It does make optimization easier but until MIR landed, many of the best optimizations weren't really possible. The problem is that a lot of type information is lost between Rust and LLVM IR, where the compiler does the really serious optimizations. For example, Rust can't tell LLVM about its pointer aliasing guarantees (immutable and mutable borrows can't be done at the same time without unsafe) so a lot of optimizations like storing a highly used value from the heap in a register are passed over because of conservative heuristics.

Now that MIR has landed, Rust will eventually get much better optimizations from both rustc and the llvm optimization passes but other things are a much higher priority like non-lexical scoping.

> For example, Rust can't tell LLVM about its pointer aliasing guarantees

False.

http://llvm.org/docs/LangRef.html#noalias-and-alias-scope-me...

From what I understand, LLVM still doesn't take as much advantage of that information as it could, given Rust input. It's too geared toward the C family of languages. (But as sibling comment says, the problem was partially the Rust compiler's fault.)

C has a pointer aliasing keyword, "restrict".

Also: "Originally implemented for C and C++, the language-agnostic design of LLVM has since spawned a wide variety of front ends: languages with compilers that use LLVM include ActionScript, Ada, C#,[4][5][6] Common Lisp, Crystal, D, Delphi, Fortran, OpenGL Shading Language, Halide, Haskell, Java bytecode, Julia, Lua, Objective-C, Pony,[7] Python, R, Ruby, Rust, CUDA, Scala,[8] and Swift."

Most of these are nothing like C when it comes to pointers and memory layout.

Again, from what I understand, `restrict` is not enough to convey everything Rust knows.

Further, those other languages may be nothing like C, but they're even less like Rust. So because of its heritage, even given the information Rust knows, LLVM simply doesn't have the optimization passes to take full advantage of it.

Ok. Like what ?

The point was that Rust _can_ (more easily) tell LLVM now that MIR has landed.

It's not only a MIR issue, it's also an issue of how many optimizations should be allowed when you also need to be able to make some assumptions about your data in unsafe blocks. For example if you simply added the obvious attributes everywhere, interior mutability (Cell, RefCell, UnsafeCell) would most certainly break. The exact rules around unsafe rules are still being discussed (see https://github.com/nikomatsakis/rust-memory-model), so until that is done some of these optimizations are very unlikely to be implemented because they can make a lot of unsafe code illegal.

Note that the compiler already adds "obvious" attributes to function calls, being careful to not emit them for things that contain an `UnsafeCell`. This is why that type exists, as the building block for interior mutability that the compiler understand and can optimise around.

I guess having taken a compilers class a decade ago has given me a Dunning-Kruger sense of optimism but the explanation sounds kind of like bullshit to me. So what if LLVM doesn't have the type information? There's no type information at all in the machine code… optimize stuff yourself before you hand it to LLVM?

LLVM is literally designed as an production quality optimisation/code-generation backend, so it's seems perfectly reasonable for projects to rely on LLVM's optimisations without doing their own, especially when there are other things people can work on than the large effort to optimise some programs slightly better (and even fewer programs a lot better, but these are often microbenchmarks, or tight loops like the OP that can be diagnosed with a profiler relatively easily and rewritten) by doing language-specific optimisations. It's definitely a good long-term goal, but it requires a lot of infrastructure, and ends up requiring duplicating a lot of the optimisations and analysis already in LLVM (e.g. inlining is a critical transformation for enabling other optimisations) before one can get great results from the custom optimisations.

In any case, compiler optimisations are usually easy to implement on specific styles of internal representations, and this is MIR for Rust, which was only introduced recently, after requiring a very large refactoring of the compiler's internal pipeline.

As I understand it, this is partially the plan. Historically it has just been waiting for a compiler refactor to MIR (which is now complete). I'm not sure why more optimization RFCs haven't been created and prioritized.