Exploring SIMD instructions in Rust

Hello 👋,

I started exploring Rust a week ago due to more and more web services being written in Rust at my work place and so far I like the language, it’s simple, fast and beautiful.

My day to day programming tasks are in Python, Go and Java. I’ve also used C# for some personal projects. Picking up Rust is not that easy for me since everything I write is web services or Java programs that don’t have “low-level” logic.

In this article I will talk about my journey with Rust and my experience of using std::simd. I find things that I don’t have much experience with fascinating. I also tried using SIMD with Go but that’s above my skill level, I could not get it to work.

What is SIMD?

The SIMD stands for Single Instruction/Multiple Data. Given a vector of 128 elements.

[1,2,3, …, 128]

When you’re adding the elements, you will loop through them. A typical code will lock like this:

int sum(int * arr, int size) {
    int sum = 0;
    for (int i = 0; i < size; i++) {
        sum = sum + *(arr + i);
    }
    return 0;
}

The generated assembly will look something like this:

The code in RED is a the loop part, as you can see we have lots of instructions. Sometimes when the size is known, the compiler may choose to do loop unrolling, this means that instead of using JMP instructions and looping the compiler will do something like this, if the size is 3 for example:

int sum(int * arr, int size) {
    int sum = 0;
    sum = sum + *(arr + 0);
    sum = sum + *(arr + 1);
    sum = sum + *(arr + 2);
    return 0;
}

With SIMD instructions your CPU has support for assembly instructions that can work with vectors in an efficient manner. You may read more about SIMD and how it works in this excellent article Crunching Numbers with AVX and AVX2.

Basically given two vectors a and b you’re telling the compiler to do a mathematical operation on them and it will be efficient. Imagine that instead of looping though each element and executing the + operation; all elements are added at once:

Now that we have a basic understanding of how SIMD works we can move to Rust.

Rust Program to compute hamming distance

Note:

if you want to follow along or run the code from this article you will need to install rust nightly version and set it as default:

rustup install nightly
rustup default nightly


when you're done:
rustup default stable

The problem we want to solve with SIMD instructions is to compute hamming distance on two vectors equal in size. The hamming distance between [1, 2, 3] and [1, 3, 3] is equal to 1 because 2 != 3, the rest of the elements are equal.

The non-SIMD Rust function looks like this:

#[inline(never)]
fn hamming_distance_classic(a: &mut Vec<i32>, b: &mut Vec<i32>) -> u64 {
    let mut distance = 0;
    let length = a.len();

    for i in 0..length {
        if a[i] != b[i] {
            distance += 1
        }
    }

    return distance;
}

To preview the assembly a colleague recommended the cargo-show-asm crate. You can install it and run the following command to show the assembly code for the given function

cargo asm --bin rust_simd "rust_simd::hamming_distance_classic"

The generated assembly looks like a simple loop:

.LBB12_4:
        cmp r8, rcx

        jae .LBB12_5

        mov r10d, dword ptr [rdi + 4*r9]

        xor r11d, r11d
        cmp r10d, dword ptr [rdx + 4*r9]

        lea r10, [r9 + 1]

        setne r11b
        add rax, r11

        mov r9, r10

        cmp rsi, r10

        jne .LBB12_4

        pop rcx
        .cfi_def_cfa_offset 8
        ret

Next, I’ll show my implementation that uses simd instructions:

#[inline(never)]
fn hamming_distance_simd(a: &mut Vec
, b: &mut Vec) -> u32 {
    let mut distance: u32 = 0;
    let lanes = 16;
    let chunks = a.len() / lanes;


    for i in 0..chunks {
        // create simd vectors
        let a: i32x16 = Simd::from_slice(&a[i*lanes..i*lanes+lanes]);
        let b: i32x16 = Simd::from_slice(&b[i*lanes..i*lanes+lanes]);
        // simd XOR
        let z = a.simd_ne(b);
        distance += z.to_bitmask().count_ones();
    }
    return distance;
}

