[remcob]

Not necessarily. Because C allows the pointer manipulation, the compiler can in general not make assumptions about pointer aliasing. This prevents some optimizations.

In Rust, the compiler has more information/control over memory layout/lifetime and can therefore make stronger optimizations.

Automatic vectorization is an area where this helps a lot, and raytracing can benefit a lot here. 20% sounds reasonable to me.

[bjourne]

Well, generally, perhaps. But any performance oriented C programmer worth his or her salt would be aware of aliasing issues and write code in such a way that it doesn't cause problems for the compiler. Plus, this is a toy benchmark of a few hundred lines so the compiler can do full-program analysis. So the 20% difference is indeed a smell.

Looking at the crb*.c files, structs are passed as pointers and not by value. This makes it harder for the compiler to analyze the data flow which I would bet is part of the reason Rust is faster here.

[cesarb]

> pointer aliasing

Unfortunately, due to LLVM bugs, the Rust developers had to disable that optimization, more than once. I don't know whether the "1.13.0-nightly" he used has that optimization enabled or disabled. (See https://github.com/rust-lang/rust/issues/31681 and https://github.com/rust-lang/rust/issues/54878 for the relevant Rust issues.)

[oconnor663]

That's a good point, I didn't think about the effect on autovectorization. Do you think that's what's happening here? My impression was that getting good vector code out of the compiler usually requires manually tuning things.