Feature #6602: Tail call optimization: enable by default?

• Status: Feedback
• Assignee: ko1 (Koichi Sasada)
• Target version: -

[ko1 (Koichi Sasada)]

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

[nobu (Nobuyoshi Nakada)]

• Description updated (diff)

[trans (Thomas Sawyer)]

I'd rather have the backtrace information and depend on Moore's law for "enhancements".

However, in the the case of tail-recursion, it could be ok since the stack entry is essentially a facsimile of the previous.

[ko1 (Koichi Sasada)]

(2012/06/18 20:29), trans (Thomas Sawyer) wrote:

However, in the the case of tail-recursion, it could be ok since the stack entry is essentially a facsimile of the previous.

Do you mean the following code isn't applied tail call optimization?

   # double recursion (foo -> bar -> foo -> bar -> ...)
   def foo
      bar()
   end
 
   def bar()
      foo()
   end

I agree it is one option for this problem.

--
// SASADA Koichi at atdot dot net

[mame (Yusuke Endoh)]

• Status changed from Open to Assigned

[headius (Charles Nutter)]

FWIW, JRuby will not be able to support TCO until the JVM supports TCO, so it won't work across implementations. I don't say that to hold back progress...just stating facts.

[alexeymuranov (Alexey Muranov)]

+1 for goto foo()

Tail call optimization looks to me like a tamed form of goto.

[ko1 (Koichi Sasada)]

• Target version changed from 2.0.0 to 2.6

Alexey: to introduce new syntax (or method), we need more discussion (mainly on name ;)).

I changed target to next minor.

[mame (Yusuke Endoh)]

• Assignee changed from mame (Yusuke Endoh) to ko1 (Koichi Sasada)

[mame (Yusuke Endoh)]

• Related to Feature #12543: explicit tail call syntax: foo() then return added

[ko1 (Koichi Sasada)]

• Description updated (diff)
• Status changed from Assigned to Feedback

I hope someone propose smart answers.

[Eregon (Benoit Daloze)]

I think losing the backtrace is fairly bad in a mostly-imperative language.
Tail calls would allow tail-recursion, but this is not very frequent in Ruby.

In my opinion, recursion becomes really interesting when you can invoke it from different points,
like walking over a tree, but this is not tail-recursive anymore.

[mame (Yusuke Endoh)]

Benoit Daloze wrote:

I think losing the backtrace is fairly bad in a mostly-imperative language.
Tail calls would allow tail-recursion, but this is not very frequent in Ruby.

Agreed. There are few cases in Ruby where tail-call optimization is really useful. I still believe that any explicit syntax to mark such a case (#12543) is a good way.

[naruse (Yui NARUSE)]

• Target version deleted (2.6)

[dsisnero (Dominic Sisneros)]

I think it is not used because it is not optimized. If it was optimized, people would use it.