The assembly code for this function looks like this:

.LBB11_2:
        cmp r8, rsi
        ja .LBB11_6


        cmp r8, rcx
        ja .LBB11_7


        movdqu xmm1, xmmword ptr [rdi + 4*r8 - 64]
        movdqu xmm2, xmmword ptr [rdi + 4*r8 - 48]
        movdqu xmm3, xmmword ptr [rdi + 4*r8 - 32]
        movdqu xmm4, xmmword ptr [rdi + 4*r8 - 16]


        movdqu xmm5, xmmword ptr [rdx + 4*r8 - 64]


        pcmpeqd xmm5, xmm1


        movdqu xmm1, xmmword ptr [rdx + 4*r8 - 48]


        pcmpeqd xmm1, xmm2


        movdqu xmm2, xmmword ptr [rdx + 4*r8 - 32]


        pcmpeqd xmm2, xmm3


        movdqu xmm3, xmmword ptr [rdx + 4*r8 - 16]


        pcmpeqd xmm3, xmm4
        pxor xmm1, xmm0
        pxor xmm5, xmm0
        packssdw xmm5, xmm1
        pxor xmm2, xmm0
        pxor xmm3, xmm0
        packssdw xmm2, xmm3
        packsswb xmm5, xmm2
        pmovmskb r10d, xmm5


        mov r11d, r10d
        shr r11d
        and r11d, 21845
        sub r10d, r11d


        mov r11d, r10d
        and r11d, 13107
        shr r10d, 2
        and r10d, 13107
        add r10d, r11d
        mov r11d, r10d
        shr r11d, 4
        add r11d, r10d
        and r11d, 3855
        mov r10d, r11d
        shr r10d, 8
        add r10d, r11d
        movzx r10d, r10b


        add eax, r10d


        add r8, 16


        dec r9


        jne .LBB11_2

We can see that XMM registers are being used.

Benchmarking the code

To benchmark the code I’ve used a block of code that I’ve copied from a Rust SIMD page[1] and modified slightly.

mod bench {
    extern crate test;
    use self::test::Bencher;
    use std::iter;
    static BENCH_SIZE: usize = 128;

    macro_rules! bench {
        ($name:ident, $func:ident) => {
            #[bench]
            fn $name(b: &mut Bencher) {
                let mut x: Vec<_> = iter::repeat(1i32)
                                        .take(BENCH_SIZE)
                                        .collect();
                let mut y: Vec<_> = iter::repeat(1i32)
                                        .take(BENCH_SIZE)
                                        .collect();

                b.iter(|| {
                    super::$func(&mut x, &mut y);
                })
            }
        }
    }

    bench!(simd, hamming_distance_simd);
    bench!(vanilla, hamming_distance_classic);
}

The results are the following. The code written to use SIMD wins:

/home/denis/.cargo/bin/cargo bench --color=always --package rust_simd --bin rust_simd bench
    Finished bench [optimized] target(s) in 0.00s
     Running unittests src/main.rs (target/release/deps/rust_simd-2477d04109c1c861)

running 2 tests
test bench::simd    ... bench:          14 ns/iter (+/- 0)
test bench::vanilla ... bench:          20 ns/iter (+/- 0)

Inlining the functions

If we remove #[inline(never)] then something interesting happens. The compiler sees that both functions are used only once and it decides to inline them.

