Repeating an enum does not generate a memset

[Noratrieb]

I tried this code: godbolt

pub fn create_array() -> [Option<u8>; 256] {
    [None; 256]
}

I expected to see this happen: This should compile to a memset, or the equivalent unrolled SIMD stores

LLVM IR

Instead, this happened: It compiles to a bunch of movs.

I think this happens because we transform this to a bunch of stores of the discriminant, without touching the data part of the option, making it impossible for LLVM to memset this. Ideally, we'd write 0 to the data part as well here, allowing this to use memset.

[Noratrieb]

@rustbot label A-codegen

[danielrab]

there is also an issue which is probably related, where this doesn't get vectorized at all

pub fn create_array() -> [Result<u16, u8>; 512] {
    [Ok(0); 512] 
}

godbolt

[clubby789]

This regressed between nightly-2022-02-25 -> nightly-2022-02-26 (bisected to b7e5597)

[crlf0710]

This works fine though.

#![feature(inline_const)]

pub fn create_array() -> [Result<u16, u8>; 512] {
    const { [Ok(0); 512] }
}

[veera-sivarajan]

Fixed in nightly (Mar 3, 2025): https://rust.godbolt.org/z/9dbfeYrj9

@rustbot label +e-needs-test