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Bug #17209

closed

String#slice allocates Range object in ruby 3.0.0preview1

Added by bruno.escherl@xing.com (Bruno Escherl) about 4 years ago. Updated about 4 years ago.

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
[ruby-core:100262]

Description

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?

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