How does the putspecialobject opcode work in RubyVM?

From insns.def:

DEFINE_INSN putspecialobject (rb_num_t value_type) () (VALUE val)
{
  switch (value_type) {
    case VM_SPECIAL_OBJECT_VMCORE:
      val = rb_mRubyVMFrozenCore;
      break;
    case VM_SPECIAL_OBJECT_CBASE:
      val = vm_get_cbase (GET_ISEQ (), GET_LFP (), GET_DFP ());
      break;
    default:
      rb_bug ("putspecialobject insn: unknown value_type");
  }
}

Where:

VM_SPECIAL_OBJECT_VMCORE = 1
VM_SPECIAL_OBJECT_CBASE = 2

In other words, if the argument is 1, then it is pushed on the stack rb_mRubyVMFrozenCore

, which is an object created at the start of VM startup that implements some language features as methods:

set_method_alias (klass,: new,: old) alias new old
set_variable_alias (: $ new,: $ old) alias $ new $ old
undef_method (klass,: foo) undef foo
define_method (klass,: foo, & block) def foo ... end
define_singleton_method (obj,: foo, & block) def obj.foo ... end
set_postexe (& block) END {...}

If the argument is 2, the class / module for the current class context is pushed onto the stack. That the lexical scope in which constants and def

and are allowed alias

takes effect. It is also a class passed as "class" for the methods above.

So,

def foo
  ...
end

compiles:

[: putspecialobject, 1],
[: putspecialobject, 2],
[: putobject,: foo],
[: putiseq, [...]],
[: send,: "core # define_method", 3, nil, 0, nil]

This is as much as I could redesign, which is not easy with this code.