Why is == faster than eql?

Question

I read in the documentation for the String class that eql? is a strict equality operator, without type conversion, and == is a equality operator which tries to convert second its argument to a String, and, the C source code for this methods confirms that:

The eql? source code:

static VALUE
rb_str_eql(VALUE str1, VALUE str2)
{
    if (str1 == str2) return Qtrue;
    if (TYPE(str2) != T_STRING) return Qfalse;
    return str_eql(str1, str2);
}

The == source code:

VALUE
rb_str_equal(VALUE str1, VALUE str2)
{
    if (str1 == str2) return Qtrue;
    if (TYPE(str2) != T_STRING) {
        if (!rb_respond_to(str2, rb_intern("to_str"))) {
            return Qfalse;
        }
        return rb_equal(str2, str1);
    }
    return str_eql(str1, str2);
}

But when I tried to benchmark these methods, I was suprised that == is faster than eql? by up to 20%! My benchmark code is:

require "benchmark"

RUN_COUNT = 100000000
first_string = "Woooooha"
second_string = "Woooooha"

time = Benchmark.measure do
  RUN_COUNT.times do |i|
    first_string.eql?(second_string)
  end
end
puts time

time = Benchmark.measure do
  RUN_COUNT.times do |i|
    first_string == second_string
  end
end
puts time

And results:

Ruby 1.9.3-p125:

26.420000   0.250000  26.670000 ( 26.820762)
21.520000   0.200000  21.720000 ( 21.843723)

Ruby 1.9.2-p290:

25.930000   0.280000  26.210000 ( 26.318998)
19.800000   0.130000  19.930000 ( 19.991929)

So, can anyone explain why the more simple eql? method is slower than == method in the case when I run it for two similar strings?

Answer 1

The reason you are seeing a difference is not related to the implementation of == vs eql? but is due to the fact that Ruby optimizes operators (like ==) to avoid going through the normal method lookup when possible.

We can verify this in two ways:

• Create an alias for == and call that instead. You'll get similar results to eql? and thus slower results than ==.

• Compare using send :== and send :eql? instead and you'll get similar timings; the speed difference disappears because Ruby will only use the optimization for direct calls to the operators, not with using send or __send__.

Here's code that shows both:

require 'fruity'
first = "Woooooha"
second = "Woooooha"
class String
  alias same_value? ==
end

compare do
  with_operator   { first == second }
  with_same_value { first.same_value? second }
  with_eql        { first.eql? second }
end

compare do
  with_send_op    { first.send :==, second }
  with_send_eql   { first.send :eql?, second }
end

Results:

with_operator is faster than with_same_value by 2x ± 0.1
with_same_value is similar to with_eql
with_send_eql is similar to with_send_op

If you're the curious, the optimizations for operators are in insns.def.

Note: this answer applies only to Ruby MRI, I would be surprised if there was a speed difference in JRuby / rubinius, for instance.