PR 13948 and Miranda Methods

Tue May 4 15:39:00 GMT 2004

Ranjit Mathew wrote:
>>Ranjit> 2. Miranda methods are needed to work with older buggy VMs
>>Ranjit> that need an implicit method declaration in an abstract
>>Ranjit> class for a method from an implemented interface (right?).
>>>>An abstract class isn't required to declare methods inherited from
>>interfaces. However, when we build vtables these methods have to
>>come from somewhere. So, we introduce these fake Miranda methods to
>>make vtable construction work out right. We do this in the compiler
>>when generating .o files and in the runtime for other classes. So,
>>it isn't a compatibility thing -- it is really needed.
>>>>>>I'm not totally sure, but can't this be treated in the way that
>multiple inheritance is in the C++ ABI? (Of which I know zilch,
>so never mind...)
>>
MI is basically implemented in C++ by sticking the vtables of your 
superclasses together and twiddling the vtable pointers - its pretty 
icky and I don't think it would solve any problems - Java interfaces 
don't have vtables. The "miranda methods" problem is just a matter of 
making sure that methods implicitly inherited from interfaces in an 
abstract class get vtable slots. Note that the issue only applies to the 
current ABI - the problem dissapears with the BC-ABI.
>>Ranjit> However, layout_class_methods() calls add_miranda_methods()
>>Ranjit> anyways - so isn't the call on line 2097 made redundant by his
>>Ranjit> patch?
>>>>I don't know, but it would appear so.
>>>>>>Hmmm... Bryce/Andrew - what're your thoughts on this?
>>I removed that line (i.e. 2097 in class.c) and recompiled
>libjava and ran the testsuite (including Jacks), without
>any regressions. Would anyone have any objections to
>removing it?
>>
No objections here, I think you're right - the add_miranda_methods 
recursive call is redundant because layout_class_methods() calls it anyway.
Regards
Bryce