High-performance Ruby: faster Symbol.to_s (See related posts)

hmm, nice Idea. but to make this code a little bit more beatiful, you can do this:

class Symbol
  alias_method :__orig_to_s, :to_s
  def to_s
    @str_rep ||= __orig_to_s.freeze
  end
end

Hi NoKarma. Tried it. My way is faster, but you can try it yourself:

require 'benchmark'

class Symbol
  def to_s_fast
    @str_rep ||= to_s
  end
  def to_s_faster
    @str_rep || (@str_rep = to_s)
  end
end

n = 1000000
Benchmark.bm do |x|
  x.report {n.times {:hello.to_s}}
  x.report {n.times {:hello.to_s_fast}}
  x.report {n.times {:hello.to_s_faster}}
end

Reporting in with verification that the to_s_faster method is indeed the fastest on three different machines of varying architecture (PPC, x86, amd64). This kind of blew my mind because I had assumed that the to_s_fast and to_s_faster methods had the same semantics and were treated identically by the runtime.

My current theory as to why the to_s_faster methods performs better is because the instance variable assignment is performed each time in the to_s_fast case (albeit to itself) but is not in the to_s_faster case. i.e. the following to_s_fast methods are all roughly equivalent (semantically):

def to_s_fast
  @str_rep ||= to_s
end

def to_s_fast
  @str_rep = @str_rep || to_s
end

def to_s_fast
  instance_variable_set(:@str_rep, instance_variable_get(:@str_rep) || to_s)
end

def to_s_fast
  result = instance_variable_get(:str_rep)
  result = to_s if result.nil?
  instance_variable_set(:@str_rep)
end

What I'm trying to call out is that the instance_variable_set method may be invoked each time. Compare to these (again, roughly equivalent versions of the to_s_faster) method:

def to_s_faster
  @str_rep || (@str_rep = to_s)
end

def to_s_faster
  return @str_rep unless @str_rep.nil?
  @str_rep = to_s
end

def to_s_faster
  instance_variable_get(:@str_rep) || instance_variable_set(:@str_rep, to_s)
end

Can anyone confirm or deny this? I use "@x ||= y" quite frequently and had assumed the variable assignment didn't take place if the LHS of the || was truthful.

You might try:

class Symbol
def to_s
@to_s || (@to_s = id2name)
end
end

That way you don't need to alias to_s first.

Thanks trans. I updated the snippet.

don't use this trick because it makes a conflict with ruby/date. you would go a frozen string error.