`maximum!` allocates

e3c6 · August 28, 2018, 3:16pm

I get allocations, but I don’t understand why since it is writing the results in a preallocated array.

X = rand(10,3,4); x= rand(1,3,4);
@btime maximum!($x,$X)
# 452.492 ns (6 allocations: 240 bytes)

Paethon · August 28, 2018, 3:39pm

Deep down, following lots of function calls, maximum! calls the function mapfirst! and that allocates memory

github.com

JuliaLang/julia/blob/3ab56f19a81a1d658a02750b48abbd63950ef248/base/reducedim.jl#L221


      
                  end
              end
              return lsiz
          end
          
          
"""
          Extract first entry of slices of array A into existing array R.
          """
          copyfirst!(R::AbstractArray, A::AbstractArray) = mapfirst!(identity, R, A)
          
          
function mapfirst!(f, R::AbstractArray, A::AbstractArray)
              lsiz = check_reducedims(R, A)
              iA = axes(A)
              iR = axes(R)
              t = []
              for i in 1:length(iR)
                  iAi = iA[i]
                  push!(t, iAi == iR[i] ? iAi : first(iAi))
              end
              map!(f, R, view(A, t...))
          end

You can prevent that by calling maximum!(x, X, init=false) but I think you have to ensure manually that x does not contain a value bigger than any value in X (but I am not completely sure about that to be honest)

e3c6 · August 28, 2018, 3:44pm

I raised an issue, maximum! allocates · Issue #28928 · JuliaLang/julia · GitHub

Paethon · August 28, 2018, 4:04pm

I am not really sure this is an issue. maximum! never claims to not allocate memory. And those are all just temporary allocations that are freed as soon as mapfirst! has finished …

e3c6 · August 28, 2018, 4:13pm

In principle the allocation could affect performance.
Those allocations don’t have to wait for GC?