Optimized BLAS libraries pack, copying elements into preallocated blocks for better locality.
Here is a comment explaining how important it is to performance, by mratsim who implemented a high performance BLAS in Nim.
In Fortran, the gfortran compiler uses views when they are contiguous, but copies otherwise. This is often a good heuristic, but can prevent vectorization if not inlined when using a struct-of-arrays style memory layout (where what would be the fields to the struct are distributed across the columns of a matrix). If inlined, the compiler will hopefully make the correct perform- vs eliminate-the-copy decision.
If the calling function mutates the view, gfortran also emits an unpack, to maintain the same behavior in both versions.
Perhaps a view is only ever up to several times slower than a copy, but couldn’t the same normally be said about a copy, barring fairly extreme cases, such as
foo(x) = x[1] + 1
function bar(x)
s = zero(eltype(x))
@inbounds @simd for i ∈ eachindex(x)
#s += @views foo(x[i:i])
s += foo(x[i:i])
end
s
end