Feature #6602

Tail call optimization: enable by default?

Added by Koichi Sasada almost 2 years ago. Updated over 1 year ago.

[ruby-core:45694]
Status:Assigned
Priority:Normal
Assignee:Koichi Sasada
Category:core
Target version:next minor

Description

=begin
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) settracefunc(): 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:
*
*
*
*

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
=end

History

#1 Updated by Nobuyoshi Nakada almost 2 years ago

  • Description updated (diff)

#2 Updated by Thomas Sawyer almost 2 years ago

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.

#3 Updated by Koichi Sasada almost 2 years ago

(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

#4 Updated by Yusuke Endoh almost 2 years ago

  • Status changed from Open to Assigned

#5 Updated by Charles Nutter over 1 year ago

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.

#6 Updated by Alexey Muranov over 1 year ago

+1 for goto foo()

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

#7 Updated by Koichi Sasada over 1 year ago

  • Target version changed from 2.0.0 to next minor

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

I changed target to next minor.

#8 Updated by Yusuke Endoh over 1 year ago

  • Assignee changed from Yusuke Endoh to Koichi Sasada

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