[JDK8] RFR (XS): JSR292: IncompatibleClassChangeError in LambdaForm for CharSequence.toString() method handle type converter

[Vladimir Ivanov]

Chris,

Thanks for looking into this. See my answers inline.

Best regards,
Vladimir Ivanov

On 1/15/14 9:51 PM, Christian Thalinger wrote:
> [I’m resending something I sent earlier today to Vladimir directly.]
>
> On Jan 15, 2014, at 7:31 AM, Vladimir Ivanov <vladimir.x.ivanov at oracle.com> wrote:
>
>> http://cr.openjdk.java.net/~vlivanov/8031502/webrev.00/
>> https://bugs.openjdk.java.net/browse/JDK-8031502
>>
>> InvokeBytecodeGenerator can produce incorrect bytecode for a LambdaForm
>> when invoking a method from Object declared in an interface.
>>
>> The problem is the following:
>>    (1) java.lang.CharSequence interface declares abstract method "String
>> toString()";
>>
>>    (2) after 8014013 fix, VM resolves
>> CharSequence::toString()/invokeInterface to
>> CharSequence::toString()/invokeVirtual;
>
> Without having looked at the changes of 8014013, why did the invoke type change?  Is it an optimization that was added?
>

After 8014013, LinkResolver::resolve_interface_call returns virtual 
(_call_kind = CallInfo::vtable_call), instead of interface method and 
MethodHandles::init_method_MemberName uses it as is (without any fine 
tuning, which was done before).

It's caused by the following:
   - LinkResolver::linktime_resolve_interface_method returns 
CharSequence::toString method, but it has vtable index, instead of 
itable index;

   - LinkResolver::runtime_resolve_interface_method looks at resolved 
method and sets _call_kind to vtable_call, since resolved method doesn't 
have itable index.

   - then MethodHandles::init_method_MemberName looks at CallInfo passed 
in and sets the reference kind flag to JVM_REF_invokeVirtual.

That's how the conversion from invokeInterface to invokeVirtual occurs.

I did a quick debugging session with pre-8014013 hotspot to check how it 
worked before, but don't remember all the details now.

>>
>>    (3) during LambdaForm compilation, CharSequence is considered
>> statically invocable (see
>> InvokeBytecodeGenerator::isStaticallyInvocable) and invokevirtual for
>> CharSequence::toString() is issued, which is wrong (invokevirtual throws
>> ICCE if it references an interface);
>>
>> The fix is straightforward: during LambdaForm compilation, switch back
>> from invokevirtual to invokeinterface instruction when invoking a method
>> on an interface.
>
> I find this risky.  Right now MemberName is only used in a couple places in java.lang.invoke but in the future it might be used for other things (e.g. java.lang.reflect).  The information MemberName contains should be correct and usable without changing.  Otherwise all users have to patch the information the same way as you do in your patch.
>
> I would like to see the VM passing correct information (whatever the definition of correct is here).
>

It's an interesting question what kind of correctness is required for 
MemberName and I don't know the answer. Looking through the code, I got 
an impression MemberName isn't designed to provide information to be 
used "as is" for bytecode generation, because, at least:
   - MemberName::referenceKindIsConsistent already expects 
(clazz.isInterface() && refKind == REF_invokeVirtual && 
isObjectPublicMethod()) case;

   - InvokeBytecodeGenerator::emitStaticInvoke already has a special 
case for REF_invokeSpecial referenceKind, converting it to 
REF_invokeVirtual;

Best regards,
Vladimir Ivanov