Feature #13587: Improve performance of string interpolation

• Status: Closed
• Assignee: nobu (Nobuyoshi Nakada)
• Target version: -

[south37 (Nao Minami)]

This patch will add pre-allocation in string interpolation.
By this, unnecessary capacity resizing is avoided.

For small strings, optimized rb_str_resurrect operation is faster, so pre-allocation is done only when concatenated strings are large.
MIN_PRE_ALLOC_SIZE was decided by experimenting with local machine (x86_64-apple-darwin 16.5.0, Apple LLVM version 8.1.0 (clang - 802.0.42)).

String interpolation will be faster around 72% when large string is created.

Before

Calculating -------------------------------------
Large string interpolation
                          1.276M (± 5.9%) i/s -      6.358M in   5.002022s
Small string interpolation
                          5.156M (± 5.5%) i/s -     25.728M in   5.005731s

After

Calculating -------------------------------------
Large string interpolation
                          2.201M (± 5.8%) i/s -     11.063M in   5.043724s
Small string interpolation
                          5.192M (± 5.7%) i/s -     25.971M in   5.020516s

Test code

require 'benchmark/ips'

Benchmark.ips do |x|
  x.report "Large string interpolation" do |t|
    a = "Hellooooooooooooooooooooooooooooooooooooooooooooooooooo"
    b = "Wooooooooooooooooooooooooooooooooooooooooooooooooooorld"

    t.times do
      "#{a}, #{b}!"
    end
  end

  x.report "Small string interpolation" do |t|
    a = "Hello"
    b = "World"

    t.times do
      "#{a}, #{b}!"
    end
  end
end

Patch
https://github.com/ruby/ruby/pull/1626

[k0kubun (Takashi Kokubun)]

• Status changed from Open to Closed
• Assignee set to nobu (Nobuyoshi Nakada)

This was applied in r60320.