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by oconnor663 2618 days ago
Different languages' benchmarks might not be equally well-written / optimized. In particular, I'd expect C and Rust to be very close to each other, and a 20% gap between them is a red flag.

Rules like "code should be simple, as in, easy to read and understand" are also hard to judge, especially near the top of the list where there's a lot of pressure to optimize. Is SIMD easy to understand? What if it's in a library? What if the library was written specifically for this benchmark? Etc. I think https://benchmarksgame-team.pages.debian.net/benchmarksgame/ has to deal with every possible permutation of this debate.
1 comments
remcob 2618 days ago
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.

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bjourne 2618 days ago
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.

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cesarb 2618 days ago
> 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.)

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oconnor663 2618 days ago
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.
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