I don't know if it has to do with generics, but you can't blame it on LLVM.
/usr/src/rust/src/libsyntax % time /usr/src/rust/x/x86_64-apple-darwin/stage2/bin/rustc lib.rs -o /tmp/x.dylib --crate-type dylib -C prefer-dynamic -Z time-passes
[most output removed...]
time: 6.186 s type checking
time: 5.084 s translation
time: 7.221 s LLVM passes
/usr/src/rust/x/x86_64-apple-darwin/stage2/bin/rustc lib.rs -o /tmp/x.dylib 22.44s user 0.76s system 99% cpu 23.301 total
First of all: no more than 1/3 to 1/2 of the time is spent in LLVM in this particular example. Okay, that's cheating because there is no -O, as I'm guessing your statement supposed, but 22 seconds is already a huge amount of time to compile 30,000 lines of code, so unoptimized builds are relevant to claims that rustc is slow. The other points apply regardless of optimization setting.
Second: rustc will take this long every time to recompile libsyntax every time anything in it changes. If this were written in C, most changes would only require recompiling one source file, even though, again, header files are not treated well (something that Rust does not need to replicate). In practice this means that typical latency between changing something and seeing the output in a C/C++ program is an order of magnitude faster.
Third: The same separation that makes incremental compilation work in C/C++ allows parallelism (make -j) in full builds. rustc uses only one core per crate. Again, headers reduce the gains but Rust doesn't have that problem.
Fourth: If we compare to C rather than C++, we're off by sometimes an order of magnitude regardless of parallelism. Here is some random C program (Apple as), a total of 26929 lines, compiling in 0.90 seconds:
clang -o foo -I ../include -I ../include/gnu -I. -DNeXT_MOD -DI386 -Di486 0.73s user 0.16s system 98% cpu 0.904 total
(With optimizations it is 2.426 seconds.)
Or libpng, 32433 lines in 0.83 seconds:
clang -o png *.c -I. -lz 0.70s user 0.12s system 98% cpu 0.831 total
With the Linux kernel, compiled without -j and at -O1 using GCC (both of which make it slower), it's not an order of magnitude off, but it's still significantly faster than Rust:
make ARCH=arm zImage 540.25s user 73.71s system 96% cpu 10:33.81 total
It compiled 1572713 lines of code; normalizing to 30,000 gives us about 12 seconds.
On the Rust side, linking libstd, about the same size, takes 6 seconds, but librustc, three times the size, took 216 seconds (10 times as long as libsyntax) to get to linking. So it varies, but I guess libsyntax is representative.
I do not know enough to have a definitive opinion of why this difference exists, but judging from the difference between C and C++, I wouldn't be surprised if it were related to Rust's complexity, which includes generics.
> First of all: no more than 1/3 to 1/2 of the time is spent in LLVM in this particular example.
"translation" is mostly LLVM (in particular, allocation of LLVM data structures).
> I do not know enough to have a definitive opinion of why this difference exists, but judging from the difference between C and C++, I wouldn't be surprised if it were related to Rust's complexity, which includes generics.
If you look at the amount of code in a typical large Rust program that is related to generic instantiations, then it's pretty miniscule (less than 10%).
Typechecking is known to be slow because the way we do method lookups is not well optimized. I don't believe that this is a fundamental issue; it's more that nobody has gotten around to improving it yet.
I don't think the rust compiler has been particularly extensively optimised at this point. There's no blindingly obvious hotspots (last I profiled the biggest single time sink was hashmap lookups in type checking), but there's not been any real effort in reducing compile time yet, as far as I know.
Second: rustc will take this long every time to recompile libsyntax every time anything in it changes. If this were written in C, most changes would only require recompiling one source file, even though, again, header files are not treated well (something that Rust does not need to replicate). In practice this means that typical latency between changing something and seeing the output in a C/C++ program is an order of magnitude faster.
Third: The same separation that makes incremental compilation work in C/C++ allows parallelism (make -j) in full builds. rustc uses only one core per crate. Again, headers reduce the gains but Rust doesn't have that problem.
Fourth: If we compare to C rather than C++, we're off by sometimes an order of magnitude regardless of parallelism. Here is some random C program (Apple as), a total of 26929 lines, compiling in 0.90 seconds:
(With optimizations it is 2.426 seconds.)Or libpng, 32433 lines in 0.83 seconds:
With the Linux kernel, compiled without -j and at -O1 using GCC (both of which make it slower), it's not an order of magnitude off, but it's still significantly faster than Rust: make ARCH=arm zImage 540.25s user 73.71s system 96% cpu 10:33.81 totalIt compiled 1572713 lines of code; normalizing to 30,000 gives us about 12 seconds.
On the Rust side, linking libstd, about the same size, takes 6 seconds, but librustc, three times the size, took 216 seconds (10 times as long as libsyntax) to get to linking. So it varies, but I guess libsyntax is representative.
I do not know enough to have a definitive opinion of why this difference exists, but judging from the difference between C and C++, I wouldn't be surprised if it were related to Rust's complexity, which includes generics.