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Zlib-rs is faster than C

(trifectatech.org)
341 points dochtman | 1 comments | 16 Mar 25 19:35 UTC | HN request time: 0.223s | source
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YZF ◴[16 Mar 25 20:12 UTC] No.43381858[source]▶
I found out I already know Rust:

        unsafe {
            let x_tmp0 = _mm_clmulepi64_si128(xmm_crc0, crc_fold, 0x10);
            xmm_crc0 = _mm_clmulepi64_si128(xmm_crc0, crc_fold, 0x01);
            xmm_crc1 = _mm_xor_si128(xmm_crc1, x_tmp0);
            xmm_crc1 = _mm_xor_si128(xmm_crc1, xmm_crc0);
Kidding aside, I thought the purpose of Rust was for safety but the keyword unsafe is sprinkled liberally throughout this library. At what point does it really stop mattering if this is C or Rust?

Presumably with inline assembly both languages can emit what is effectively the same machine code. Is the Rust compiler a better optimizing compiler than C compilers?

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Filligree ◴[16 Mar 25 20:18 UTC] No.43381907[source]▶
The usual answer is: You only need to verify the unsafe blocks, not every block. Though 'unsafe' in Rust is actually even less safe than regular C, if a bit more predictable, so there's a crossover point where you really shouldn't have bothered.

The Rust compiler is indeed better than the C one, largely because of having more information and doing full-program optimisation. A `vec_foo = vec_foo.into_iter().map(...).collect::Vec<foo>`, for example, isn't going to do any bounds checks or allocate.

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johnisgood ◴[16 Mar 25 20:24 UTC] No.43381960[source]▶
I have been told that "unsafe" affects code outside of that block, but hopefully steveklabnik may explain it better (again).

> isn't going to do any bounds checks or allocate.

You need to add explicit bounds check or explicitly allocate in C though. It is not there if you do not add it yourself.

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1. pornel ◴[16 Mar 25 20:55 UTC] No.43382226[source]▶
Buggy unsafe blocks can affect code anywhere (through Undefined Behavior, or breaking the API contract).

However, if you verify that the unsafe blocks are correct, and the safe API wrapping them rejects invalid inputs, then they won't be able to cause unsafety anywhere.

This does reduce how much code you need to review for memory safety issues. Once it's encapsulated in a safe API, the compiler ensures it can't be broken.

This encapsulation also prevents combinatorial explosion of complexity when multiple (unsafe) libraries interact.

I can take zlib-rs, and some multi-threaded job executor (also unsafe internally), but I don't need to specifically check how these two interact. zlib-rs needs to ensure they use slices and lifetimes correctly, the threading library needs to ensure it uses correct lifetimes and type bounds, and then the compiler will check all interactions between these two libraries for me. That's like (M+N) complexity to deal with instead of (M*N).