# Bridging the Gap Between Rust's Theoretical IR Advantages and Real-World Performance **URL:** \ **Category:** compiler\ **Created:** [January 15, 2026, 11:30am UTC](https://internals.rust-lang.org/t/bridging-the-gap-between-rusts-theoretical-ir-advantages-and-real-world-performance/23927 "2026-01-15T11:30:21Z")\ **Posts on this page:** 7\ **Page:** 1
**Author:** ![gaowujie2004](https://sea2.discourse-cdn.com/flex002/user_avatar/internals.rust-lang.org/gaowujie2004/32/13979_2.png) [@gaowujie2004](https://internals.rust-lang.org/u/gaowujie2004)\ **Post date:** [January 15, 2026, 11:30am UTC](https://internals.rust-lang.org/t/bridging-the-gap-between-rusts-theoretical-ir-advantages-and-real-world-performance/23927/1 "2026-01-15T11:30:21Z")
# Bridging the Gap Between Rust's Theoretical IR Advantages and Real-World Performance Hi everyone, I’ve been diving deep into the LLVM IR generated by `rustc` and comparing it with the output from Clang for equivalent C/C++ code. This has led me to a question regarding the current state of Rust's optimization pipeline. ## The Premise: The "Information Dividend" In theory, Rust generates LLVM IR that contains significantly richer semantic information than C/C++. This should, logically, allow the backend to perform more aggressive optimizations: 1. **Aliasing Guarantees** : Rust's `&mut T` maps to `noalias`, and `&T` maps to `readonly`. This provides stronger guarantees than C's `restrict` keyword (which is rarely used correctly) and should unlock aggressive load/store reordering, LICM, and auto-vectorization. 2. **Type Constraints** : References are guaranteed `nonnull`, and enums/booleans have strict value ranges (`noundef`, `!range` metadata). 3. **Immutability** : Shared references allow for confident Constant Subexpression Elimination (CSE). ## The Reality Despite these theoretical advantages, real-world benchmarks often show Rust performing on par with, or sometimes slightly trailing (~1-5%), highly tuned C/C++. It seems that this "information dividend" isn't fully capitalizing into runtime performance yet. ## Hypotheses on Obstacles Based on my analysis of the generated assembly and IR, I’ve hypothesized a few reasons for this. I’d love to know if these assessments align with the compiler team’s observations: **1. LLVM's "C-Bias" in Heuristics** The LLVM backend has been tuned for decades to recognize C/C++ code patterns. Rust generated IR, while valid, often produces patterns (e.g., heavy `Option`/`Result` unwrapping, complex closure expansions in iterators) that may not trigger existing optimization heuristics designed for C-style loops and branches. **2. Implicit `memcpy` & Stack Inefficiency** Due to Rust's Move semantics and the current implementation of `Box::new`, Release builds still exhibit a significant amount of `memcpy` instructions and heavy stack usage. While LLVM's `MemCpyOpt` exists, it seems insufficient to eliminate copies across function boundaries or complex control flows, leading to unnecessary memory traffic. **3. Bounds Checking Overhead** While iterators eliminate many checks, index-based access still incurs overhead. C compilers often generate tighter code by leveraging Undefined Behavior (assuming no OOB access), a luxury Rust cannot afford by default. * * * ## Questions for the Team To understand the roadmap for unlocking Rust's full potential, I am curious about the following directions: **Q1: MIR Optimizations vs. LLVM Upstreaming** Is the long-term strategy to rely more on **MIR Optimizations** (like Destination Propagation) to "clean up" the code before it reaches LLVM, or is the focus shifting towards upstreaming **Rust-specific optimization passes** (or tuning heuristics) directly into LLVM? **Q2: The "Memcpy" Problem** Regarding excessive stack copies (and `Box::new` stack overflows): Is a language-level **"Placement New"** feature still being actively explored, or is the consensus to solve this entirely through compiler smarts