[julia-users] Unexpected allocations in summing an array

[Darwin Darakananda]

Hello all,

I'm running into some unexpected memory allocations that I'm hoping someone 
can help me with.  Here's an example the demonstrates the problem:

function sum_rand(n::Int)
    s = zero(Float64)
    for i in 1:n
        s += rand(Float64)
    end
    return s
end

sum_rand(x::Vector) = sum_rand(length(x))

function sum_vector{T}(x::Vector{T})
    s = zero(T)
    for i in 1:length(x)
        s += x[i]
    end
    return s
end

x = rand(100);

@allocated sum_rand(100)            # 0
@allocated sum_rand(length(x))      # 16
@allocated sum_rand(length(x)::Int) # 0

@allocated sum_rand(x)   # 16
@allocated sum_vector(x) # 16


For some reason adding values that are read from an array allocates memory 
for the accumulating variable while summing up random values does not.I 
would have thought that the accumulating variable would be stack allocated 
for both functions.  It's not a lot of memory per function call, but I have 
code that sums up arrays in an inner loop, so the total cost adds up.

Any suggestions?

Thanks,

Darwin