If we run cargo asm –bin rust_simd the functions do not appear in the output:

Try one of those by name or a sequence number
 0 "alloc::alloc::box_free" [15]
 1 "alloc::raw_vec::RawVec
::reserve_for_push" [136]
 2 "alloc::raw_vec::finish_grow" [110]
 3 "core::ops::function::FnOnce::call_once{{vtable.shim}}" [19]
 4 "core::ptr::drop_in_place
>" [31]
 5 "core::ptr::drop_in_place
" [27]
 6 "core::ptr::drop_in_place
::{{closure}}>" [7]
 7 "main" [18]
 8 "rand::rngs::adapter::reseeding::ReseedingCore
::reseed_and_generate" [157]
 9 "rust_simd::main" [787]
10 "std::rt::lang_start" [28]
11 "std::rt::lang_start::{{closure}}" [18]
12 "std::sys_common::backtrace::__rust_begin_short_backtrace" [22]

Benchmarking the code again yields the following result:

/home/denis/.cargo/bin/cargo bench --color=always --package rust_simd --bin rust_simd bench
    Finished bench [optimized] target(s) in 0.00s
     Running unittests src/main.rs (target/release/deps/rust_simd-2477d04109c1c861)


running 2 tests
test bench::simd    ... bench:          20 ns/iter (+/- 0)
test bench::vanilla ... bench:           0 ns/iter (+/- 0)

The vanilla block (classic hamming distance) function is much faster, and if we look at the assembly code we can see that the Rust compiler is smart and it auto vectorized the code to use SIMD instructions, unlike me the compiler is smarter and it’s code is better:

.LBB11_56:
        movq xmm3, qword ptr [r15 + 4*rdx]
        movq xmm4, qword ptr [r15 + 4*rdx + 8]
        movq xmm5, qword ptr [rbp + 4*rdx]
        pcmpeqd xmm5, xmm3
        movq xmm3, qword ptr [rbp + 4*rdx + 8]
        pcmpeqd xmm3, xmm4
        pshufd xmm4, xmm5, 212
        pandn xmm4, xmm2
        paddq xmm4, xmm0
        pshufd xmm3, xmm3, 212
        pandn xmm3, xmm2
        paddq xmm3, xmm1
        movq xmm0, qword ptr [r15 + 4*rdx + 16]
        movq xmm1, qword ptr [r15 + 4*rdx + 24]
        movq xmm5, qword ptr [rbp + 4*rdx + 16]
        pcmpeqd xmm5, xmm0
        movq xmm6, qword ptr [rbp + 4*rdx + 24]
        pcmpeqd xmm6, xmm1
        pshufd xmm0, xmm5, 212
        pandn xmm0, xmm2
        paddq xmm0, xmm4
        pshufd xmm1, xmm6, 212
        pandn xmm1, xmm2
        paddq xmm1, xmm3

        add rdx, 8
        add rsi, -2
        jne .LBB11_56

        test cl, 1
        je .LBB11_59

.LBB11_58:
        movq xmm2, qword ptr [r15 + 4*rdx]
        movq xmm3, qword ptr [r15 + 4*rdx + 8]
        movq xmm4, qword ptr [rbp + 4*rdx]
        pcmpeqd xmm4, xmm2
        movq xmm2, qword ptr [rbp + 4*rdx + 8]
        pcmpeqd xmm2, xmm3
        pshufd xmm3, xmm4, 212
        movdqa xmm4, xmmword ptr [rip + .LCPI11_0]
        pandn xmm3, xmm4
        paddq xmm0, xmm3
        pshufd xmm2, xmm2, 212
        pandn xmm2, xmm4
        paddq xmm1, xmm2

.LBB11_59:
        paddq xmm0, xmm1
        pshufd xmm1, xmm0, 238
        paddq xmm1, xmm0
        movq rcx, xmm1
        cmp rdi, rax
        je .LBB11_61

The assembly for the hamming_distance_simd function looks like this:

