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Comment by steveklabnik | Hacker News Reader

It's not a big-O thing. The optimization passes aren't necessarily huge either; right now, 50% of the time is in LLVM compiling IR -> machine code. The static checks, optimization passes, and everything else are minuscule overall. You can see this with -Z time-passes.

We have some hunches, but nothing super conclusive yet; basically, right now it's all about how much IR we generate.

Don't forget the differences in compilation model; C/C++'s compilation units are much smaller, so incremental isn't as big of a deal.

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Is there a way to generate IR which is easier on LLVM? The Jai compiler compiles 60kloc with an llvm backend in seconds. Could performance like this be seen in rustc?

Possibly; we'll see. MIR optimization passes may help.

It's hard to say what exactly Jai does since it's not public yet.

Apparently that's :

https://inductive.no/jai/

> Don't forget the differences in compilation model; C/C++'s compilation units are much smaller

Not everyone compiles C++ the same way. I prefer header-only libraries and a single compilation unit. Counting lines of code with "gcc -E" to get the headers, I just compiled 20,000 lines of C++ from scratch in .6 seconds with gcc and .4 seconds with clang. I don't have a comparable project in Rust.

How much code did the preprocessor strip out?

No idea, maybe a lot from the system headers. However, it was 20,000 lines of code that survived the preprocessor, 11,000 of which were inline or template from the project itself.

- A large fraction of compiler time is spent on code generation; how many of those lines did the compiler actually generate code for? (i.e. not unused functions or uninstantiated templates)

(And in the case of uninstantiated templates, a C++ compiler doesn't even need to type check them…)

- Did you have optimizations enabled?

> how many of those lines did the compiler actually generate code for?

Not many. How many lines would a comparable Rust program have to instantiate? (should be the same)

All I was trying to do was make an apples to apples comparison by discrediting the implication that the clang++ and g++ compilers are only faster because people use separate compilation units.

I think it's fair to compare Rust and C++ this way, and if Rust is slower to compile, then it's fair to ask why.

> Did you have optimizations enabled?

Yes, -O3. Also -Wall -Wextra and -fwrapv if you're interested.

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