Bug #9080

ENV[key] produces three objects

Added by Yehuda Katz 5 months ago. Updated 5 months ago.

[ruby-core:58160]
Status:Open
Priority:Normal
Assignee:Koichi Sasada
Category:-
Target version:-
ruby -v:ruby 2.0.0p343 (2013-10-31) [x86_64-linux] Backport:1.9.3: UNKNOWN, 2.0.0: UNKNOWN

Description

This code:

ENV["HOME"]

allocates three TSTRING according to ObjectSpace.countobjects. Unless I'm missing something, it should only need to allocate one string.

History

#1 Updated by Benoit Daloze 5 months ago

A little ObjectSpace exploration gives:

n = 1
GC.disable

a = ObjectSpace.count_objects[:T_STRING]
strings = {}
ObjectSpace.each_object(String) { |s| strings[s.object_id] = s }

n.times { ENV["HOME"] }

b = ObjectSpace.count_objects[:T_STRING]
p a => b
p (b-a)/n.to_f
new_strings = ObjectSpace.each_object(String).reject {|s| strings.key? s.object_id }
p new_strings
p new_strings.size

ObjectSpace.countobjects seems unreliable in this case, sometimes the number of TSTRING is less after invocations of ENV["HOME"] yet the GC is supposed to be disabled.

The difference using ObjectSpace.each_object give us the 3 String:

["/home/me", "UTF-8", "HOME"]

I am a bit concerned about the "UTF-8" String created.
"HOME" is created each time, and could be avoided by assigning it to a variable.
After all, the compiler is not supposed to know ENV[] is not modifying its argument,
and there is no clue the String should be immutable.
(detection of literal .freeze should help, but will anyone do ENV["HOME".freeze] ?
Anyway real code would likely not invoke ENV[var] many times for the same var)

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