i have tried it with carry look ahead adder,is there any other way?? i am futher going to draw the layouts for the design of high speed 8 bit add/sub...is there any which can be used with minimum no. of gates

 

I remember from Mead & Conway, VLSI Design, 1980, an innovative adder that used multiplexers with a Kill-Propagate Block that was substantially easier than traditional implementations.