have a gap (1/2 inch) between new sub floor and 2x4 wall: Need to fill before completing the flooring

Okay you say NEW subfloor. So I presume you had some type of damage and had to have the flooring replaced, correct? Next time have the individual installing the flooring make it a "tighter" fit. You could fill it with some water putty, that would make a good tight seal. May have to put duct tape or something on the bottom side to hold the water putty until it sets, but then it is suppose to be as hard as concrete. Or you may want to use something like the foam in a can stuff. "great stuff", is one brand of the expanding foam. Spray the foam in sparringly and allow it to expand, and then trim flush with floor, or put the foam in the gap, and quickly install the finish flooring.

Remember also, the flooring at the wall is going to have a baseboard,and perhaps a shoe molding/quarter round?That alone will take the space up, also if carpeting is being installed the tackstrip will account for a few bad gap problems too.