.LBB11_40:
        cmp rdi, rax
        ja .LBB11_41

        cmp rdi, r12
        ja .LBB11_44

        movdqu xmm1, xmmword ptr [r15 + 4*rdi - 64]
        movdqu xmm2, xmmword ptr [r15 + 4*rdi - 48]
        movdqu xmm3, xmmword ptr [r15 + 4*rdi - 32]
        movdqu xmm4, xmmword ptr [r15 + 4*rdi - 16]

        movdqu xmm5, xmmword ptr [r10 + 4*rdi - 64]

        pcmpeqd xmm5, xmm1

        movdqu xmm1, xmmword ptr [r10 + 4*rdi - 48]

        pcmpeqd xmm1, xmm2

        movdqu xmm2, xmmword ptr [r10 + 4*rdi - 32]

        pcmpeqd xmm2, xmm3

        movdqu xmm3, xmmword ptr [r10 + 4*rdi - 16]

        pcmpeqd xmm3, xmm4
        pxor xmm3, xmm0
        pxor xmm2, xmm0
        packssdw xmm2, xmm3
        pxor xmm1, xmm0
        pxor xmm5, xmm0
        packssdw xmm5, xmm1
        packsswb xmm5, xmm2
        pmovmskb ebp, xmm5

        mov esi, ebp
        shr esi
        and esi, 21845
        sub ebp, esi

        mov esi, ebp
        and esi, 13107
        shr ebp, 2
        and ebp, 13107
        add ebp, esi
        mov eax, ebp
        shr eax, 4
        add eax, ebp
        and eax, 3855
        mov esi, eax
        shr esi, 8
        add esi, eax
        movzx eax, sil

        add r9d, eax

        add rdi, 16

        dec rcx
        mov rax, qword ptr [rsp + 8]

        jne .LBB11_40

We can also force a loop unrolling if we hard-code the chunks size in hamming_distance_simd:

fn hamming_distance_simd(a: &mut Vec
, b: &mut Vec) -> u32 {
    let mut distance: u32 = 0;
    let lanes = 16;
    // let chunks = a.len() / lanes;


    for i in 0..8 {
        // create simd vectors
        let a: i32x16 = Simd::from_slice(&a[i*lanes..i*lanes+lanes]);
        let b: i32x16 = Simd::from_slice(&b[i*lanes..i*lanes+lanes]);
        // simd XOR
        let z = a.simd_ne(b);
        distance += z.to_bitmask().count_ones();
    }
    return distance;
}

Then the assembly will become:

