How Ruby GC count allocated RegularExpression

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Today i learned about Ruby Garbage Collection, and do some test

def count_allocated_objects
  before = GC.stat(:total_allocated_objects)

  yield

  after = GC.stat(:total_allocated_objects)

  after - before
end

count_allocated_objects {
  s = "this is a string"
  r = /[a-z]/
} # => 1, so only the string `s` be counted

count_allocated_objects {
  s = "this is a string"
  s.gsub(/[a-z]/, "")
} # => 6, in this case, is the regex `/[a-z]/` counted ?

As you can see, it looks like the regex /[a-z]/ is not counted by the GC. is it the Ruby's rule or the GC's stat total_allocated_objects rule ?

garbage-collection ruby
1 Answers

Turn out the Regex is considered as frozen object (similar to the frozen String) and the GC will not count those frozen objects, so the Regex will not be counted. (regardless which Ruby version, at least so far)

count_allocated_objects {
  s1 = "this is a string"
  s2 = "another string"
  r = /[a-z]/
} # ruby 2.6.5 => 2, ruby 3.0.0 => 2, ruby 3.1.0 => 3

although ruby 3.1.0 return 3, but if i remove r from the above code, it still return 3

count_allocated_objects {
  s1 = "this is a string"
  s2 = "another string"
} # ruby 2.6.5 => 2, ruby 3.0.0 => 2, ruby 3.1.0 => 3

test with a frozen String

count_allocated_objects {
  s1 = "this is a string"
  s2 = "another string".freeze
} # ruby 2.6.5 => 1, ruby 3.0.0 => 1, ruby 3.1.0 => 2

For sure i checked a frozen String and Regexp in heap dump

require 'objspace'
ObjectSpace.trace_object_allocations_start

s1 = "this is a string"
s2 = "another string".freeze
r = /[a-z]/

p r.frozen? # true
p ObjectSpace.dump(r) # "{..\"type\":\"REGEXP\", ... \"frozen\":true ...
p ObjectSpace.dump(s2) # "{..\"type\":\"STRING\", ... \"frozen\":true ...
p ObjectSpace.dump(s1) # didn't contain \"frozen\"
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