Someone on Reddit recently asked:

What would make a Rust implementation of something faster than a C implementation, all things being the same?

I think this is a great and interesting question! It’s really tough because it ultimately relies on what exactly you mean by “all things being the same.” And I think this is something that makes it hard to compare languages.

Here’s some ways in which you can argue that things are “the same” but also, that they’re not, and what they imply for runtime performance.

Inline Assembly

Rust has inline assembly built into the language. C has inline assembly as a very common compiler extension, to the degree where saying it’s not part of the language is an arguable nitpick.

Here’s an example in Rust:

use std::arch::asm;

#[unsafe(no_mangle)]
pub fn rdtsc() -> u64 {
    let lo:  u32;
    let hi:  u32;
    unsafe {
        asm!(
            "rdtsc",
            out("eax") lo,
            out("edx") hi,
            options(nomem, nostack, preserves_flags),
        );
    }
    ((hi as u64) << 32) | (lo as u64)
}

This reads the time stamp counter with rdtsc, and returns its value.

Here’s the example in C:

#include <stdint.h>

uint64_t rdtsc(void)
{
    uint32_t lo, hi;
    __asm__ __volatile__ (
        "rdtsc"
        : "=a"(lo), "=d"(hi)
    );
    return ((uint64_t)hi << 32) | lo;
}

These (under -0 in both rustc 1.87.0 and clang 20.1.0) produce the same assembly:

rdtsc:
        rdtsc
        shl     rdx, 32
        mov     eax, eax
        or      rax, rdx
        ret

Here’s a link on Godbolt: https://godbolt.org/z/f7K8cfnx7

Does this count? I don’t know. I don’t think it really speaks to the question asked, but it is one way to answer the question.

Similar code, different results

Rust and C can have different semantics for similar code. Here’s a struct in Rust:

struct Rust {
    x: u32,
    y: u64,
    z: u32,
}

Here’s “the same” struct in C:

struct C {
    uint32_t x;
    uint64_t y;
    uint32_t z;
};

In Rust, this struct is 16 bytes (on x86_64, again) and in C, it is 24. This is because Rust is free to reorder the fields to optimize for size, while C is not.

Is this the same, or different?

In C, you can re-order the fields to get the same size. In Rust, you can write #[repr(C)] to get the same layout as C. Does this mean we should have written different Rust or different C to get “the same” thing?

Social Factors

Some people have reported that, thanks to Rust’s checks, they are more willing to write code that’s a bit more dangerous than in the equivalent C (or C++), where they’d do a bit more copying to play it a bit safer. This would be “the same” in the sense of the same devs on the same project, but the code would be different, due to judgement calls. You can make an argument that that’s not the same, but is different too.

An example of this from a long time ago is the Stylo project. Mozilla tried to parallelize Firefox’s style layout twice in C++, and both times the project failed. The multithreading was too tricky to get right. The third time, they used Rust, and managed to ship. This is the same project, (though not the same programmers, I believe) by the same organization, but one was possible and one was not. Is that “the same”? In some senses, but not in others.

This also goes for a similar question: assuming we have a junior developer writing Rust, and also writing C, for the same task. Are we going to get faster code in one or the other? This controls for ability, but not for the same code. Is that “the same”? I don’t know. What about an expert in each language, someone who knows Rust super well but doesn’t know C, and vice versa, being given the same task? Is that different than a junior, or an “average” developer?

Compile time vs runtime?

Another redditor asks:

I’m not Rust expert, but aren’t most (all?) of the safety checks compile time checks? They shouldn’t have any runtime impact.

A great question! Part of this is another example of defaults being different.

array[0] is valid in both languages.

In Rust, there’s a bounds check at runtime. In C, there is not. Does this mean that they’re the same? In Rust, I could write array.get_unchecked(0), and get C’s semantics. In C, I could write a bounds check to get Rust’s semantics. Are any of those “the same”?

In Rust, that check may be optimized away, if the compiler can prove it’s safe. In C, if we wrote the bounds check by hand, the check may be optimized away, if the compiler can prove it’s safe. Are any of those “the same”?

They aren’t wrong that a lot of Rust’s safety checks are at compile time. But some are at runtime. But this raises another interesting question: That compile-time check may cause you to write different code for the same task as in C. A common example is using indices rather than pointers. That may mean that the generated code performs differently. Is that check truly “at compile time”? Technically, at the micro level, yes. At the engineering level? Possibly not!

My Conclusions

I think the most important part of this question is related to possibility, that is:

1. If we assume C is the ‘fastest language,’ whatever that means.
2. Is there any inherent reason why Rust could not do the same things?

I think the answer to that is “no,” even ignoring the inline assembly case. So on that most important, fundamental level, the answer is “there’s no difference between the two.”

But we’re not usually talking about that. We’re usually talking about something in the context of engineering, a specific project, with specific developers, with specific time constraints, and so on. I think that there are so many variables that it is difficult to draw generalized conclusions.

Here’s my post about this post on BlueSky: