Thanks for your help!
Actually I don't think this is exactly what I wanted (sum of all element-wise products of two arrays), I misinterpreted the description of the function.
In the meantime I already created my own arm neon assembler-code for that purpose.
hotchip
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Ah I guess this must be the right function
https://developer.apple.com/documentation/accelerate/vdsp/3240838-add#declaration
But how do I use this with c++?