        movdqu xmm0, xmmword ptr [r15]
        movdqu xmm1, xmmword ptr [r15 + 16]
        movdqu xmm2, xmmword ptr [r15 + 32]
        movdqu xmm3, xmmword ptr [r15 + 48]
        movdqu xmm4, xmmword ptr [r14]
        pcmpeqd xmm4, xmm0
        movdqu xmm0, xmmword ptr [r14 + 16]
        pcmpeqd xmm0, xmm1
        movdqu xmm1, xmmword ptr [r14 + 32]
        pcmpeqd xmm1, xmm2
        movdqu xmm2, xmmword ptr [r14 + 48]
        pcmpeqd xmm2, xmm3
        pcmpeqd xmm3, xmm3
        pxor xmm2, xmm3
        pxor xmm1, xmm3
        packssdw xmm1, xmm2
        pxor xmm0, xmm3
        pxor xmm4, xmm3
        packssdw xmm4, xmm0
        packsswb xmm4, xmm1
        pmovmskb eax, xmm4
        mov ecx, eax
        shr ecx
        and ecx, 21845
        sub eax, ecx
        mov ecx, eax
        and ecx, 13107
        shr eax, 2
        and eax, 13107
        add eax, ecx
        mov ecx, eax
        shr ecx, 4
        add ecx, eax
        and ecx, 3855
        mov eax, ecx
        shr eax, 8
        add eax, ecx
        movzx eax, al
        movdqu xmm0, xmmword ptr [r15 + 64]
        movdqu xmm1, xmmword ptr [r15 + 80]
        movdqu xmm2, xmmword ptr [r15 + 96]
        movdqu xmm3, xmmword ptr [r15 + 112]
        movdqu xmm4, xmmword ptr [r14 + 64]
        pcmpeqd xmm4, xmm0
        movdqu xmm0, xmmword ptr [r14 + 80]
        pcmpeqd xmm0, xmm1
        movdqu xmm1, xmmword ptr [r14 + 96]
        pcmpeqd xmm1, xmm2
        movdqu xmm2, xmmword ptr [r14 + 112]
        pcmpeqd xmm2, xmm3
        pcmpeqd xmm3, xmm3
        pxor xmm2, xmm3
        pxor xmm1, xmm3
        packssdw xmm1, xmm2
        pxor xmm0, xmm3
        pxor xmm4, xmm3
        packssdw xmm4, xmm0
        packsswb xmm4, xmm1
        pmovmskb ecx, xmm4
        mov edx, ecx
        shr edx
        and edx, 21845
        sub ecx, edx
        mov edx, ecx
        and edx, 13107
        shr ecx, 2
        and ecx, 13107
        add ecx, edx
        mov edx, ecx
        shr edx, 4
        add edx, ecx
        and edx, 3855
        mov ecx, edx
        shr ecx, 8
        add ecx, edx
        movzx ecx, cl
        movzx eax, ax
        add eax, ecx
        movdqu xmm0, xmmword ptr [r15 + 128]
        movdqu xmm1, xmmword ptr [r15 + 144]
        movdqu xmm2, xmmword ptr [r15 + 160]
        movdqu xmm3, xmmword ptr [r15 + 176]
        movdqu xmm4, xmmword ptr [r14 + 128]
        pcmpeqd xmm4, xmm0
        movdqu xmm0, xmmword ptr [r14 + 144]
        pcmpeqd xmm0, xmm1
        movdqu xmm1, xmmword ptr [r14 + 160]
        pcmpeqd xmm1, xmm2
        movdqu xmm2, xmmword ptr [r14 + 176]
        pcmpeqd xmm2, xmm3
        pcmpeqd xmm3, xmm3
        pxor xmm2, xmm3
        pxor xmm1, xmm3
        packssdw xmm1, xmm2
        pxor xmm0, xmm3
        pxor xmm4, xmm3
        packssdw xmm4, xmm0
        packsswb xmm4, xmm1
        pmovmskb ecx, xmm4
        mov edx, ecx
        shr edx
        and edx, 21845
        sub ecx, edx
        mov edx, ecx
        and edx, 13107
        shr ecx, 2
        and ecx, 13107
        add ecx, edx
        mov edx, ecx
        shr edx, 4
        add edx, ecx
        and edx, 3855
        mov ecx, edx
        shr ecx, 8
        add ecx, edx
        movzx ecx, cl
        movzx eax, ax
        add eax, ecx
        movdqu xmm0, xmmword ptr [r15 + 192]
        movdqu xmm1, xmmword ptr [r15 + 208]
        movdqu xmm2, xmmword ptr [r15 + 224]
        movdqu xmm3, xmmword ptr [r15 + 240]
        movdqu xmm4, xmmword ptr [r14 + 192]
        pcmpeqd xmm4, xmm0
        movdqu xmm0, xmmword ptr [r14 + 208]
        pcmpeqd xmm0, xmm1
        movdqu xmm1, xmmword ptr [r14 + 224]
        pcmpeqd xmm1, xmm2
        movdqu xmm2, xmmword ptr [r14 + 240]
        pcmpeqd xmm2, xmm3
        pcmpeqd xmm3, xmm3
        pxor xmm2, xmm3
        pxor xmm1, xmm3
        packssdw xmm1, xmm2
        pxor xmm0, xmm3
