...
Each klass
structure already includes (allocated inline) a 2-dimensional ragged table of its statically defined interfaces. The spine of the table has pairs, <oop iklass, offset_t itable>
. The spine is terminated by a tuple <NULL, 0>.. The itable offsets are relative to the enclosing klass
(the receiver type).
Each itable is an array of the form {{ Wiki Markup methodOop
target
\[N
\]
}}, where {{N
}} is the number of methods in the interface.
Interface dispatch (for polymorphic call sites) searches the spine of the table, and dips into the matching itable, picking out the target method corresponding to the index (in 0..N-1) of the abstract method in the interface.
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Since the offset entries on the spine of the table have two words, one word of the terminating entry can have a sentinel value, and the other is free to point to a linked list of extension records. So we repurpose the second word head of a linked list of extension records: <NULL, extension>.
There is one extension record for one interface injection event. They are linked through "next" fields (in an arbitrary order), and the incoming interface {{ Wiki Markup klass
}} is matched against their "key" fields. A matching extension record has N additional words, all methodOops; those N words are exactly like a statically defined embedded itable. The extension records are actually just system arrays (type {{Object
\[
\]
}}) whose lengths are {{2+N
}}, with key at element 0, "next" link at element 1, and the implementation methods (if any) at elements 2 through {{2+N-1
}}.
In order to pack these dynamic itables, MethodHandles must be lowered to methodOops. For the special case of direct method handles, the original methodOop can be reused. For other (adapted or bound) method handles, a new methodOop must be created to wrap the method handle. This is a dark and dirty secret that nobody but the JVM will know about. (See the auto-generation of invoke methods in methodOop.cpp of meth.patch.)
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This needs a linked list search and a whole new negative logic side. As with invokeinterface, if the initial searches fail, there needs to be a backoff and upcall to the JVM, to possibly inject the interface. Since negative interface checks are too common to handle this way (via an upcall) some negative filtering needs to be put in. We could do it in a couple of ways:
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- Interface klasses which are not injectable (the vast majority) should have bits in their header which identifies them as such, so that instanceof can return false more quickly. A good way to do this may be to add a second {{
Klass::secondary_super_cache
}} just for injectable interfaces; then the secondary_super_offset for an interface will take one of two distinct values (instead of the single value it takes today), depending on which secondary_super_cache it uses. This simultaneously makes it easy to detect injectables (by {{secondary_super_offset
==
offsetof(&secondary_super_cache
\[1
\]))
}} and also allocates a word in every {{klass
}} to optimize the lookup of injected interfaces. - (Can delay this for the POC.) Introduce a negative super type cache:
Klass::secondary_non_super_cache
. Use it as a first resort, to avoid upcalls on negative type tests of injectables.
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