The Case of the Missing Line: A Ruby Mystery

A narrative investigation into a CRuby bug where a jump-to-jump peephole optimization silently removed the line event for a loop condition, causing TracePoint.new(:line) to miss reporting an executed line even though program results stayed correct. The compiler's jump-folding could bypass an instruction carrying RUBY_EVENT_LINE metadata, while line Coverage already had a guard from an earlier fix (Bug #15980) preventing the same issue. The eventual fix, merged via a GitHub pull request, extends that guard to stop the peephole optimization specifically when the skipped instruction carries RUBY_EVENT_LINE or RUBY_EVENT_COVERAGE_LINE, restoring accurate tracing at a measured interpreter cost of about 1% in a hot loop and 2.7% in a minimal guarded reader, with JITs like YJIT and ZJIT able to reorganize machine code without losing the observable event.

Source: https://www.codeotaku.com/journal/2026-08/the-case-of-the-missing-line/index. 8 Sync News only summarizes and links out; content copyright belongs to the authors and original sources.