Ruby

erb.rb: Use script encoding instead of force_encoding

The original intention of introducing _erbout.force_encoding
in r21170 was:

• "returns a string in the same character encoding as the input string."
• "When the input string has a magic comment, however, it returns a string
  in the encoding specified by the magic comment."

And they are tested by test/erb/test_erb_m17n.rb well and this patch
passes the test.
Since magic comment is always added in ERB compiled code, using ''.dup
instead of String.new will set correct encoding without calling
force_encoding method.

The benchmark results are:

• Before

$ ./ruby benchmark/run.rb --matzruby=./ruby -m bm_app_erb
MatzRuby:
ruby 2.5.0dev (2017-05-26 skip-force-enc.. 58903) [x86_64-linux]
last_commit=Skip force_encoding in compiled code of erb
Ruby:

app_erb:
matz 0.715

• After

$ ./ruby benchmark/run.rb --matzruby=./ruby -m bm_app_erb
MatzRuby:
ruby 2.5.0dev (2017-05-26 skip-force-enc.. 58903) [x86_64-linux]
last_commit=Skip force_encoding in compiled code of erb
Ruby:

app_erb:
matz 0.672

And perf(1) results are:

• Before

$ sudo perf stat ./ruby benchmark/bm_app_erb.rb

Performance counter stats for './ruby benchmark/bm_app_erb.rb':

 709.571746 task-clock (msec) # 1.000 CPUs utilized 5 context-switches # 0.007 K/sec 1 cpu-migrations # 0.001 K/sec 1,337 page-faults # 0.002 M/sec 3,088,936,521 cycles # 4.353 GHz 

stalled-cycles-frontend
stalled-cycles-backend
4,849,564,282 instructions # 1.57 insns per cycle
1,027,042,087 branches # 1447.411 M/sec
19,983,456 branch-misses # 1.95% of all branches

 0.709747823 seconds time elapsed 

• After

$ sudo perf stat ./ruby benchmark/bm_app_erb.rb

Performance counter stats for './ruby benchmark/bm_app_erb.rb':

 693.494673 task-clock (msec) # 1.000 CPUs utilized 7 context-switches # 0.010 K/sec 1 cpu-migrations # 0.001 K/sec 1,316 page-faults # 0.002 M/sec 3,025,639,349 cycles # 4.363 GHz 

stalled-cycles-frontend
stalled-cycles-backend
4,694,848,271 instructions # 1.55 insns per cycle
994,496,704 branches # 1434.037 M/sec
19,693,239 branch-misses # 1.98% of all branches

 0.693724345 seconds time elapsed 

[fix GH-1147]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@58904 b2dd03c8-39d4-4d8f-98ff-823fe69b080e