There's seemingly no way to construct a __m128i in a const fn

[shepmaster]

My ideal case would be to take a &[u8] and "fill up" a __m128:

let needle: __m128i = bytes.collect(); // Takes the first 16 bytes, zero-initialized otherwise

// or

let mut needle = __m128i::default(); // zero-initialize
needle.extend(bytes); // Takes the first 16 bytes

However, none of the "nice" traits are implemented for these types, and I've gotten the feeling that is no accident. Basically, I can use an intrinsic function to set it (e.g. _mm_setzero_si128), or I can convert from u8x16 and friends.

The intrinsics are not const, so they cannot be used in a const fn context.

The closest I could find was

u8x16::new(
    x0, x1, x2, x3, x4, x5, x6, x7,
    x8, x9, x10, x11, x12, x13, x14, x15,
).into_bits()

but into_bits is likewise not const.

[BurntSushi]

I'm having trouble connecting the dots in your issue, so I'll just vomit out two things and see what sticks. :-)

My ideal case would be to take a &[u8] and "fill up" a __m128:

Ignoring the "const" part of your request, I would use _mm_loadu_si128 or _mm_load_si128, depending on whether you're loading unaligned or aligned data, respectively.

There's seemingly no way to construct a __m128i in a const fn

I think the way to do this (right now at least) is via a union

union u8x16 {
    vector: __m128i,
    bytes: [u8; 16],
}

const ZERO: __m128i = unsafe { (u8x16 { bytes: [0; 16] }).vector };

playground link

[gnzlbg]

I think the way to do this (right now at least) is via a union

Yes, in the future mem::uninitialized and mem::zero should become const fns, which will allow you to transmute a std::simd::u8x16 type (which has const fn constructors) directly into an __m128i. But that's blocked on rust-lang/rfcs#411 being fixed, and on the portable packed vector types RFC being merged: rust-lang/rfcs#2366

[shepmaster]

mem::uninitialized and mem::zero should become const fns, which will allow you to transmute a std::simd::u8x16 type (which has const fn constructors) directly into an __m128i

I think I'm missing a step here. How do mem::uninitialized or mem::zero allow for transmuting?

[gnzlbg]

I think I'm missing a step here. How do mem::uninitialized or mem::zero allow for transmuting?

Oh sorry, mem::transmute is what should become const fn here.

[alexcrichton]

I think this has basically been resolved, so closing