[nine_k]

To sum up: LLVM is heavily optimized for C code, not so much for Rust. So Rust code has to imitate certain C mannerisms for the optimizer to kick in.

You have to pay the price of compatibility with the existing toolchain even if it's not your explicit goal.

[oconnor663]

I'm not sure this is the optimizer's fault. If I'm doing a bounds-checked array access that might panic, that panic is an observable side effect that cannot be reordered with respect to other observable side effects. That puts constraints on what the optimizer can do, not because it's not smart enough, but because it has to respect the meaning of the program.

[gameswithgo]

there are aspects of Rust that are easier for llvm to optimize with than c. there are things rust isn't yet doing that it will do to make llvm be able to optimize it better as well.

the primary reason THIS code didn't optimize as well is that you can't pass ffast-math to llvm from rust in any useful way (yet)