>>> [1] http://mail.openjdk.java.net/pipermail/valhalla-dev/2017-July/003007.html