From: "Marcin Mielżyński" <lopx@...>
Date: 2008-04-02T00:50:09+09:00
Subject: Re: Why Ruby do not optimize code at all?

ts wrote:
> Marcin Mielżyński wrote:
>> Not true. The instruction itself is responsible for the fallback (just 
>> not implemented yet).
> 
> vgs% ./ruby -ve 'case "a"; when :a; p :ok; end'
> ruby 1.9.0 (2008-04-01 revision 15881) [i686-linux]
> vgs%

Actually, same happens for 1.8.6 (more Symbol#=== issue), but I'm aware 
of your point.

> 
>  It's a bug :  it's useless to add an optimisation which *break*
>  some code and in this case the fallback is *precisely* to call
>  Symbol#===, i.e. to bypass the optimisation.

Well, that's why I'd rather call it temporary breakage since the sources 
say the author's been aware of it from very beginning (chance the TODO: 
comment). Just, opt_case_dispatch impl is not complete _yet_, still 
being faster by default by not using funcall dispatch.

Actually a bug (but different) I'd call a compiler issue generating 
opt_case_dispatch even with specialized_instruction option turned off, so:

opts = VM::InstructionSequence.compile_option
opts[:specialized_instruction] = false
VM::InstructionSequence.compile_option = opts

s = VM::InstructionSequence.new %{
   class Symbol
       def === other
           puts "foo"
       end
   end
   case :s
       when :s
   end
}

s.eval


s.disam:

0000 putnil                                                           (   2)
0001 putnil
0002 defineclass      :Symbol, <class:Symbol>, 0
0006 pop
0007 putobject        :s                                              (   7)
0009 dup
0010 opt_case_dispatch <cdhash>, 25
0013 putobject        :s                                              (   8)
0015 topn             1
0017 send             :===, 1, nil, 0, <ic>
0023 branchif         29
0025 pop                                                              (   7)
0026 putnil
0027 leave
0028 pop
0029 pop                                                              (   9)
0030 putnil
0031 leave                                                            (   8)
== disasm: <ISeq:<class:Symbol>@<compiled>>=============================
0000 putnil                                                           (   3)
0001 definemethod     :===, ===, 0
0005 putnil
0006 leave
== disasm: <ISeq:===@<compiled>>========================================
local table (size: 2, argc: 1 [opts: 0, rest: -1, post: 0, block: -1] s1)
[ 2] other<Arg>
0000 putnil                                                           (   4)
0001 putstring        "foo"
0003 send             :puts, 1, nil, 8, <ic>
0009 leave


lopex