Eval vs defined? for a class

benchmark.rb

require 'benchmark/ips'

def eval_eval_true
  name = "Benchmark"
  eval("defined?(#{name}) && #{name}.class").to_s.eql?('Module')
end

def eval_eval_false
  name = "XABCDE"
  eval("defined?(#{name}) && #{name}.class").to_s.eql?('Module')
end

def eval_raise_true
  name = "Benchmark"
  eval(name + ".class").to_s.eql?('Module')
rescue NameError
  false
end

def eval_raise_false
  name = "XABCDE"
  eval(name + ".class").to_s.eql?('Module')
rescue NameError
  false
end

def def_eval_true
  name = "Benchmark"
  defined?(Benchmark) && eval(name + ".class").to_s.eql?('Module')
end

def def_eval_true_interp
  name = "Benchmark"
  defined?(Benchmark) && eval("#{name}.class").to_s.eql?('Module')
end

def def_eval_false
  name = "XABCDE"
  defined?(XABCDE) && eval(name + ".class").to_s.eql?('Module')
end

def def_true
  defined?(Benchmark) && Benchmark.class == Module
end

def def_false
  defined?(XABCDE) && XABCDE.class == Module
end

def direct_raise_true
  Benchmark.class == Module
rescue NameError
  false
end

def direct_raise_false
  XABCDE.class == Module
rescue NameError
  false
end

def def_direct_true
  defined?(Benchmark) && Benchmark.class == Module
end

def def_direct_false
  defined?(XABCDE) && XABCDE.class == Module
end

# -----
# sanity checking

raise "NO" if !eval_eval_true
raise "NO" if !eval_raise_true
raise "NO" if !def_eval_true
raise "NO" if !def_eval_true_interp
raise "NO" if !direct_raise_true
raise "NO" if !def_direct_true
raise "NO" if !def_true
raise "NO" if eval_eval_false
raise "NO" if eval_eval_false
raise "NO" if def_eval_false
raise "NO" if eval_raise_false
raise "NO" if direct_raise_false
raise "NO" if def_direct_false
raise "NO" if def_false

Benchmark.ips do |x|
  x.report("eval_eval_true")   { eval_eval_true }
  x.report("eval_raise_true")  { eval_raise_true }
  x.report("def_eval_true")    { def_eval_true }
  x.report("def_eval_true_i")  { def_eval_true_interp }
  x.report("direct_raise_true") { direct_raise_true }
  x.report("def_direct_true")  { def_direct_true }
  x.report("def_true")         { def_true }
  x.compare!
end

Benchmark.ips do |x|
  x.report("eval_eval_false")     { eval_eval_false }
  x.report("eval_raise_false")     { eval_eval_false }
  x.report("def_eval_false")    { def_eval_false }
  x.report("eval_raise_false")  { eval_raise_false }
  x.report("direct_raise_false") { direct_raise_false }
  x.report("def_direct_false")  { def_direct_false }
  x.report("def_false")         { def_false }
  x.compare!
end

results.txt

uby define-vs-eval.rb 
Warming up --------------------------------------
      eval_eval_true    13.706k i/100ms
     eval_raise_true    17.766k i/100ms
       def_eval_true    16.619k i/100ms
     def_eval_true_i    17.423k i/100ms
   direct_raise_true   270.068k i/100ms
     def_direct_true   261.766k i/100ms
            def_true   258.549k i/100ms
Calculating -------------------------------------
      eval_eval_true    143.672k (± 4.7%) i/s -    726.418k in   5.067286s
     eval_raise_true    190.764k (± 3.6%) i/s -    959.364k in   5.035629s
       def_eval_true    189.151k (± 4.5%) i/s -    947.283k in   5.018542s
     def_eval_true_i    186.567k (± 3.6%) i/s -    940.842k in   5.049468s
   direct_raise_true      8.298M (± 5.4%) i/s -     41.590M in   5.028062s
     def_direct_true      6.328M (± 5.3%) i/s -     31.674M in   5.020321s
            def_true      6.376M (± 5.2%) i/s -     31.802M in   5.002057s

Comparison:
   direct_raise_true:  8297936.9 i/s
            def_true:  6376020.5 i/s - 1.30x slower
     def_direct_true:  6327824.8 i/s - 1.31x slower
     eval_raise_true:   190763.9 i/s - 43.50x slower
       def_eval_true:   189151.1 i/s - 43.87x slower
     def_eval_true_i:   186566.8 i/s - 44.48x slower
      eval_eval_true:   143671.6 i/s - 57.76x slower

Warming up --------------------------------------
     eval_eval_false    13.818k i/100ms
    eval_raise_false    13.863k i/100ms
      def_eval_false   249.870k i/100ms
    eval_raise_false     3.854k i/100ms
  direct_raise_false    55.493k i/100ms
    def_direct_false   276.104k i/100ms
           def_false   273.045k i/100ms
Calculating -------------------------------------
     eval_eval_false    149.025k (± 4.6%) i/s -    746.172k in   5.017757s
    eval_raise_false    148.935k (± 4.4%) i/s -    748.602k in   5.036183s
      def_eval_false      6.071M (± 5.5%) i/s -     30.484M in   5.037580s
    eval_raise_false     39.355k (± 4.2%) i/s -    196.554k in   5.003303s
  direct_raise_false    666.719k (± 4.8%) i/s -      3.330M in   5.006008s
    def_direct_false      7.565M (± 5.8%) i/s -     37.826M in   5.018291s
           def_false      7.569M (± 6.2%) i/s -     37.953M in   5.034977s

Comparison:
           def_false:  7569137.3 i/s
    def_direct_false:  7565112.3 i/s - same-ish: difference falls within error
      def_eval_false:  6070722.0 i/s - 1.25x slower
  direct_raise_false:   666719.4 i/s - 11.35x slower
     eval_eval_false:   149025.0 i/s - 50.79x slower
    eval_raise_false:    39355.5 i/s - 192.33x slower