Well, not really "you don't care about safe rounding behavior", more just "you have specified a specific operation order that happens to be more susceptible to being vectorizable". Implementing a float sum that way has the completely-safe completely-well-defined portable behavior of summing strides for any given size.

Both float multiplication and float addition are equally bad for optimizations though - both are non-associative: https://play.rust-lang.org/?version=stable&mode=debug&editio... ; and indeed changing the aligned-sum example to f64, neither .sum() nor .product() get vectorized.

And e.g. here's a plain rust loop autovectorizing both addition and multiplication (though of course not a reduction): https://rust.godbolt.org/z/6hEcj8zfx