Ruby Inline

February 28, 2008, by Alexis Bernard

Sometimes applications need to be fast. Unfortunalty scripting languages like Ruby are slow. However rewriting an entire application isn’t the most efficient way. Furthemore the part that need to be faster isn’t really of an outstanding size in terms of code. That’s why Ruby inline is an excellent solution.

The goal is to replace the slow Ruby code by C code. It could be 5 or 20 lines in most of cases. Thanks to Ruby Inline it’s possible to rewrite methods in C directly in the ruby source. The C code is compiled on the fly only once. In less than 10 lines I decrease execution time of 40% in the example of this article. The most impressive is that it took me 15 minutes to do it without knowing Ruby Inline before. I think Ruby Inline is a really good tool.

For example we will try to optimise the class Hash32. This class computes the hash of a string by mixing characters by block of 4 bytes. The result is an integer of 32 bytes. The following sample code is extracted from file hash32.rb.

class Hash32
  def compute(str)
    i = 0
    sum = 0
    size = str.size
    str =
    missing = 4 - (size % 4)
    str << '_' * missing if missing != 4
    while i < size
      sum ^= mix(str[i], str[i+1], str[i+2], str[i+3])
      i += 4

  def mix(a, b, c, d)
    (a << 24) + (b << 16) + (c << 8) + d

I decided to optimise only the method mix because it don’t required a lot of knowledge about Ruby C API. I decided to write the fast version in an other file, called hash32.c.rb, to be more elegant and useful.

class Hash32
  require 'inline'
  c_inline = <<__INLINE__
VALUE mix(unsigned int a, unsigned int b, unsigned int c, unsigned int d)
  return UINT2NUM((a << 24) + (b << 16) + (c << 8) + d);
  inline do |builder| builder.c(c_inline) end
rescue LoadError => e

First of all we have to load the inline library. Then we write the C code for the mix method. Ruby Inline translates most of basic types. However in this case we need the macro UINT2NUM. I advice you to read the chapter Extending Ruby of the programming Ruby book. You d’ont have to be a Ruby C API guru to optimize with Ruby Inline. But it’s better to have some basic knowledge about it.

Ruby Inline is not included in the Ruby standard library. Moreover Ruby Inline requires a C compiler like gcc and Ruby headers to work. That’s why it may not work on all platforms. The best solution is to use the C version where Ruby Inline is available otherwise the slow version of mix. This job is done by the rescue LoadError if require ‘inline’ failes.

You can install Ruby Inline via gem.

gem install RubyInline

And don’t forget to install Ruby headers too. You can download the complete source code. There is a file named collisions.rb that tests the hash sum with about 90 000 words. The script is executed in 7.2 seconds in pure Ruby and in 4.3 seconds with the inline version of mix.

Wishing you nice optimisations with Ruby Inline.