Optimization of REXML::Parsers::SAX2Parser#get_namespace (#300)

## Benchmark ``` before after before(YJIT) after(YJIT) sax 1.415k 2.297k 1.755k 2.958k i/s - 100.000 times in 0.070687s 0.043539s 0.056966s 0.033810s Comparison: sax after(YJIT): 2957.7 i/s after: 2296.8 i/s - 1.29x slower before(YJIT): 1755.4 i/s - 1.68x slower before: 1414.7 i/s - 2.09x slower ``` - YJIT=ON : 1.68x faster - YJIT=OFF : 1.62x faster

Author: naitoh

benchmark/sax.yaml

@@ -0,0 +1,34 @@
+loop_count: 100
+contexts:
+ - gems:
+ rexml: 3.2.6
+ require: false
+ prelude: require 'rexml'
+ - name: master
+ prelude: |
+ $LOAD_PATH.unshift(File.expand_path("lib"))
+ require 'rexml'
+ - name: 3.2.6(YJIT)
+ gems:
+ rexml: 3.2.6
+ require: false
+ prelude: |
+ require 'rexml'
+ RubyVM::YJIT.enable
+ - name: master(YJIT)
+ prelude: |
+ $LOAD_PATH.unshift(File.expand_path("lib"))
+ require 'rexml'
+ RubyVM::YJIT.enable
+
+prelude: |
+ require 'rexml/parsers/sax2parser'
+
+ DEPTH = 100
+ xml = '<a>' * DEPTH + '</a>' * DEPTH
+
+benchmark:
+ 'sax' : |
+ parser = REXML::Parsers::SAX2Parser.new(xml)
+ parser.listen(:start_element){|uri, localname, qname, attrs|}
+ parser.parse

lib/rexml/parsers/sax2parser.rb

@@ -12,7 +12,7 @@ def initialize source
  @parser = BaseParser.new(source)
  @listeners = []
  @procs = []
- @namespace_stack = []
+ @namespace_stack = [{}]
  @has_listeners = false
  @tag_stack = []
  @entities = {}
@@ -122,7 +122,7 @@ def parse
  event[2].each {|n, v| event[2][n] = @parser.normalize(v)}
  nsdecl = event[2].find_all { |n, value| n =~ /^xmlns(:|$)/ }
  nsdecl.collect! { |n, value| [ n[6..-1], value ] }
- @namespace_stack.push({})
+ @namespace_stack.push(@namespace_stack[-1].dup)
  nsdecl.each do |n,v|
  @namespace_stack[-1][n] = v
  # notify observers of namespaces
@@ -259,11 +259,8 @@ def add( pair )
  end
 
  def get_namespace( prefix )
- return nil if @namespace_stack.empty?
-
- uris = (@namespace_stack.find_all { |ns| not ns[prefix].nil? }) ||
- (@namespace_stack.find { |ns| not ns[nil].nil? })
- uris[-1][prefix] unless uris.nil? or 0 == uris.size
+ current_namespace = @namespace_stack[-1]
+ current_namespace[prefix] || current_namespace[nil]
  end
  end
  end