It appears my 1st attempt didn't make it through :(
The first two examples produce 16 4-input LUTs on a Spartan 3 and the third produces only 8 4-input LUTs. Is there a way to get ISE to use only 8 LUTs without adding flipflops or resorting to a structural (XORCY, MUXCY, etc.) description?
Note: to use this as a subtract-with-borrow, invert the carry input.
module adder_test_16_luts_1(a, b, c, result); input[7:0] a, b; input c; output[8:0] result;
assign result = a + (~b) + c; endmodule
module adder_test_16_luts_2(a, b, c, result); input[7:0] a, b; input c; output[8:0] result;
wire[8:0] inv_b = ~b; assign result = a + inv_b + c; endmodule
module adder_test_8_luts(clk, a, b, c, result); input clk; input[7:0] a, b; input c; output[8:0] result;
reg[8:0] inv_b; always @(posedge clk) inv_b