Question

Recently, I had seen some D flip-flop RTL code in verilog like this:

    module d_ff(
            input d,
            input clk,
            input reset,
            input we,
            output q
    );

    always @(posedge clk) begin
            if (~reset) begin
                    q <= 1'b0;
            end
            else if (we) begin
                    q <= d;
            end
            else begin
                    q <= q;
            end
    end
    endmodule

Does the statement q <= q; necessary?

Answer 1

Does the statement q <= q; necessary?

No it isn’t, and in the case of an ASIC it may actually increase area and power consumption. I’m not sure how modern FPGAs handle this. During synthesis the tool will see that statement and require that q be updated on every positive clock edge. Without that final else clause the tool is free to only update q whenever the given conditions are met.

On an ASIC this means the synthesis tool may insert a clock gate(provided the library has one) instead of mux. For a single DFF this may actually be worse since a clock gate typically is much larger than a mux but if q is 32 bits then the savings can be very significant. Modern tools can automatically detect if the number of DFFs using a shared enable meets a certain threshold and then choose a clock gate or mux appropriately.

In this case the tool needs 3 muxes plus extra routing

always @(posedge CLK or negedge RESET)
  if(~RESET)
    COUNT <= 0;
  else if(INC)
    COUNT <= COUNT + 1;
  else
    COUNT <= COUNT;

Here the tool uses a single clock gate for all DFFs

always @(posedge CLK or negedge RESET)
  if(~RESET)
    COUNT <= 0;
  else if(INC)
    COUNT <= COUNT + 1;

Images from here