Bug #17209: String#slice allocates Range object in ruby 3.0.0preview1

• Status: Closed
• Assignee: ko1 (Koichi Sasada)
• Target version: -
• ruby -v: ruby 3.0.0preview1 (2020-09-25 master 0096d2b895) [x86_64-darwin19]
• Backport: 2.5: UNKNOWN, 2.6: UNKNOWN, 2.7: UNKNOWN

[[e-mail] (Bruno Escherl)]

While trying to optimise a use case, where I need to get parts of a string extracted, ignoring a constant length prefix, I noticed that ruby 3.0.0preview1 allocates an additional Range object for String#slice, that ruby 2.7.1 didn't allocate.

Test script using memory_profiler gem:

require 'bundler/inline'

gemfile do
  source 'https://rubygems.org'
  gem 'memory_profiler'
end

string = 'const:prefix:and:the:rest'

MemoryProfiler.report do
  10000.times do
    string.slice(13..-1)
  end
end.pretty_print

Output for ruby 3.0.0preview1

Total allocated: 800000 bytes (20000 objects)
Total retained:  0 bytes (0 objects)

allocated memory by gem
-----------------------------------
    800000  other

allocated memory by file
-----------------------------------
    800000  profile_string_slice.rb

allocated memory by location
-----------------------------------
    800000  profile_string_slice.rb:12

allocated memory by class
-----------------------------------
    400000  Range
    400000  String

allocated objects by gem
-----------------------------------
     20000  other

allocated objects by file
-----------------------------------
     20000  profile_string_slice.rb

allocated objects by location
-----------------------------------
     20000  profile_string_slice.rb:12

allocated objects by class
-----------------------------------
     10000  Range
     10000  String

Output for ruby 2.7.1

Total allocated: 400000 bytes (10000 objects)
Total retained:  0 bytes (0 objects)

allocated memory by gem
-----------------------------------
    400000  other

allocated memory by file
-----------------------------------
    400000  profile_string_slice.rb

allocated memory by location
-----------------------------------
    400000  profile_string_slice.rb:12

allocated memory by class
-----------------------------------
    400000  String

allocated objects by gem
-----------------------------------
     10000  other

allocated objects by file
-----------------------------------
     10000  profile_string_slice.rb

allocated objects by location
-----------------------------------
     10000  profile_string_slice.rb:12

allocated objects by class
-----------------------------------
     10000  String

Is this a regression in ruby 3 or something that memory_profiler doesn't record correctly in 2.7?

[mame (Yusuke Endoh)]

• Status changed from Open to Assigned
• Assignee set to ko1 (Koichi Sasada)

The optimization has been disabled temporarily because of Ractor.

https://github.com/ruby/ruby/blob/c881678cd75432f47903a5d1d8b86a7a723cb023/compile.c#L8647-L8648

        // TODO: Ractor can not use cached Range objects
	if (0 && optimizable_range_item_p(b) && optimizable_range_item_p(e)) {

As #15504 was accepted, now can we remove this restriction? @ko1 (Koichi Sasada)

[ko1 (Koichi Sasada)]

• Status changed from Assigned to Closed

I forgot to merge it: https://github.com/ruby/ruby/pull/3587