Julia High Performance - Second Edition

Allocations and in-place operations

Consider the following trivial function, xpow, which takes an integer as input and returns the first few powers of the number. Another function, xpow_loop, uses the first function to compute the sum of squares of a large sequence of numbers as follows:

function xpow(x)    return [x x^2 x^3 x^4] end  function xpow_loop(n)    s = 0    for i = 1:n       s = s + xpow(i)[2]    end    return s end 

Benchmarking this function for large input shows that this function is quite slow:

julia> @btime xpow_loop($1000000)  100.594 ms (4999441 allocations: 167.84 MiB)

The clue is in the number of allocations displayed in the preceding output. Within the xpow function, a four-element array is allocated for each invocation of this function. ...