[julia-users] Two (silly) questions about code performance

[Eduardo Lenz]

I have two silly questions about the example code below.

1) There is a small, but noticeable speed up in this code if I define the 
alias
    colptr = A.colptr
    rowval = A.rowval
    nzval  = A.nzval

2) Also, I would not expect a speed up from the @simd macro since the loop

 @simd for k=colptr[col]:(colptr[col+1]-1)
            @inbounds soma += B[rowval[k]] * nzval[k]
  end

does not have a unitary stride, but the diference is also noticiable.

As there are no mentions to alias in the Performance Tips I would like to 
known if there
is a logical explanation for this. 


# Test Code ( Y = A*B, A is sparse) 
function Test( A::SparseMatrixCSC{Float64,Int64}, B::Array{Float64}, 
Y::Array{Float64})

    # Lets assume it is square
    n = size(A,1)

    # Local alias 
    colptr = A.colptr
    rowval = A.rowval
    nzval  = A.nzval

    #Loops
    @inbounds for col = 1:n
        const s = 0.0
        @simd for k=colptr[col]:(colptr[col+1]-1)
            @inbounds s += B[rowval[k]] * nzval[k]
        end
        Y[col] = s
    end

end