Preheat oven to 325 degrees. In a bowl, combine sage, salt, pepper, and garlic powder. Rub thoroughly all over pork. Place pork in an uncovered roasting pan on the middle oven rack. Bake in the preheated oven approximately 3 hours. Meanwhile, place sugar, cornstarch, water, and soy sauce in a small saucepan. Heat, stirring occasionally, until mixture begins to bubble and thicken slightly. Brush roast with glaze 3 or 4 times during the last 1/2 hour of cooking. Pour remaining glaze over roast, and serve.