Seems that[0] clang doesn't succeed at vectorizing that, instead just doing a bunch of boring regular unrolling. I'd assume it wouldn't be too hard for clang to be able to understand that pattern, just noone has bothered to (and it won't pop up much in regular code because usually it'd be done with vector pushes and not a preallocated buffer).

[0]: https://godbolt.org/z/hzsdhGorr