invokeinterface

invokeinterface bytecode is used to call an interface method. The mechanism used to implement this is slightly different from invokevirtual. We cannot assign a unique virtual_index to an interface method as a class may implement two different interfaces creating a conflict in virtual_index. So virtual_index of an interface method varies from its implementation in one class to another (i.e virtual_index depends on method and class). So how can we find the virtual_index of a method given "this"? This information is stored in a hash table called itable. Each bucket in an itable is a list of itable_entry structures. Each itable_entry has a c_method structure which points to the vm_method structure of the implementation method (the method to be called). So an invokeinterface call boils down to:-

• Searching this->itable[method->itable_index] for the required method. The itable_index for a method is the same across classes. The result of this search is c_method->virtual_index.
  
• Call this->vtable[c_method->virtual_index].

The LIR generated for invokeinterface is produced by following code.

/* object class */
select_insn(s, tree, membase_reg_insn(INSN_MOV_MEMBASE_REG,
call_target, offsetof(struct vm_object, class), call_target));

/* itable entry */
select_insn(s, tree, imm_reg_insn(INSN_ADD_IMM_REG,
offsetof(struct vm_class, itable) + method->itable_index * sizeof(void *),
call_target));

/* hidden parameter to the conflict resolution stub */
select_insn(s, tree, imm_reg_insn(INSN_MOV_IMM_REG,
(unsigned long) method, eax));

/* invoke method */
call_insn = reverse_reg_insn(INSN_CALL_REG, call_target);

This call instruction transfers control to itable conflict resolver generated by emit_itable_resolver_stub(). This function emits code to do a binary search on the appropriate list and jump to the correct vtable entry (See emit_itable_bsearch()).