Issue #21650 has been reported by koilanetroc (Oleg Tolmashov). ---------------------------------------- Bug #21650: Performance regression: `Rational#floor(ndigits)` extremely slow for huge ndigits in Ruby 3.4 (ok in 3.2) https://bugs.ruby-lang.org/issues/21650 * Author: koilanetroc (Oleg Tolmashov) * Status: Open * ruby -v: ruby 3.4.7 (2025-10-08 revision 7a5688e2a2) +PRISM [arm64-darwin24] * Backport: 3.2: UNKNOWN, 3.3: UNKNOWN, 3.4: UNKNOWN ---------------------------------------- ## Summary `Rational#floor(ndigits)` with a very large positive ndigits takes tens of seconds in Ruby 3.4, while it returns essentially instantly in Ruby 3.2. Reproducible on macOS and Linux. Looks like a missing fast‑path for rationals whose decimal expansion terminates. ## Steps to reproduce ```ruby require "benchmark" puts RUBY_DESCRIPTION t = Benchmark.realtime { (2 ** -3).floor(2 ** 31) } puts "elapsed: #{t.round(3)}s" ``` Also reproduces with the explicit rational form: ```ruby Benchmark.realtime { Rational(1, 8).floor(2 ** 31) } ``` ### Results on my machine Ruby 3.2.8: ``` ruby 3.2.8 (2025-03-26 revision 13f495dc2c) [arm64-darwin24] slow_math.rb:4: warning: in a**b, b may be too big elapsed: 0.0s ``` Ruby 3.4.7: ``` ruby 3.4.7 (2025-10-08 revision 7a5688e2a2) +PRISM [arm64-darwin24] elapsed: 39.214s ``` ## Actual behavior On Ruby 3.4.x this call takes ~tens of seconds (e.g., ~40s on my machine), consuming CPU. Same on macOS and Linux. ## Expected behavior The method should return quickly (ideally O(1)). -- https://bugs.ruby-lang.org/