Hi,

Thank you!

There are no person who read YARV in detail :)  I'm very glad to
describe YARV internal.


Charles Oliver Nutter wrote:
> 1. What do getspecial/setspecial manipulate in the VM? I understand the
> other get/set instructions.

to access $_, $~, and other variables.  In current Ruby prepare
variables for $_ and $~ each stack frame.  But most of method invocation
 doesn't need it.  So $_ or $~ variable are created on demand.

> 2. I see that getdynamic/setdynamic take idx and level. I assume this
> means that in YARV block variable scoping is determined statically. We
> have planned to make this change in JRuby as well. Am I correct about
> how this works?

Yes.

> 3. If I am correct in #2, Is the level zero-based? Is the first
> (topmost) block index zero, and the first nested block inside it index 1?

Yes.

> 4. What is the declp operand to setclassvariable?

Ah, it is obsolete variable.  rb_cvar_set C API need it, but not affect
any behavior.


> 5. I see putnil and putself. Why no puttrue, putfalse, as in the 1.8 AST?

"puttrue" => "putobject true"
"putfalse" => "putobject false"

putself can not be represented by "putobject ??".
And putnil can be represented by "putobject nil", but many times this
patterns are occurred (in compiler).  So I prepare it.

Motivation of this decision is to small VM as possible as I can.  And
puttrue and putfalse can be made by operand unification optimization.


> 6. putundef takes no operands and returns a val; what does it do, and
> what is it returning?

putundef return Qundef.  This instruction is made for block inlining,
but I think it is bad idea.  So I will remove it.


> 7. putobject pushes an object on the stack, correct? Is pushstring just
> doing the same thing with a string value? What's the difference between
> putobject with a string val and putstring with a string val?

putstring insn duplicate value.

Try it:

  # putstring
  loop{
    obj = ""
    break if obj.respond_to? :foo
    def obj.foo; end
    p :continue
  }
  p :exit

----

  # putobject //
  loop{
    obj = //
    break if obj.respond_to? :foo
    def obj.foo; end
    p :continue
  }
  p :exit


> 8. I assume concatstrings concatenates the topmost "num" strings from
> the stack and pushes the result back. Is the topmost string the base
> (leftmost) string for concatenation? I.e. does the first string popped
> sit at the front of the resulting string?

Try it:

  a = b = c = 'xyz'
  puts "#{a}#{b}#{c}", a


> 9. Shouldn't duparray take an array from the top of the stack and push
> it and a copy back down? Online it looks like it takes an array operand,
> which seems odd.

Ah, I see.  This instruction means "putarray" (like "putstring").  This
instruction works like putstring.


> 10. What is the difference between expandarray and splatarray? Is
> expandarray equivalent to multi-assignment in 1.8? Does num specify the
> number of values to multi-assign? What is flag for?

This is because ruby's strange massign behavior :)

And I can't describe now enough ^^;  Too complicated.  If you can
eliminate something around this, please teach me.


> 11. I assume concatarray and checkincludearray are for internal use and
> not used directly as the implementation of Array#include? and Array#<<
> or concat, correct?

Almost same.  But Array#include? can be replaced.


> 12. newhash pulls keys/values off the stack. What is the sequence of
> keys and values?

You can check with YARV easily:

 1. write some program to "yarvsrc/test.rb"
 2. do "make parse" on build directory
 3. you can see disassembled result


For example, "p({:a=>:b, :c=>:d})" program is disassembled as followed.

== disasm: <ISeq:<main> c:/ko1/src/rubyext/trunk/test.rb>===============
local scope table (size: 1, argc: 0)

0000 putself                                                        (1)
0001 putobject        :a
0003 putobject        :b
0005 putobject        :c
0007 putobject        :d
0009 newhash          4
0011 send             :p, 1, nil, 4, <ic>
0017 leave


> 13. When the "stack" column on the YARV instruction table lists multiple
> elements, is the top of the stack on the left or right?

I can't understand what is your question, but you can check translated C
code in builddirectory/vm.inc.  I think that makes you clear than my
poor English.


> 14. Is there a specification for the format of a compiled YARV script? I
> would like to start generating some test scripts and building a machine
> to execute them.

What format?  YARVCore::InstructionSequence.compile or compile_file
methods help you?

-- 
// SASADA Koichi at atdot dot net

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