SmallVec<[u8; 16]>::clone is not optimized to a memcpy

[saethlin]

Playground example: https://play.rust-lang.org/?version=stable&mode=release&edition=2018&gist=64230bf4215b7206e80b1f37198cf611

When I'm cloning a small SmallVec I don't mind this at all, but there should probably be a branch for the spilled case.

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I'm not amazing at reading assembly but I think it's actually worse than "not a memcpy", pretty sure I see the increment for the SetLenOnDrop in there still 😬

[mbrubeck]

Note: You can work around this for now by enabling the nightly-only "specialization" feature, or by replacing buf.clone() with:

SmallVec::from_slice(buf)

[saethlin]

Interesting. So would it be better to implement Clone using SmallVec::from_slice?

[mbrubeck]

That's what the specialization feature does. Without specialization, that's hard to do because from_slice (and memcpy) only works for T: Copy, while clone works for any T: Clone.