        pxor xmm4, xmm3
        packssdw xmm4, xmm0
        packsswb xmm4, xmm1
        pmovmskb ecx, xmm4
        mov edx, ecx
        shr edx
        and edx, 21845
        sub ecx, edx
        mov edx, ecx
        and edx, 13107
        shr ecx, 2
        and ecx, 13107
        add ecx, edx
        mov edx, ecx
        shr edx, 4
        add edx, ecx
        and edx, 3855
        mov ecx, edx
        shr ecx, 8
        add ecx, edx
        movzx ecx, cl
        movzx eax, ax
        add eax, ecx
        movdqu xmm0, xmmword ptr [r15 + 256]
        movdqu xmm1, xmmword ptr [r15 + 272]
        movdqu xmm2, xmmword ptr [r15 + 288]
        movdqu xmm3, xmmword ptr [r15 + 304]
        movdqu xmm4, xmmword ptr [r14 + 256]
        pcmpeqd xmm4, xmm0
        movdqu xmm0, xmmword ptr [r14 + 272]
        pcmpeqd xmm0, xmm1
        movdqu xmm1, xmmword ptr [r14 + 288]
        pcmpeqd xmm1, xmm2
        movdqu xmm2, xmmword ptr [r14 + 304]
        pcmpeqd xmm2, xmm3
        pcmpeqd xmm3, xmm3
        pxor xmm2, xmm3
        pxor xmm1, xmm3
        packssdw xmm1, xmm2
        pxor xmm0, xmm3
        pxor xmm4, xmm3
        packssdw xmm4, xmm0
        packsswb xmm4, xmm1
        pmovmskb ecx, xmm4
        mov edx, ecx
        shr edx
        and edx, 21845
        sub ecx, edx
        mov edx, ecx
        and edx, 13107
        shr ecx, 2
        and ecx, 13107
        add ecx, edx
        mov edx, ecx
        shr edx, 4
        add edx, ecx
        and edx, 3855
        mov ecx, edx
        shr ecx, 8
        add ecx, edx
        movzx ecx, cl
        movzx eax, ax
        add eax, ecx
        movdqu xmm0, xmmword ptr [r15 + 320]
        movdqu xmm1, xmmword ptr [r15 + 336]
        movdqu xmm2, xmmword ptr [r15 + 352]
        movdqu xmm3, xmmword ptr [r15 + 368]
        movdqu xmm4, xmmword ptr [r14 + 320]
        pcmpeqd xmm4, xmm0
        movdqu xmm0, xmmword ptr [r14 + 336]
        pcmpeqd xmm0, xmm1
        movdqu xmm1, xmmword ptr [r14 + 352]
        pcmpeqd xmm1, xmm2
        movdqu xmm2, xmmword ptr [r14 + 368]
        pcmpeqd xmm2, xmm3
        pcmpeqd xmm3, xmm3
        pxor xmm2, xmm3
        pxor xmm1, xmm3
        packssdw xmm1, xmm2
        pxor xmm0, xmm3
        pxor xmm4, xmm3
        packssdw xmm4, xmm0
        packsswb xmm4, xmm1
        pmovmskb ecx, xmm4
        mov edx, ecx
        shr edx
        and edx, 21845
        sub ecx, edx
        mov edx, ecx
        and edx, 13107
        shr ecx, 2
        and ecx, 13107
        add ecx, edx
        mov edx, ecx
        shr edx, 4
        add edx, ecx
        and edx, 3855
        mov ecx, edx
        shr ecx, 8
        add ecx, edx
        movzx ecx, cl
        movzx eax, ax
        add eax, ecx
        movdqu xmm0, xmmword ptr [r15 + 384]
        movdqu xmm1, xmmword ptr [r15 + 400]
        movdqu xmm2, xmmword ptr [r15 + 416]
        movdqu xmm3, xmmword ptr [r15 + 432]
        movdqu xmm4, xmmword ptr [r14 + 384]
        pcmpeqd xmm4, xmm0
        movdqu xmm0, xmmword ptr [r14 + 400]
        pcmpeqd xmm0, xmm1
        movdqu xmm1, xmmword ptr [r14 + 416]
        pcmpeqd xmm1, xmm2
        movdqu xmm2, xmmword ptr [r14 + 432]
        pcmpeqd xmm2, xmm3
        pcmpeqd xmm3, xmm3
        pxor xmm2, xmm3
        pxor xmm1, xmm3
        packssdw xmm1, xmm2
        pxor xmm0, xmm3
        pxor xmm4, xmm3
        packssdw xmm4, xmm0
        packsswb xmm4, xmm1
        pmovmskb ecx, xmm4
        mov edx, ecx
        shr edx
        and edx, 21845
        sub ecx, edx
        mov edx, ecx
        and edx, 13107
        shr ecx, 2
        and ecx, 13107
        add ecx, edx
        mov edx, ecx
        shr edx, 4
        add edx, ecx
        and edx, 3855
        mov ecx, edx
        shr ecx, 8
        add ecx, edx
        movzx ecx, cl
        movzx eax, ax
        add eax, ecx


