I'm glad to see the donations from big companies to open source maintainers are making measurable difference to the Rust experience. Telling these companies that their employees spend 5% less time waiting for compilation might motivate future investment in people like Nick and the others mentioned.
Doing stuff isn't free. For instance, Go compiles relatively quickly for a modern language, but the biggest reason for that is that it does less stuff than most compilers... less optimization, less checking, and some stuff built into the language to avoid some of the problems with having to read lots of headers just to compile a file and other ways of doing less stuff, but mostly the key is it does less stuff, in both the good and bad senses of that.
If you want something like Rust that offers guarantees and checks and cross-checks by the boatload, it adds up. Macros, monomorphization, implicit code generation with traits and all those other things add up too. And you can't always get O(n) or O(n log n) code to implement those checks. Maybe it can be sped up and maybe there's tricks here or there, but at the Pareto frontier, a language that has more checks will be slower to compile than one that has fewer.
And that's not a bad thing or a deficit in Rust, it's just the nature of the beast.
Rust wants badly to have its cake and eat it too. I get it, but it's not the sensibilities I have. The rustc compiler, if it really can achieve everything all at once, will be a beast like none other, making C++ compilers look simple by comparison.
I'd love something halfway. Go is maybe a bit radical in some regards, but also, with the news of the new SIMD package for Go, it has occured to me just how little I missed having things like, say, autovectorization.
(I know also that some people have tried halfway, but the big thing is figuring out how to keep a relatively simple type system that can still support a borrow checker. Even if there is some middleground, is it truly worth it? As nice as it sounds, I've been more skeptical. Go seems to exist in a very narrow space where its simplifications barely can be made to work.)
And since you bring up Go and contrast the compile time with Rust, it's been experienced at Google (and also Volvo and other places) that Rust and Go teams are as productive whereas C++ is less than half as productive: https://www.youtube.com/watch?t=27012&v=6mZRWFQRvmw&feature=...
So the focus on Rust compile time is misplaced. It's not a big deal in terms of overall productivity.
I don't think that follows - just that compile times aren't the only thing that matters. Maybe rust could be twice as productive as go if compile times went to zero for instance (or maybe not - just disputing that the evidence proves the claim here).
But it can't go to zero, that was the point -- at some level you have to pay for what you get. I'm not saying the compile times couldn't be better, but Rust can't be Go in terms of compile times if it wants to offer the guarantees and options it does. Meanwhile if Go wants to offer more options and stronger guarantees, it won't be able to preserve the short compile times everyone points to.
The Cranelift backend is extremely fast. Most of the slowness is all the crazy optimizations that LLVM does + other things (generating debug info etc).
Zero-cost abstractions aren't zero cost in compilation time. High-level abstractions translate to a lot of boilerplate that the compiler has to optimize out.
In unoptimized builds often the linker is the bottleneck. Rust/Cargo can parallelize most of the build, generating tons of code and debug info, but then the poor linker has to consume all of it at once. The object/exe formats were designed in ancient times, so they're hard to build incrementally or in parallel (some linkers are trying).
Is there any experimentation with new ABIs? I mean certain linker flags like --f-lto literally hijack the linker protocol to dump an AST into the backend.
At least for the fully static binary part of rust, there should be some optimizations there w.r.t. compilation. Sure you're not going to interface with shared libraries well but maybe a small experimental feature for fully owned projects? Idk.
I once heard (and don't know if it's still true) that the biggest compile-time sinks are macros and codegen.
Both are kind of outside the Rust compiler's influence. Macros can be almost arbitrarily complex: you pay for what you order. Codegen is LLVM, and that's a fixed choice. You can use Cranelift to get around it, but then you pay elsewhere.
Also, generics and monomorphization regularly come up in these discussion, while common wisdom seems to be that cost for the additional static analysis over other languages like C++ is no a major contributor.
Regardless, it's nice to see performance improvements in the compiler, even if you have to cooperate to benefit from them (e.g. by keeping your macros light and use less generics).
First of all, the fact that the article talks about "speeding up" the rust compiler doesn't automatically mean that the compiler is "slow"[0].
Now, is rustc slower than e.g. clang? by how much? why?
Those are different (and complicated) questions. It really depends on what you're compiling, but I'd say rustc can be 1-5x slower (maybe more at times?).
The reasons are many and varied, but in general rust compilation is slower because the compiler is doing way more things compared to C (monomorphization, complex trait resolution + type inference, borrow checker..)
[0]: Also I'd argue that "slow" without a concrete point of reference is a meaningless term in this context.
The go compiler is considered fast, yet Google spends effort speeding up the go compiler. Not to the same extent, but Google has a lot of go code, so it does make a difference.
Its similar to C++ compilers, there is no one reason. It is a language that tries to optimize a lot, its a big language, it does safety checks, it uses llvm which is a bit slow, its a language that makes use of generics which generate extra code etc etc.
This is not actually the main reason, most of the time.
Generics/monomorphization and how iterators work results in a lot of compiler bytecode that has to be churned through. More bytecode = longer compilation. It increases the size of the (debug) binaries, the debuginfo in general, causes performance issues with debug binaries in some situations unless you bump the optimization level, causes more IO, etc.
