--- title: Rust and the Case of the Redundant Comparison | daily.dev description: A first-person account of tracking down a redundant comparison instruction generated by the Rust compiler's Vec::clear() implementation, investigating why it... canonical: https://daily.dev/posts/rust-and-the-case-of-the-redundant-comparison-0vxeoijtb twitter:card: summary_large_image twitter:site: @dailydotdev og:type: website og:site_name: daily.dev og:title: Rust and the Case of the Redundant Comparison | daily.dev og:description: A first-person account of tracking down a redundant comparison instruction generated by the Rust compiler's Vec::clear() implementation, investigating why it... og:url: https://daily.dev/posts/rust-and-the-case-of-the-redundant-comparison-0vxeoijtb og:image: https://api.daily.dev/og/posts/0vXeOijTb.png og:image:alt: Rust and the Case of the Redundant Comparison og:image:width: 1200 og:image:height: 630 og:locale: 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. # Rust and the Case of the Redundant Comparison **[Lobsters](https://daily.dev/sources/lobsters)** · 14 min read · 0 upvotes · 0 comments ## Summary A first-person account of tracking down a redundant comparison instruction generated by the Rust compiler's Vec::clear() implementation, investigating why it differed from the faster set_len(0) approach used in hyper, and ultimately fixing it with a small patch to the standard library's truncate() function using the SetLenOnDrop pattern to work around an LLVM alias-analysis limitation. Covers building the Rust compiler locally, writing a codegen test, and navigating the PR review and bors merge process. The fix shipped for Rust 1.30. ## Full article daily.dev links to this article rather than hosting it. Read it at the original source: ## Questions this post answers ### Why does Rust's Vec::clear() generate a comparison instruction while set_len(0) doesn't? Vec::clear() calls truncate(0), which historically set the length before dropping elements in a loop, causing LLVM's alias analysis to think the drop calls could modify the length field, so it inserted a redundant load and comparison. set_len(0) directly writes zero with no check. A 2018 patch to Rust's standard library (merged for Rust 1.30) reordered the logic using a SetLenOnDrop helper struct so the length is only written once, eliminating the extra comparison and matching set_len's output. _Developers chasing compiler-generated code differences can track Rust standard library changes like this on daily.dev._ ### What is the SetLenOnDrop pattern used for in Rust's Vec implementation? SetLenOnDrop is a helper struct in Rust's standard library that stores a local copy of a Vec's length and writes it back to the real length field only when the struct is dropped. This works around a known LLVM alias-analysis limitation (issue #32155) where the optimizer assumed writes to the vector's data pointer could alias the length field, preventing it from eliminating redundant length checks in functions like truncate(). _Anyone debugging surprising codegen from safe Rust abstractions can follow patterns like this on daily.dev._ ### How do I build only stage 1 of the Rust compiler without rebuilding stage 0 dependencies? Run ./x.py --keep-stage 0 --stage 1 from the Rust compiler source repository. This skips rebuilding stage 0 artifacts like libstd that would otherwise trigger a cascade of unnecessary rebuilds, letting you iterate on compiler changes faster. The full bootstrapping process and stage definitions are documented in the repository's bootstrapping README. _Contributors setting up a local rustc build can find workflow tips like this on daily.dev._ ## Similar posts on daily.dev - [Parsa's Blog](https://daily.dev/posts/parsa-s-blog-zqofvaklq) · Lobsters · 3 upvotes · 0 comments - [Rust-proof your code with our new Testing Handbook chapter](https://daily.dev/posts/rust-proof-your-code-with-our-new-testing-handbook-chapter-syjpwdje5) · Trail of Bits Blog · 1 upvotes · 0 comments --- Tags: [#general-programming](https://daily.dev/tags/general-programming), [#open-source](https://daily.dev/tags/open-source), [#rust](https://daily.dev/tags/rust), [#assembly](https://daily.dev/tags/assembly), [#compiler-optimization](https://daily.dev/tags/compiler-optimization) [View this post on daily.dev](https://daily.dev/posts/rust-and-the-case-of-the-redundant-comparison-0vxeoijtb) ```json {"@context":"https://schema.org","@graph":[{"@type":"Organization","@id":"https://daily.dev/#organization","name":"daily.dev","url":"https://daily.dev","logo":{"@type":"ImageObject","url":"https://daily.dev/apple-touch-icon.png","width":180,"height":180},"sameAs":["https://twitter.com/dailydotdev","https://github.com/dailydotdev","https://www.linkedin.com/company/daily-dev-ltd"]},{"@type":"WebSite","@id":"https://daily.dev/#website","url":"https://daily.dev","name":"daily.dev","publisher":{"@id":"https://daily.dev/#organization"},"potentialAction":{"@type":"SearchAction","target":{"@type":"EntryPoint","urlTemplate":"https://daily.dev/search?q={search_term_string}"},"query-input":"required name=search_term_string"}}]} {"@context":"https://schema.org","@type":"TechArticle","headline":"Rust and the Case of the Redundant Comparison","url":"https://daily.dev/posts/rust-and-the-case-of-the-redundant-comparison-0vxeoijtb","mainEntityOfPage":{"@type":"WebPage","@id":"https://daily.dev/posts/rust-and-the-case-of-the-redundant-comparison-0vxeoijtb"},"datePublished":"2026-08-12T11:53:49.412Z","dateModified":"2026-08-12T11:54:14.770Z","description":"A first-person account of tracking down a redundant comparison instruction generated by the Rust compiler's Vec::clear() implementation, investigating why it...","image":"https://media.daily.dev/image/upload/s--VDukGCjf--/f_auto/v1722860399/public/Placeholder%2002","thumbnailUrl":"https://media.daily.dev/image/upload/s--VDukGCjf--/f_auto/v1722860399/public/Placeholder%2002","isAccessibleForFree":true,"articleSection":"Lobsters","inLanguage":"en","publisher":{"@type":"Organization","name":"daily.dev","url":"https://daily.dev","logo":{"@type":"ImageObject","url":"https://daily.dev/apple-touch-icon.png","width":180,"height":180}},"author":{"@type":"Organization","name":"Lobsters","logo":"https://media.daily.dev/image/upload/s--tl8v_Fku--/f_auto,t_logo/v1698841318/logos/lobste.jpg","url":"https://daily.dev/sources/lobsters"},"commentCount":0,"discussionUrl":"https://daily.dev/posts/rust-and-the-case-of-the-redundant-comparison-0vxeoijtb","interactionStatistic":[{"@type":"InteractionCounter","interactionType":{"@type":"LikeAction"},"userInteractionCount":0},{"@type":"InteractionCounter","interactionType":{"@type":"CommentAction"},"userInteractionCount":0}],"keywords":"general-programming,open-source,rust,assembly,compiler-optimization","timeRequired":"PT14M"} {"@context":"https://schema.org","@type":"BreadcrumbList","itemListElement":[{"@type":"ListItem","position":1,"name":"Home","item":"https://daily.dev"},{"@type":"ListItem","position":2,"name":"Lobsters","item":"https://daily.dev/sources/lobsters"},{"@type":"ListItem","position":3,"name":"Rust and the Case of the Redundant Comparison"}]} {"@context":"https://schema.org","@type":"FAQPage","@id":"https://daily.dev/posts/rust-and-the-case-of-the-redundant-comparison-0vxeoijtb#faq","mainEntity":[{"@type":"Question","name":"Why does Rust's Vec::clear() generate a comparison instruction while set_len(0) doesn't?","acceptedAnswer":{"@type":"Answer","text":"Vec::clear() calls truncate(0), which historically set the length before dropping elements in a loop, causing LLVM's alias analysis to think the drop calls could modify the length field, so it inserted a redundant load and comparison. set_len(0) directly writes zero with no check. A 2018 patch to Rust's standard library (merged for Rust 1.30) reordered the logic using a SetLenOnDrop helper struct so the length is only written once, eliminating the extra comparison and matching set_len's output. Developers chasing compiler-generated code differences can track Rust standard library changes like this on daily.dev."}},{"@type":"Question","name":"What is the SetLenOnDrop pattern used for in Rust's Vec implementation?","acceptedAnswer":{"@type":"Answer","text":"SetLenOnDrop is a helper struct in Rust's standard library that stores a local copy of a Vec's length and writes it back to the real length field only when the struct is dropped. This works around a known LLVM alias-analysis limitation (issue #32155) where the optimizer assumed writes to the vector's data pointer could alias the length field, preventing it from eliminating redundant length checks in functions like truncate(). 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