How does explicit unrolling differ from iterating through elements one-by-one? (ndarray example)

• rust-ndarray

  20 3,319 8.2 Rust

  ndarray: an N-dimensional array with array views, multidimensional slicing, and efficient operations

  

While looking through ndarrays src, I came across a set of functions that explicitly unroll 8 variables on each iteration of a loop, with the comment eightfold unrolled so that floating point can be vectorized (even with strict floating point accuracy semantics). I don't understand why floats would be affected by unrolling, and in general I'm confused as to how explicit unrolling differs from iterating through each element one by one. I assumed this would be a scenario where the compiler would optimize best anyway, which seems to be confirmed (at least in the context of using iter() rather than for) here. Could anyone give a little context into what this, or any explicit unrolling achieves?

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