Reduce string allocations with better alternatives

[ashmaroli]

Rationales

• interpolation results in a new object allocation for each call. So, interpolation within an iteration block is wasteful unless the interpolated variable is the block parameter.
• Regarding the changes to frontmatter_defaults.rb:
  • Private method #path_is_subpath? checks a derived string against a string value derived from the second argument to the method. Instead of generating a new string from a Pathname object, its better to just pass String objects directly.
    

    jekyll/lib/jekyll/frontmatter_defaults.rb

    Lines 136 to 139 in 8673cdc

    	def path_is_subpath?(path, parent_path)
    	path.ascend do |ascended_path|
    	return true if ascended_path.to_s == parent_path.to_s
    	end

  • slashed_coll_dir = collections_dir.empty? ? "/" : "#{collections_dir}/" is better since it avoids allocating interpolated strings for default configuration. Surprisingly, its a lot faster too..!

    # frozen_string_literal: true
    
    require 'benchmark/ips'
    
    def interpolate(collections_dir)
      "#{collections_dir}/"
    end
    
    def conditional(collections_dir)
      collections_dir.empty? ? "/" : "#{collections_dir}/"
    end
    
    ["", "foo"].each do |dir|
      return unless interpolate(dir) == conditional(dir)
    
      Benchmark.ips do |x|
        x.report("interpolate '#{dir}'") { interpolate(dir) }
        x.report("conditional '#{dir}'") { conditional(dir) }
        x.compare!
      end
    end

    Warming up --------------------------------------
          interpolate ''    73.729k i/100ms
          conditional ''    88.071k i/100ms
    Calculating -------------------------------------
          interpolate ''      1.991M (A± 0.7%) i/s -      9.953M in   5.000580s
          conditional ''      3.412M (A± 2.9%) i/s -     17.086M in   5.013017s
    
    Comparison:
          conditional '':  3411816.5 i/s
          interpolate '':  1990549.5 i/s - 1.71x  slower
    
    Warming up --------------------------------------
       interpolate 'foo'    73.693k i/100ms
       conditional 'foo'    72.454k i/100ms
    Calculating -------------------------------------
       interpolate 'foo'      1.992M (A± 0.6%) i/s -     10.022M in   5.031876s
       conditional 'foo'      1.911M (A± 0.8%) i/s -      9.564M in   5.006282s
    
    Comparison:
       interpolate 'foo':  1991828.9 i/s
       conditional 'foo':  1910513.6 i/s - 1.04x  slower
    

Context

String allocations will be further reduced by shipping #7406

[ashmaroli]

Profiler summary

--- master branch https://travis-ci.org/jekyll/jekyll/jobs/532959217
+++ PR branch     https://travis-ci.org/jekyll/jekyll/jobs/532974783

- Total allocated: 532.65 MB (5301707 objects)
- Total retained:  18.34 MB (96237 objects)
+ Total allocated: 532.07 MB (5287525 objects)
+ Total retained:  18.34 MB (96241 objects)

[mattr-]

@jekyllbot: merge +minor