        movdqu xmm0, xmmword ptr [r15 + 448]
        movdqu xmm1, xmmword ptr [r15 + 464]
        movdqu xmm2, xmmword ptr [r15 + 480]
        movdqu xmm3, xmmword ptr [r15 + 496]


        movdqu xmm4, xmmword ptr [r14 + 448]


        pcmpeqd xmm4, xmm0


        movdqu xmm0, xmmword ptr [r14 + 464]


        pcmpeqd xmm0, xmm1


        movdqu xmm1, xmmword ptr [r14 + 480]


        pcmpeqd xmm1, xmm2


        movdqu xmm2, xmmword ptr [r14 + 496]


        pcmpeqd xmm2, xmm3
        pcmpeqd xmm3, xmm3
        pxor xmm2, xmm3
        pxor xmm1, xmm3
        packssdw xmm1, xmm2
        pxor xmm0, xmm3
        pxor xmm4, xmm3
        packssdw xmm4, xmm0
        packsswb xmm4, xmm1
        pmovmskb ecx, xmm4


        mov edx, ecx
        shr edx
        and edx, 21845
        sub ecx, edx


        mov edx, ecx
        and edx, 13107
        shr ecx, 2
        and ecx, 13107
        add ecx, edx
        mov edx, ecx
        shr edx, 4
        add edx, ecx
        and edx, 3855
        mov ecx, edx
        shr ecx, 8
        add ecx, edx
        movzx ecx, cl


        add ecx, eax


        mov dword ptr [rsp + 108], ecx
        lea rax, [rsp + 108]


        mov qword ptr [rsp + 120], rax
        mov rax, qword ptr [rip + core::fmt::num::imp::
::fmt@GOTPCREL]

You can see that by unrolling the loop there are exactly 8 code blocks that use SIMD instructions. The performance is slightly better but not by much:

/home/denis/.cargo/bin/cargo bench --color=always --package rust_simd --bin rust_simd bench
    Finished bench [optimized] target(s) in 0.00s
     Running unittests src/main.rs (target/release/deps/rust_simd-2477d04109c1c861)

running 2 tests
test bench::simd    ... bench:          17 ns/iter (+/- 1)
test bench::vanilla ... bench:           0 ns/iter (+/- 0)

Conclusions

I’m still a beginner rustacean and my journey is just getting started. I’ve tried to outsmart the Rust compiler by using SIMD instructions manually but that didn’t work that well. Rust figured out how to optimize and vectorize the loop code that wasn’t written with SIMD instructions in mind and in the end it gave much better performance.

If you have suggestions on how to improve the code please let me know in the comments. 😀

Thanks for reading! Happy hacking!

Code

https://gist.github.com/dnutiu/8fc529d09d2872595cf2bf23d40e1cd2

Refferences

• [1] https://www.cs.brandeis.edu/~cs146a/rust/rustbyexample-02-21-2015/simd.html
• https://en.wikipedia.org/wiki/Single_instruction,_multiple_data
• https://en.wikipedia.org/wiki/Hamming_distance
• https://en.wikibooks.org/wiki/X86_Assembly/SSE