It's unlikely that many people are using Rust without using Option<T> or Result<T, U> a fair bit. Idiomatic Rust fundamentally uses a lot of generics. And a basic for loop expands into quite a lot of intermediate representation due to Iterator.
Other languages support generics, monomorphization, and iterators (e.g. Zig or D), but they're not as slow as Rust to compile, what's the reason for that?
It's likely easier to answer for specific languages
All three of those words can mean "just like Rust" but equally "Not at all like Rust" for different languages.
Two examples to contrast: In C++ the iterators are basically a pointer analog (in some cases they're just literally pointers) and that's a very difference "feature" but it's still definitely iterators. In Ginger Bill's Odin, the iterators are a function, possibly generic, which returns a pair, the next item and a boolean telling you whether the iterator was exhausted.
Incremental seems to keep winning the easy wins, so the interesting part is whether the remaining compile-time still lives in the same places as last year.
The EverInitializedPlaces example really stands out. Going from ~1.5M to ~90K apply_effects_in_block calls by changing the CFG traversal is a reminder that the biggest compiler optimizations often come from changing the algorithm, not optimizing the hot loop itself.
It also seems like the new Polonius/trait-solver work is pushing compiler performance toward a more interesting problem: doing expensive analysis only when it is actually needed.
4.57% mean wall-time reduction across 629 benchmarks in two months is pretty remarkable. Great progress.
> reminder that the biggest compiler optimizations often come from changing the algorithm, not optimizing the hot loop itself.
Isn't this true for most optimizations, not just in compilers? My usual goto process for optimizing is "Find stuff we're doing that we don't have to do, re-evaluate what data structures we use and then re-evaluate what algorithms we use" basically, with minor changes depending on the results. Served me well so far, and haven't (intentionally) written any compilers.
I'm glad to see the donations from big companies to open source maintainers are making measurable difference to the Rust experience. Telling these companies that their employees spend 5% less time waiting for compilation might motivate future investment in people like Nick and the others mentioned.
Really nice to see that the 5% speedup is despite making the borrow checker better, validating code that previously would have tripped it up.
Sometimes we really can have our cake and eat it too.
Why is the compiler slow in the first place? I have no rust knowledge, how slow us slow, lets say in comparison to a c compiler?
What is the performance killer?
Doing stuff isn't free. For instance, Go compiles relatively quickly for a modern language, but the biggest reason for that is that it does less stuff than most compilers... less optimization, less checking, and some stuff built into the language to avoid some of the problems with having to read lots of headers just to compile a file and other ways of doing less stuff, but mostly the key is it does less stuff, in both the good and bad senses of that.
If you want something like Rust that offers guarantees and checks and cross-checks by the boatload, it adds up. Macros, monomorphization, implicit code generation with traits and all those other things add up too. And you can't always get O(n) or O(n log n) code to implement those checks. Maybe it can be sped up and maybe there's tricks here or there, but at the Pareto frontier, a language that has more checks will be slower to compile than one that has fewer.
And that's not a bad thing or a deficit in Rust, it's just the nature of the beast.
Rust wants badly to have its cake and eat it too. I get it, but it's not the sensibilities I have. The rustc compiler, if it really can achieve everything all at once, will be a beast like none other, making C++ compilers look simple by comparison.
I'd love something halfway. Go is maybe a bit radical in some regards, but also, with the news of the new SIMD package for Go, it has occured to me just how little I missed having things like, say, autovectorization.
(I know also that some people have tried halfway, but the big thing is figuring out how to keep a relatively simple type system that can still support a borrow checker. Even if there is some middleground, is it truly worth it? As nice as it sounds, I've been more skeptical. Go seems to exist in a very narrow space where its simplifications barely can be made to work.)
Yeah doing optimizations in a compiler on things like loops, addition, or string layouts is never free.
And since you bring up Go and contrast the compile time with Rust, it's been experienced at Google (and also Volvo and other places) that Rust and Go teams are as productive whereas C++ is less than half as productive: https://www.youtube.com/watch?t=27012&v=6mZRWFQRvmw&feature=...
So the focus on Rust compile time is misplaced. It's not a big deal in terms of overall productivity.
I don't think that follows - just that compile times aren't the only thing that matters. Maybe rust could be twice as productive as go if compile times went to zero for instance (or maybe not - just disputing that the evidence proves the claim here).
But it can't go to zero, that was the point -- at some level you have to pay for what you get. I'm not saying the compile times couldn't be better, but Rust can't be Go in terms of compile times if it wants to offer the guarantees and options it does. Meanwhile if Go wants to offer more options and stronger guarantees, it won't be able to preserve the short compile times everyone points to.
The Cranelift backend is extremely fast. Most of the slowness is all the crazy optimizations that LLVM does + other things (generating debug info etc).
Zero-cost abstractions aren't zero cost in compilation time. High-level abstractions translate to a lot of boilerplate that the compiler has to optimize out.
