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Comment by pcwalton | Hacker News Reader

> There were compilers in the 90s which ran a million lines per second (on much slower computers).

Those compilers performed nowhere near the level of optimization that modern compilers do.

> I'm sure some of the LLVM optimization passes are expensive, but those are used in clang++ too, and it's not terribly slow.

clang++ is sure slow if it has to rebuild an entire codebase from scratch.

The main reason that C++ compilers get away with not having partial compilation is that it forces programmers to do the division into code units manually through separate .h and .cpp files. Rust doesn't have this, except at the crate level, so it needs partial compilation instead. It turns out that partial compilation is better in the first place, because compilation units tend to be fairly coarse-grained: .cpp files frequently grow very large.

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> Those compilers performed nowhere near the level of optimization that modern compilers do.

I think this gets at Wirth's law a bit. Yes compilers perform lots of optimizations now, but in Rust's case it is at least partly because they need to in order to claw back the performance that the Rust gives up with its abstraction (as seen in debug builds).

I do much prefer Rust's safe abstractions to writing more direct code and hoping I get it right. I hope (and do believe) that the MIR effort will somewhat rein in the burden shunted over to LLVM, as it is super painful for some of us.

> I think this gets at Wirth's law a bit.

I think Proebsting's law is also relevant. Compilers did an impressive job of optimization in the 90s, and while yes there have been advancements in the last 18 years, it's foolish to over-estimate the progress.

Also, considering how much hardware changes have improved performance (speculative execution, register renaming, etc...), it wouldn't surprise me if many compiler optimizations have become less important with time. For instance, peep-hole instruction scheduling has to matter less than it once did.

Have you guys spend any time with the incremental compilation and linking of Visual C++ or C++ Builder?

It really does make a difference versus traditional UNIX compilers.

clang++ is slow, but it certainly feels faster compared to rustc (and I typically use the write everything in one include C++ style). I’m curious, has there been any outreach to the clang community about rustc performance issues?

Not going to budge an inch, eh? It must be as good as it will ever get.

I don't know what this means.

It means things don't usually get better until you admit they're not as good as they can be.

The rust compiler provide huge guaranties no other provide, like for memory safety. This has a performance impact.

The memory safety checking (or more generally, the type checking) isn't what makes the compiler slow. That part is pretty fast because it's intentionally modular.

It's the cross-module optimizations that make the compiler slow. Those optimizations are in service of C++-inspired ideal of being able to use layers of abstraction to make your code cleaner, but having the compiler flatten them down so your program still runs fast.

I hope you see the irony in accusing them of that in a post announcing that the compiler just got better.

It's not ironic. I see this kind of thing all the time. Rather than cleanup and speedup an implementation, engineers will make it multi-threaded or send it off to a GPU, generally achieving limited improvements and greater complexity. In this case, rather than make the compiler faster, they hack it up with incremental compilation. It's still slow, but they apply the slow code to less data and call it a win.

Nobody who reads the rustc commit history would agree that this is what is going on. The number of straightforward changes that are constantly made in the service of small performance improvements far outweighs the work on incremental compilation.

I’m pretty sure no one in their right mind claims that rustc is “as good as can be.”

It’s called incremental progress.

rustc could definitely be faster.

I'm simply pushing back on the idea that there's some obvious low-hanging fruit that clang did that rustc didn't. Compiler performance has more or less much been the #1 goal for well over a year now. The Rust compiler team (which hasn't included me for years, by the way) is fantastic. We'd all like for there to be some magic bullet that makes the compiler faster, but at this point I don't believe it exists. It's hard work from here on out.

> I'm simply pushing back on the idea that there's some obvious low-hanging fruit that clang did that rustc didn't. [...] magic bullet

I didn't suggest low hanging fruit or a magic bullet. If anything, incremental compilation is being promoted as that kind of thing, and I just wanted call it into question.

Nobody thinks incremental compilation is a magic bullet! The article specifically says "This is still not the end story for compiler performance generally, nor incremental compilation specifically."

Is it they who didn't "budge an inch", or you, that despite a reasoned explanation, insist on the original accusation?

Not to mention the underlying insinuation, that the Rust compiler developers are dumb for not being able to get at the 80s compiler's level of speed -- since you don't seem to accept that this is a byproduct of the advanced optimizations and security guarantees they do.

My criticisms are meant to point out areas for actual improvement.

Try to be scientific. If you've got a conjecture, test it and see if it holds water. One excuse was that C++ is only faster because people build C++ projects with separate compilation units. Well, you can compile a project in C++ with a single compilation unit, and it's not horribly slow. Now you need a new conjecture.

The type system is comparable to Standard ML from the 70s. The lifetime analysis is a solid improvement over mainstream languages, but I have a tough time believing it needs to be an order of magnitude slower than const correctness. The optimizations are on par with C++ and Fortran (mostly because they're leveraging the guts from an existing C++ compiler). SML, C++, and Fortran do not compile as slowly as Rust. There's probably another reason.

I don't think the Rust compiler developers are dumb, and I never said that. I think several of them are brilliant but young (and occasionally very arrogant).

Rust is so close to being a great replacement for C++. I read every one of these threads about new releases hoping they'll eventually fix the problems. Compilation speed and memory use are lesser issues, but they're still important, and things like incremental compilation dodge the issues instead of fixing them.

Borrow checking is a lot more involved than const correctness. It's not even in the same league. Const correctness just adds a few more types, whereas borrow checking involves generating sets of constraints and solving them with a constraint solver, something C++ doesn't really have to do at all.

The type system of Rust goes significantly beyond that of Standard ML, because of traits/typeclasses among many other features. Also, Standard ML is usually not implemented with monomorphization, which affects compile time.

> Compilation speed and memory use are lesser issues, but they're still important, and things like incremental compilation dodge the issues instead of fixing them.

Incremental compilation is a way of addressing the compile time and memory usage issues. It's just not the way you seem to want them to be fixed. Nobody did incremental compilation for fun; it was done because without it we didn't believe we could reach our compiler performance goals.

Again, you're asking for a magic bullet, and I'm saying it's not likely that there is going to be one. If one existed, it probably would have been found already. There's lots of hard work that has been done, and lots of hard work ahead.

If I had to guess what the most important issue is, it's that idiomatic Rust generates a lot more LLVM IR than idiomatic C++ does, because it leans on abstractions for safety and developer ergonomics. Think about how many more closures Rust code uses, what for loops expand to, what pointer arithmetic compiles to (ptr::offset), etc...

I'm not going to dive into your points about constraint solvers, type classes, or "monomorphization". Unless those things take more than a few percent of the compile time, they're just more red herrings.

> Incremental compilation is a way of addressing the compile time and memory usage issues. It's just not the way you seem to want them to be fixed. Nobody did incremental compilation for fun.

I'm well aware that my opinion doesn't matter, but I doubt incremental compilation will make any real strides towards making things as good as they could be. It sounds like the real problem is here:

> If I had to guess what the most important issue is, it's that idiomatic Rust generates a lot more LLVM IR than idiomatic C++ does

I suspect every single feature in idiomatic Rust has a one-to-one translation to some bit of C++ that isn't too horrible. You can do lambdas, iterators, generators, or whatever else it takes to do Rust style for loops and closures in C++, and it won't choke the compiler. So while I don't doubt rustc is generating a lot more IR, I doubt it needs to.

Maybe the MIR optimization pass will move in that direction.

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