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

[Karen Kinnear]

Maurizio,

Can you clarify for me where we ended up?
__Value is not checked at the java level if neither EnableMVT or EnableValhalla flag -> system property is set.

Does __Value contain ;Q syntax?
Do bytecodes that refer to it need ;Q syntax? Or did we end up with a workaround here?

I just want to be sure I understand what is generated when we do enable either valhalla version for other folks
generating bytecodes.

thanks,
Karen

> On Jul 28, 2017, at 10:45 AM, Maurizio Cimadamore <maurizio.cimadamore at oracle.com> wrote:
> 
> 
> 
> 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
>