YES

(format LCTRS :logic QF_LIA)
(fun return 1 :sort (Int Result))
(fun sum 1 :sort (Int Result))
(fun w 2 :sort (Int Result Result))

(rule (w x_0 (return y_1)) (return (+ x_0 y_1)) :vars ((x_0 Int) (y_1 Int)))
(rule (sum x_2) (w x_2 (sum (- x_2 1))) :guard (<= 0 (- x_2 1)) :vars ((x_2 Int)))
(rule (sum x_3) (return 0) :guard (<= x_3 0) :vars ((x_3 Int)))

Confluent by Parallel Closedness with proof:
no critical pairs

Elapsed Time:  11.13 ms