From: tenderlove@...
Date: 2015-12-03T21:43:24+00:00
Subject: [ruby-core:71818] [Ruby trunk - Feature #11769] [Open] optimize	case / when for `nil`

Issue #11769 has been reported by Aaron Patterson.

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Feature #11769: optimize case / when for `nil`
https://bugs.ruby-lang.org/issues/11769

* Author: Aaron Patterson
* Status: Open
* Priority: Normal
* Assignee: 
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Hi,

I've noticed that when there are certain values in a case / when statement it gets optimized to a hash lookup.  For example, code like this:

~~~
def foo socket
  case str = socket.read_nonblock(10, exception: false)
  when :wait_readable
    # do something
  else
    str
  end
end

puts RubyVM::InstructionSequence.of(method(:foo)).disasm
~~~

The above code will use `opt_case_dispatch` instruction with a hash.  However, if I use `nil` in the case statement like this:

~~~
def foo socket
  case str = socket.read_nonblock(10, exception: false)
  when :wait_readable
    # do something
  when nil
    # got an EOF
  else
    str
  end
end

puts RubyVM::InstructionSequence.of(method(:foo)).disasm
~~~

Then the optimization is lost.

I've attached a patch that adds `nil` to the optimized case such that the above code will use `opt_case_dispatch`.  My patch defines `===` on `nil`, then adds nil to the list of "optimizable literals".

---Files--------------------------------
0001-optimize-case-when-for-nil.patch (3.4 KB)


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