Issue #6602 has been reported by ko1 (Koichi Sasada).

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Feature #6602: Tail call optimization: enable by default?
https://bugs.ruby-lang.org/issues/6602

Author: ko1 (Koichi Sasada)
Status: Open
Priority: Normal
Assignee: mame (Yusuke Endoh)
Category: core
Target version: 2.0.0


Hi,

Some hours ago, Matz proposed turning on "tail call optimization" by default from Ruby 2.0.

What do you think about it?


= Background

Tail call: Method invocation at last of method body.

Tail call optimization: Eliminating the new stack frame creation when method invocation is "tail call".


For exmaple, the method bar() is located at last of method foo(), so bar() is "tail call".

  def foo()
    ...
    bar()
  end

In this case, after invocation of method bar(), foo()'s method frame information (which contains local variables, program counter, stack pointer and so on) is no longer needed because method foo() doesn't work after that (correctly, method foo() only does "return").

Next example, a simple recursion code by foo().  Of course, foo() is "tail call".

  def foo()
    ...
    foo()
  end

Current Ruby causes stack overflow error because such recursion consumes the (VM) stack.  However, using tail call optimization, VM doesn't consume stack frame any more.

Such recursion can be converted to simple loop:

  def foo
    while true
      foo()
    end
  end

Someone calls tail-call opt as "tail recursion optimization" because recursion is famous use-case (*1).

*1: Generally, tail-recursion optimization includes another optimization technique - "call" to "jump" translation.  In my opinion, it is difficult to apply this optimization because recognizing "recursion" is difficult in Ruby's world.

Next example.  fact() method invocation in "else" clause is *not* a "tail call".

  def fact(n)
    if n < 2
      1
    else
      n * fact(n-1)
    end
  end

If you want to use tail-call optimization on fact() method, you need to change fact() method as follows (continuation passing style).

  def fact(n, r)
    if n < 2
      r
    else
      fact(n-1, n*r)
    end
  end

In this case, fact() is tail-call (and a bit difficult to read/write).

Of course, the following code is easy to understand and short.

  (1..n).inject(:*)

Last examples.  Recognizing tail-call is a bit difficult.  

  def foo
    begin
      bar2()  # not a tail-call
    rescue
      bar3()  # not a tail-call
    rescue
      bar4()  # not a tail-call
    ensure
      bar5()  # tail-call!
    end
  end

  def foo
    while true
      return bar("break") # tail-call? (current CRuby can't handle "break" in eval().
    end
  end

CRuby 1.9 has a code tail-call optimization (not tested yet.  maybe there are several bugs).  However, it is off by default because of several problems described in next section.


= Problems:

* (1) backtrace: Eliminating method frame means eliminating backtrace.
* (2) set_trace_func(): It is difficult to probe "return" event for tail-call methods.
* (3) semantics: It is difficult to define tail-call in document (half is joking,  but half is serious)

References:
* [ruby-core:20273]
* [ruby-core:20307]
* [ruby-core:22736]
* [ruby-core:22790] 

Maybe (1) has big impact for ordinal users.

For example:

  def foo
    bar()
  end
  
  def bar
    baz()
  end
  
  def baz
    raise("somethig error")
  end

In this case, backtrace information only include "baz", because bar() in foo and baz() in bar are "tail-call".  Users can't see eliminated frame information in backtrace.

This is why we don't introduce them by default to Ruby 1.9.


= Discussion

Many people ask us that "why don't you introduce tail-call optimization?  it is very easy technique."  I wrote reasons above.

Matz said "it seems small impact enough.  Go ahead". (I doubt it ;P )

Yusuke Endo proposed that introducing special form (for example, send_tail(:foo, ...)) to declare tail call.  Users only use this special form when the backtrace information can be eliminated (*2).

(*2) Special form "goto foo()" is nice joking feature :)  I like it but I believe Matz will reject it.

Akira Tanaka introduced that special backtrace notation like:

  baz
  ... (eliminated by tail call optimization)
  main

to represent eliminating method invocation information.  We can know they were eliminated (good) but we can't know what method frames were eliminated (bad).


= Conclusion

Matz wanted to introduce it.  However it has several problems.  Should we turn on this optimization by default?

Sorry for long (and poor English) article.  Comments and proposals are welcome (with short English, long Ruby codes ;p).


Thanks,
Koichi



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