--- title: "The Case of the Missing Line: A Ruby Mystery" url: https://daily.dev/posts/the-case-of-the-missing-line-a-ruby-mystery-z7t9cbv0f source_url: https://www.codeotaku.com/journal/2026-08/the-case-of-the-missing-line/index type: article source: "RUBYLAND" published: 2026-08-24T06:51:50.139Z updated: 2026-08-24T06:52:25.005Z tags: ["ruby"] reading_time: 7 upvotes: 0 comments: 0 language: en --- > ## Documentation Index > Fetch the complete documentation index at: https://daily.dev/llms.txt > Use this file to discover all available pages before exploring further. # The Case of the Missing Line: A Ruby Mystery **[RUBYLAND](https://daily.dev/sources/rubyla)** · 7 min read · 0 upvotes · 0 comments ## Summary 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. ## Full article daily.dev links to this article rather than hosting it. Read it at the original source: ## Questions this post answers ### Why does TracePoint.new(:line) sometimes fail to report a loop condition line in Ruby even though it clearly executed? A jump-to-jump peephole optimization in CRuby's compiler can retarget a branch directly to a loop body, skipping the intermediate instruction that carried the line event for the loop condition. This happens even though the condition executes and affects the result. The behavior was reproduced on Ruby 3.3, 3.4, and 4.0 and tracked as Bug #22218 on the Ruby bug tracker. _daily.dev surfaces deep interpreter internals discussions like this for developers debugging tracing tools._ ### Why did line coverage show the correct line but TracePoint miss it for the same Ruby loop condition? Coverage already had a safeguard from an earlier fix, Bug #15980, that stops the peephole jump optimization whenever line coverage is enabled and the skipped instruction carries event metadata. Plain TracePoint line events had no equivalent protection, so the compiler produced different bytecode depending on which observer requested it, causing the two tools to disagree. _Developers comparing instrumentation tool behavior can follow such Ruby internals fixes on daily.dev._ ### What is the performance cost of fixing the Ruby TracePoint line event bug that retains an extra jump instruction? Retaining the jump instruction that carries the line event slows the CRuby interpreter by about one percent in a hot loop and 2.7 percent in a minimal guarded reader benchmark, according to targeted benchmarks concentrating the affected pattern. JITs like YJIT and ZJIT showed no repeatable regression since they can reorganize machine control flow after bytecode preserves the event. _daily.dev helps engineers weigh interpreter tradeoffs like this before adopting a Ruby patch._ ## Similar posts on daily.dev - [The Case of the Vanishing Fiber: A Ruby Mystery](https://daily.dev/posts/the-case-of-the-vanishing-fiber-a-ruby-mystery-dgqpxvnp4) · RUBYLAND · 1 upvotes · 0 comments - [Choosing the Right Debugger: TracePoint, ISeq, and why your choice of debugger affects more than just comfort](https://daily.dev/posts/choosing-the-right-debugger-tracepoint-iseq-and-why-your-choice-of-debugger-affects-more-than-jus-nulc1umad) · RUBYLAND · 0 upvotes · 0 comments - [Never Escape This Ruby Loop. Also, Here's How.](https://daily.dev/posts/never-escape-this-ruby-loop-also-here-s-how--4pbofog8d) · RUBYLAND · 0 upvotes · 0 comments --- Tags: [#ruby](https://daily.dev/tags/ruby) [View this post on daily.dev](https://daily.dev/posts/the-case-of-the-missing-line-a-ruby-mystery-z7t9cbv0f)