This law will vary from state to state. I believe that if a cop pulls you over for it he can either cite you or give you a ticket. What the officer chooses to do probably depends on how damaged your windshield is.
Generally, you aren't allowed to drive a car with a crack in the windshield that is in the area of wiper blade coverage, or that hinders the driver's vision. You can get a fine. Exact rules vary from province to province.