Bug #22092: `Array#sum` takes slow path, does not perform compensated summation of Float elements when init argument is a Float

• Status: Closed
• Assignee: -
• Target version: -
• ruby -v: ruby 4.0.1 (2026-01-13 revision e04267a14b) +PRISM [arm64-darwin24]
• Backport: 3.3: REQUIRED, 3.4: REQUIRED, 4.0: DONE

[georgeclaghorn (George Claghorn)]

% time ruby -e 'puts Array.new(1_000_000_000, 0.1).sum'
100000000.0
ruby -e 'puts Array.new(1_000_000_000, 0.1).sum'  2.67s user 2.02s system 71% cpu 6.577 total

% time ruby -e 'puts Array.new(1_000_000_000, 0.1).sum(0.0)'
99999998.74541782
ruby -e 'puts Array.new(1_000_000_000, 0.1).sum(0.0)'  22.72s user 1.80s system 96% cpu 25.332 total

Both the fast path and compensated summation are undocumented implementation details as far as I know, but this appears to be a possible oversight.

[nobu (Nobuyoshi Nakada)]

• Status changed from Open to Closed

Applied in changeset git|be696c9c241840aa366fd7444b2297f1db55d8f3.

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[Bug #22092] Improve Array#sum when the initial value is a Float

[nobu (Nobuyoshi Nakada)]

• Backport changed from 3.3: UNKNOWN, 3.4: UNKNOWN, 4.0: UNKNOWN to 3.3: REQUIRED, 3.4: REQUIRED, 4.0: REQUIRED

[etienne (Étienne Barrié)]

4.0: DONE

Edit: Sorry I tried editing custom properties to note that this was backported to 4.0 but it seems that it's not the way 😅

[byroot (Jean Boussier)]

• Backport changed from 3.3: REQUIRED, 3.4: REQUIRED, 4.0: REQUIRED to 3.3: REQUIRED, 3.4: REQUIRED, 4.0: DONE