like improved **NRVO** and **Destination Propagation**? **Q3: Rust-Specific LLVM Tuning** Are there active efforts to collaborate with the LLVM team to introduce heuristics specifically for Rust IR patterns? For example, better handling of the complex branch/switch structures often generated by `match` expressions on enums. **Q4: Code Bloat & Polymorphization** What is the current status of **Polymorphization**? Is it viewed as the primary solution to Rust's binary size bloat and Instruction Cache pressure caused by aggressive monomorphization? I would greatly appreciate any insights from the compiler team or contributors working on these fronts. Thanks! ---
**Author:** ![bjorn3](https://sea2.discourse-cdn.com/flex002/user_avatar/internals.rust-lang.org/bjorn3/32/2736_2.png) [@bjorn3](https://internals.rust-lang.org/u/bjorn3)\ **Post date:** [January 15, 2026, 12:55pm UTC](https://internals.rust-lang.org/t/bridging-the-gap-between-rusts-theoretical-ir-advantages-and-real-world-performance/23927/2 "2026-01-15T12:55:21Z")
The old polymorphization implenentation got removed per > > > \# Proposal > > I am writing this because I keep tripping over the polymorphization …implementation while working on post-mono MIR optimizations. I want there to be a point in the compiler where entire MIR bodies are fully monomorphized and then they stay monomorphized. The current polymorphization implementation breaks that mental model, in at least two ways: > \* Anything that has been touched by polymorphization must not be re-monomorphized. This is surprisingly fiddly to uphold, because the current monomorphization strategy is that each rustc crate defines its own \`fn monomorphize\` and uses it to monomorphize individual consts, operands, etc. > \* This comment is entirely correct; polymorphization means we cannot use \`TypingEnv::fully\_monomorphized\` even though this is after monomorphization. https://github.com/rust-lang/rust/blob/b19329a37cedf2027517ae22c87cf201f93d776e/compiler/rustc\_ty\_utils/src/abi.rs#L48-L63 > > So there's this rake, and I keep stepping on it, and it's quite annoying. > > I've done a GitHub code search for the flag and I cannot find any actual users: https://github.com/search?type=code&q=-Zpolymorphize+NOT+repo%3Arust-lang%2Frust++NOT+repo%3Amatthiaskrgr%2Ficemaker+NOT+repo%3Amatthiaskrgr%2Fglacier+NOT+repo%3Amatthiaskrgr%2Fglacier2+NOT+repo%3Arust-lang%2Fzulip\_archive+NOT+path%3A%C2%B7tests%2Fui%2F\*+NOT+path%3Atests%2Fui%2F\*%2F\*+NOT+path%3Atests%2Fui%2F\*%2F\*%2F\*+NOT+path%3A%C2%B7src%2Ftest%2Fui%2F\*+NOT+path%3Asrc%2Ftest%2Fui%2F\*%2F\*+NOT+path%3Asrc%2Ftest%2Fui%2F\*%2F\*%2F\*+NOT+repo%3Adavidtwco%2Fzulip\_archive+NOT+path%3Atests%2Fcrashes%2F\*.rs&p=1 > > It doesn't make a whole lot of sense to be spending maintenance effort on a feature that may have users we can't see, but they are so few that none are public on GitHub. > > There are a few more details about the current state and history in the feature's tracking issue, including a link to a sketch of a redesign: https://github.com/rust-lang/rust/issues/124962 > > \# Mentors or Reviewers > > \*If you have a reviewer or mentor in mind for this work, mention them > here. You can put your own name here if you are planning to mentor the > work.\* > > \# Process > > The main points of the \[Major Change Process\]\[MCP\] are as follows: > > \* \[x\] File an issue describing the proposal. > \* \[\] A compiler team member or contributor who is knowledgeable in the area can \*\*second\*\* by writing \`@rustbot second\`. > \* Finding a "second" suffices for internal changes. If however, you are proposing a new public-facing feature, such as a \`-C flag\`, then full team check-off is required. > \* Compiler team members can initiate a check-off via \`@rfcbot fcp merge\` on either the MCP or the PR. > \* \[\] Once an MCP is seconded, the Final Comment Period begins. If no objections are raised after 10 days, the MCP is considered \*\*approved\*\*. > > You can read \[more about Major Change Proposals on forge\]\[MCP\]. > > \[MCP\]: https://forge.rust-lang.org/compiler/mcp.html > > \# Comments > > \*\*This issue is not meant to be used for technical discussion. There is a Zulip stream for that. Use this issue to leave procedural comments, such as volunteering to review, indicating that you second the proposal (or third, etc), or raising a concern that you would like to be addressed.\*\* ---
