From: "rdil (Reece Dunham) via ruby-core" <ruby-core@...>
Date: 2026-11-14T16:45:52+00:00
Subject: [ruby-core:127491] [Ruby Bug#22551] YJIT: String#<< clears the receiver's coderange

Issue #22551 has been reported by rdil (Reece Dunham).

----------------------------------------
Bug #22551: YJIT: String#<< clears the receiver's coderange
https://bugs.ruby-lang.org/issues/22551

* Author: rdil (Reece Dunham)
* Status: Open
* ruby -v: 4.0.7
* Backport: 3.3: UNKNOWN, 3.4: UNKNOWN, 4.0: UNKNOWN
----------------------------------------
With YJIT, `str << other` leaves `str` with an unknown coderange when both strings have the same encoding. The interpreter keeps the coderange. Any later operation that needs it (`length`, `[]`, `valid_encoding?`) then rescans the whole string, so code that appends and reads in a loop is much slower with YJIT than without.

```ruby
# Run twice: `ruby repro.rb` and `ruby --yjit repro.rb`
require 'objspace'
require 'json'

def append(buf, chunk) = buf << chunk
def coderange(str) = JSON.parse(ObjectSpace.dump(str))['coderange'] || 'unknown'

chunk = '�' * 100 # valid UTF-8, not ASCII
chunk.valid_encoding?

buf = +'�'
buf.valid_encoding?
1_000.times { append(+'�', chunk) } # let YJIT compile append
append(buf, chunk)
puts RUBY_DESCRIPTION
puts "coderange after <<: #{coderange(buf)}"

buf = +''
started = Process.clock_gettime(Process::CLOCK_MONOTONIC)
2_000.times do
  append(buf, chunk)
  buf.length
end
puts format('2,000 appends, each followed by #length: %.0f ms',
            (Process.clock_gettime(Process::CLOCK_MONOTONIC) - started) * 1000)
```

Results on arm64 macOS:

| ruby   | without YJIT | with --yjit     |
|--------|--------------|-----------------|
| 3.3.13 | valid, 17 ms | unknown, 400 ms |
| 3.4.9  | valid, 17 ms | unknown, 477 ms |
| 4.0.5  | valid, 17 ms | unknown, 479 ms |
| 4.0.7  | valid, 17 ms | unknown, 504 ms |



-- 
https://bugs.ruby-lang.org/
______________________________________________
 ruby-core mailing list -- ruby-core@ml.ruby-lang.org
 To unsubscribe send an email to ruby-core-leave@ml.ruby-lang.org
 ruby-core info -- https://ml.ruby-lang.org/mailman3/lists/ruby-core.ml.ruby-lang.org/