potential performance issue without move_factors_into_sums

[blegat]

I thought move_factors_into_sums is about x + 2 * sum(...) to be rewritten as x + sum(2 * ...) but I didn't expect it to also have the following effect:

julia> MA.rewrite(:(x + 2x))
(Symbol("##341"), :(var"##341" = begin
          #= /home/blegat/.julia/packages/MutableArithmetics/Zy1nV/src/rewrite.jl:321 =#
          let
              #= /home/blegat/.julia/packages/MutableArithmetics/Zy1nV/src/rewrite.jl:322 =#
              begin
                  begin
                      var"##344" = MutableArithmetics.Zero()
                      var"##343" = (MutableArithmetics.operate!!)(MutableArithmetics.add_mul, var"##344", $(Expr(:escape, :x)))
                  end
                  begin
                      var"##342" = (MutableArithmetics.operate!!)(MutableArithmetics.add_mul, var"##343", $(Expr(:escape, 2)), $(Expr(:escape, :x)))
                  end
              end
              #= /home/blegat/.julia/packages/MutableArithmetics/Zy1nV/src/rewrite.jl:323 =#
              var"##342"
          end
      end))

julia> MA.rewrite(:(x + 2x), move_factors_into_sums = false)
(Symbol("##346"), :(var"##346" = begin
          #= /home/blegat/.julia/packages/MutableArithmetics/Zy1nV/src/rewrite.jl:321 =#
          let
              #= /home/blegat/.julia/packages/MutableArithmetics/Zy1nV/src/rewrite.jl:322 =#
              begin
                  #= /home/blegat/.julia/packages/MutableArithmetics/Zy1nV/src/rewrite.jl:317 =#
                  var"##347" = (MutableArithmetics.copy_if_mutable)($(Expr(:escape, :x)))
                  var"##348" = (MutableArithmetics.operate!!)(*, 2, $(Expr(:escape, :x)))
                  var"##349" = (MutableArithmetics.operate!!)(MutableArithmetics.add_mul, var"##347", var"##348")
              end
              #= /home/blegat/.julia/packages/MutableArithmetics/Zy1nV/src/rewrite.jl:323 =#
              var"##349"
          end
      end))

The line var"##348" = (MutableArithmetics.operate!!)(*, 2, $(Expr(:escape, :x))) creates a temporary expression containing the term 2x. This will have quite a bad effect on the performance of JuMP.AffExpr and JuMP.QuadExpr.

[odow]

move_factors_into_sums was perhaps a bad name. It's really: don't mess with the expression graph.

This add_mul issue could be fixed though.