In unoptimized builds often the linker is the bottleneck. Rust/Cargo can parallelize most of the build, generating tons of code and debug info, but then the poor linker has to consume all of it at once. The object/exe formats were designed in ancient times, so they're hard to build incrementally or in parallel (some linkers are trying).
Is there any experimentation with new ABIs? I mean certain linker flags like --f-lto literally hijack the linker protocol to dump an AST into the backend.
At least for the fully static binary part of rust, there should be some optimizations there w.r.t. compilation. Sure you're not going to interface with shared libraries well but maybe a small experimental feature for fully owned projects? Idk.
I once heard (and don't know if it's still true) that the biggest compile-time sinks are macros and codegen.
Both are kind of outside the Rust compiler's influence. Macros can be almost arbitrarily complex: you pay for what you order. Codegen is LLVM, and that's a fixed choice. You can use Cranelift to get around it, but then you pay elsewhere.
Also, generics and monomorphization regularly come up in these discussion, while common wisdom seems to be that cost for the additional static analysis over other languages like C++ is no a major contributor.
Regardless, it's nice to see performance improvements in the compiler, even if you have to cooperate to benefit from them (e.g. by keeping your macros light and use less generics).
First of all, the fact that the article talks about "speeding up" the rust compiler doesn't automatically mean that the compiler is "slow"[0].
Now, is rustc slower than e.g. clang? by how much? why?
Those are different (and complicated) questions. It really depends on what you're compiling, but I'd say rustc can be 1-5x slower (maybe more at times?).
The reasons are many and varied, but in general rust compilation is slower because the compiler is doing way more things compared to C (monomorphization, complex trait resolution + type inference, borrow checker..)
[0]: Also I'd argue that "slow" without a concrete point of reference is a meaningless term in this context.
> the fact that the article talks about "speeding up" the rust compiler doesn't automatically mean that the compiler is "slow"
Well, if it weren't "slow" for some definition of "slow", nobody would bother speeding it up, would they?
The go compiler is considered fast, yet Google spends effort speeding up the go compiler. Not to the same extent, but Google has a lot of go code, so it does make a difference.
Its similar to C++ compilers, there is no one reason. It is a language that tries to optimize a lot, its a big language, it does safety checks, it uses llvm which is a bit slow, its a language that makes use of generics which generate extra code etc etc.
It's doing static analysis that many other languages don't do at compile time.
This is not actually the main reason, most of the time.
Generics/monomorphization and how iterators work results in a lot of compiler bytecode that has to be churned through. More bytecode = longer compilation. It increases the size of the (debug) binaries, the debuginfo in general, causes performance issues with debug binaries in some situations unless you bump the optimization level, causes more IO, etc.
If you don't use generics and monomorphization, then what explains it?
It's unlikely that many people are using Rust without using Option<T> or Result<T, U> a fair bit. Idiomatic Rust fundamentally uses a lot of generics. And a basic for loop expands into quite a lot of intermediate representation due to Iterator.
Other languages support generics, monomorphization, and iterators (e.g. Zig or D), but they're not as slow as Rust to compile, what's the reason for that?
It's likely easier to answer for specific languages
All three of those words can mean "just like Rust" but equally "Not at all like Rust" for different languages.
Two examples to contrast: In C++ the iterators are basically a pointer analog (in some cases they're just literally pointers) and that's a very difference "feature" but it's still definitely iterators. In Ginger Bill's Odin, the iterators are a function, possibly generic, which returns a pair, the next item and a boolean telling you whether the iterator was exhausted.
AFAIK it's because of trait solving. And also not every language does monomorphization.
Not sure why you are being downvoted here.
I'm surprised the OpenAI Codex team doesn't donate like 10B tokens or something to the Rust team for performance
LLM written code is disallowed by the rust project policy
https://forge.rust-lang.org/policies/llm-usage.html
It doesn't seem to be quite that simple?
https://forge.rust-lang.org/policies/llm-usage.html#experime...
Biggest win for me was splitting one fat crate into three, parallelism got real.
Gonna have to name my next project Penelope Hammertime, or at least Pineapple Häagen-Dazs.
Incremental seems to keep winning the easy wins, so the interesting part is whether the remaining compile-time still lives in the same places as last year.
The EverInitializedPlaces example really stands out. Going from ~1.5M to ~90K apply_effects_in_block calls by changing the CFG traversal is a reminder that the biggest compiler optimizations often come from changing the algorithm, not optimizing the hot loop itself.
It also seems like the new Polonius/trait-solver work is pushing compiler performance toward a more interesting problem: doing expensive analysis only when it is actually needed.
4.57% mean wall-time reduction across 629 benchmarks in two months is pretty remarkable. Great progress.
> reminder that the biggest compiler optimizations often come from changing the algorithm, not optimizing the hot loop itself.
Isn't this true for most optimizations, not just in compilers? My usual goto process for optimizing is "Find stuff we're doing that we don't have to do, re-evaluate what data structures we use and then re-evaluate what algorithms we use" basically, with minor changes depending on the results. Served me well so far, and haven't (intentionally) written any compilers.