Julia High Performance - Second Edition

Fixing performance issues with globals

A simple way to get the performance back is to declare the global variable a const declaration:

const p2 = 2 function pow_array2(x::Vector{Float64})     s = 0.0     for y in x         s = s + y^p2     end     return s end 

This one change will get us a little under two orders of magnitude performance gain on the following function:

julia> @btime pow_array2($t)  103.506 μs (0 allocations: 0 bytes)33240.25981204582

Global constThe const declaration in Julia means something different from the similar keyword in C. In Julia, a global variable declared as const can change its value (a warning is printed). However, what it cannot do is change its type. Also, note that you cannot explicitly declare the type of a global variable. ...