~3.5x speedup of String#blank? for empty strings

See the rationale in the comment in this patch.

To benchmark this I ran a number of variations, ultimately narrowing to

    require 'benchmark/ips'

    str = ''
    regexp = /\A[[:space:]]*\z/

    Benchmark.ips do |x|
      x.report('regexp') { regexp === str }
      x.report('empty')  { str.empty? || regexp === str }
      x.compare!
    end

This benchmark has consistently reported speedups around 3.5x:

    Calculating -------------------------------------
                  regexp    69.197k i/100ms
                   empty   115.468k i/100ms
    -------------------------------------------------
                  regexp      2. 6.3%) i/s -     13.839M
                   empty      9. 8.8%) i/s -     47.804M

    Comparison:
                   empty:  9642607.6 i/s
                  regexp:  2768351.9 i/s - 3.48x slower

Sometimes even reaching 4x.

Running the same bechmark on strings of 10 or 100 characters (with
whitespace or present) has shown a slowdown of just about 1.01/1.02.
Marginal, we seem to have a worthwhile trade-off here.

1 files changed, 6 insertions, 1 deletions

diff --git a/activesupport/lib/active_support/core_ext/object/blank.rb b/activesupport/lib/active_support/core_ext/object/blank.rb
index 039c50a4a2..71d411b6d6 100644
--- a/activesupport/lib/active_support/core_ext/object/blank.rb
+++ b/activesupport/lib/active_support/core_ext/object/blank.rb
@@ -112,7 +112,12 @@ class String
   #
   # @return [true, false]
   def blank?
-    BLANK_RE === self
+    # In practice, the majority of blank strings are empty. As of this writing
+    # checking for empty? is about 3.5x faster than matching against the regexp
+    # in MRI, so we call the predicate first, and then fallback.
+    #
+    # The penalty for blank strings with whitespace or present ones is marginal.
+    empty? || BLANK_RE === self
   end
 end