cext.c

#include "ruby.h"

VALUE CExt = Qnil;

VALUE method_floatmul(VALUE self);
VALUE method_loop_floatmul(VALUE self);

void Init_CExt() {
	CExt = rb_define_module("CExt");
	rb_define_singleton_method(CExt, "floatmul", method_floatmul, 0);
	rb_define_singleton_method(CExt, "loop_floatmul", method_loop_floatmul, 0);
}

VALUE method_floatmul(VALUE self) {
	double a = 3.0f, b = 2.0f, c;
	c = a * b;
	return rb_float_new(c);
}

VALUE method_loop_floatmul(VALUE self) {
	double a = 3.0f, b = 2.0f, c;
	int i;
	for (i = 0; i < 1000000; i++) {
		c = a * b;
	}
	return rb_float_new(c);
}

extconf.rb

require 'mkmf'

extension_name = 'CExt'

dir_config = extension_name

create_makefile(extension_name)

external.rb

require_relative 'cext/CExt'

CExt.loop_floatmul

mixed.rb

require_relative 'cext/CExt'

1000000.times do 
  CExt.floatmul
end

pure.c

int main() {
	double a = 3.0f, b = 2.0f, c;
	int i;
	for (i = 0; i < 1000000; i++) {
		c = a * b;
	}
	return 0;
}

pure.rb

a, b = 3.0, 2.0
c = 0.0
1000000.times do
  c = a * b
end

results.sh

time ruby pure.rb        # ruby pure.rb      0.18s user 0.01s system 92% cpu 0.203 total
time ruby mixed.rb       # ruby mixed.rb     0.20s user 0.01s system 92% cpu 0.230 total
time ruby external.sh    # ruby external.rb  0.01s user 0.01s system 60% cpu 0.025 total
time purec               # ./purec           0.00s user 0.00s system 48% cpu 0.011 total