Some built-in functions (like
Exp) give an arbitrary precision result, even when the argument is a machine precision number. Example:
Exp[-10.] // Precision
Exp[-1000.] // Precision
I can of course
Chop the result to a machine precision number, but I would like to avoid the cost of arbitrary precision calculations (and unpacking, if the argument is a packed array) in the first place. For example here:
x = Range[-10.^6, 10.^6]; Timing[y = Exp[-x^2/2];] PackedArrayQ[y]
The calculation takes 7s on my PC (machine precision calculation would take ~0.1s) and the result isn't a packed array.