what's the fastest way to reset a julia array to all zeros?

The best way to do it in one line is:

x .= zero(y)

or

fill!(x, zero(y))

where y is the type of number you want it to be like. The reason why this way is good is that it works in all cases. If x is any type, using this will work as long as y matches the type (indeed, you can use y = x[1]).

When I mean any type, I mean that this also works for odd number types like SIUnits. If you use this command, a package could support SIUnits without ever having to import the package since this will apply the correct units to the x values (as long as all of its operations are correct in units), while fill!(x, 0.0) will error due to wrong units (you can also use fill!(x, zeros(y)). Otherwise, you'd have to check for units and all sorts of things.

You can do

fill!(x, 0.0)

This overwrites the contents with zeros.

This should be just as efficient as just doing a for loop, though:

for i in 1:length(x)
    x[i] = 0.0
end

Indeed, if you do @edit fill!(x, 0.0), you will see that this is basically what it is doing (except that it uses @inbounds for more efficiency).