Place cherries into a saucepan over medium heat, and cover the pan; heat cherries until they release their juice and come to a simmer, 10 to 15 minutes. Stir often.
In a bowl, whisk the sugar with cornstarch until smooth; pour the mixture into the hot cherries and juice, and thoroughly combine. Return to low heat, bring to a simmer, and cook until the filling has thickened, about 2 minutes; remove from heat, let cool, and use as pie filling.
