RUBY_ENGINE is always defined

Author: eregon

examples/benchmark_read_write_lock.rb

@@ -34,7 +34,7 @@
 end.parse!
 
 def jruby?
-  defined?(RUBY_ENGINE) && RUBY_ENGINE == "jruby"
+  RUBY_ENGINE == "jruby"
 end
 
 # for performance comparison with ReadWriteLock

lib/concurrent-ruby/concurrent/thread_safe/util/cheap_lockable.rb

@@ -32,8 +32,7 @@ module Util
       # @!visibility private
       module CheapLockable
         private
-        engine = defined?(RUBY_ENGINE) && RUBY_ENGINE
-        if engine == 'rbx'
+        if Concurrent.on_rbx?
           # Making use of the Rubinius' ability to lock via object headers to avoid the overhead of the extra Mutex objects.
           def cheap_synchronize
             Rubinius.lock(self)
@@ -70,7 +69,7 @@ def cheap_broadcast
             waiters.shift << true until waiters.empty?
             self
           end
-        elsif engine == 'jruby'
+        elsif Concurrent.on_jruby?
           # Use Java's native synchronized (this) { wait(); notifyAll(); } to avoid the overhead of the extra Mutex objects
           require 'jruby'
 

lib/concurrent-ruby/concurrent/utility/engine.rb

@@ -4,23 +4,23 @@ module Utility
     # @!visibility private
     module EngineDetector
       def on_jruby?
-        ruby_engine == 'jruby'
+        RUBY_ENGINE == 'jruby'
       end
 
       def on_jruby_9000?
         on_jruby? && ruby_version(JRUBY_VERSION, :>=, 9, 0, 0)
       end
 
       def on_cruby?
-        ruby_engine == 'ruby'
+        RUBY_ENGINE == 'ruby'
       end
 
       def on_rbx?
-        ruby_engine == 'rbx'
+        RUBY_ENGINE == 'rbx'
       end
 
       def on_truffleruby?
-        ruby_engine == 'truffleruby'
+        RUBY_ENGINE == 'truffleruby'
       end
 
       def on_windows?
@@ -35,10 +35,6 @@ def on_linux?
         !(RbConfig::CONFIG['host_os'] =~ /linux/).nil?
       end
 
-      def ruby_engine
-        defined?(RUBY_ENGINE) ? RUBY_ENGINE : 'ruby'
-      end
-
       def ruby_version(version = RUBY_VERSION, comparison, major, minor, patch)
         result      = (version.split('.').map(&:to_i) <=> [major, minor, patch])
         comparisons = { :== => [0],