1. Cook sugar in a 2½- to 3-quart heavy saucepan over moderate heat, undisturbed, until it begins to melt. Continue to cook, stirring occasionally with a fork, until sugar is melted to a deep golden caramel. Tilt pan and carefully pour in whipped topping (caramel will harden and steam vigorously). Cook over moderately low heat, stirring, until caramel is dissolved. Remove from heat and stir in margarine, vanilla, salt, and pecans.
2. Immediately spread topping over cooled crust and bake until bubbling, about 20 minutes. Cool completely in pan on rack for about two hours.
3. Cut into 2-inch triangles, diamonds, or squares (for neat slicing, run a heavy knife under hot water, then wipe dry).