**Author:** ![Noratrieb](https://sea2.discourse-cdn.com/flex002/user_avatar/internals.rust-lang.org/noratrieb/32/12440_2.png) [@Noratrieb](https://internals.rust-lang.org/u/Noratrieb)\ **Post date:** [January 15, 2026, 5:07pm UTC](https://internals.rust-lang.org/t/bridging-the-gap-between-rusts-theoretical-ir-advantages-and-real-world-performance/23927/3 "2026-01-15T17:07:03Z")
Please don't use large language models to write your posts. It creates a large asymmetry between the time put in by you and the time expected from others. ---
**Author:** ![scottmcm](https://sea2.discourse-cdn.com/flex002/user_avatar/internals.rust-lang.org/scottmcm/32/2355_2.png) [@scottmcm](https://internals.rust-lang.org/u/scottmcm)\ **Post date:** [January 15, 2026, 5:31pm UTC](https://internals.rust-lang.org/t/bridging-the-gap-between-rusts-theoretical-ir-advantages-and-real-world-performance/23927/4 "2026-01-15T17:31:59Z")
> [@gaowujie2004](#): > > Despite these theoretical advantages, real-world benchmarks often show Rust performing on par with, or sometimes slightly trailing (~1-5%), highly tuned C/C++. Do you have an example? Speaking in the abstract often goes less well than just looking specifically at one particular comparison where there can be a specific thing to address. > [@gaowujie2004](#): > > Is the long-term strategy to rely more on **MIR Optimizations** (like Destination Propagation) to "clean up" the code before it reaches LLVM, or is the focus shifting towards upstreaming **Rust-specific optimization passes** (or tuning heuristics) directly into LLVM? MIR is a poor IR for writing complex optimizations, so anything more than relatively-simple ones are best done in LLVM (or potentially in a hypotheitical "LIR" Low-level IR). ---
**Author:** ![gaowujie2004](https://sea2.discourse-cdn.com/flex002/user_avatar/internals.rust-lang.org/gaowujie2004/32/13979_2.png) [@gaowujie2004](https://internals.rust-lang.org/u/gaowujie2004)\ **Post date:** [January 16, 2026, 6:18am UTC](https://internals.rust-lang.org/t/bridging-the-gap-between-rusts-theoretical-ir-advantages-and-real-world-performance/23927/6 "2026-01-16T06:18:26Z")
我让ai帮我翻译，因为我不懂英文 ..... 抱歉 ---
**Author:** ![Noratrieb](https://sea2.discourse-cdn.com/flex002/user_avatar/internals.rust-lang.org/noratrieb/32/12440_2.png) [@Noratrieb](https://internals.rust-lang.org/u/Noratrieb)\ **Post date:** [January 16, 2026, 12:14pm UTC](https://internals.rust-lang.org/t/bridging-the-gap-between-rusts-theoretical-ir-advantages-and-real-world-performance/23927/7 "2026-01-16T12:14:25Z")
Using an automated translator is fine, but please use a traditional translation tool like Google Translate instead of an LLM, as LLMs tend to bad things to the text that mess up the meaning and styling while Google Translate will keep the translated version as close as possible. ---
**Author:** ![thehatless](https://sea2.discourse-cdn.com/flex002/user_avatar/internals.rust-lang.org/thehatless/32/10317_2.png) [@thehatless](https://internals.rust-lang.org/u/thehatless)\ **Post date:** [January 16, 2026, 5:43pm UTC](https://internals.rust-lang.org/t/bridging-the-gap-between-rusts-theoretical-ir-advantages-and-real-world-performance/23927/8 "2026-01-16T17:43:55Z")
Google translate is an llm that's optimised for speed rather than quality. In my experience, it mangles the meaning of text much more often than other tools. (But it adopted the technology earlier than most, so people think of it as Virtuous Old rather than Frightening New, I guess.)