JuliaLang / julia / 1306

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Commit Message

abstractarray: Copy same-shape arrays with one Cartesian loop (#64552)

When `dest` is `IndexCartesian` and `src` is `IndexLinear`, their `eachindex`
iterators differ, so `copyto_unaliased!` zipped a `CartesianIndices` with a
linear range into a single flat loop. That loop carries the multidimensional
index by hand and does not vectorize. When the axes are equal, iterate one
`CartesianIndices` and index both arrays with it instead: the inner loop is
then contiguous and vectorizes.

This is the path taken by `copyto!(view(B, 2:257, 2:257), A)` and by
`B[1:end-1, 1:end-1] .= A` (through the `identity` shortcut in
`broadcast!`), the `array/subarray_assign` runtime benchmark. On an M4 Pro
each goes from 22 µs to 8 µs for 256×256 `Float64`. Copies whose shapes
differ, such as a vector into a matrix view, keep the zipped loop.

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