What is the difference between these 2 counters?

Question

What is the difference between these two statements if they are inside an always_ff @(posedge clk) block?

if(~Intf.DataFull) begin
   rWrPageCntr <= rWrPageCntr - 1;
end

vs

rWrPageCntr <= rWrPageCntr - ~Intf.DataFull;

Answer 1

Under the following assumptions regarding signal bit widths, there is a big difference:

module tb;

bit DataFull, clk;
bit [2:0] rWrPageCntr;
bit [2:0] rWrPageCntr2;

always #5 clk++;

always_ff @(posedge clk)
    if(~DataFull) begin
        rWrPageCntr <= rWrPageCntr - 1;
    end

always_ff @(posedge clk)
        rWrPageCntr2 <= rWrPageCntr2 - ~DataFull;
           
always @(negedge clk) $display("full=%b %d %d", DataFull, rWrPageCntr, rWrPageCntr2);

initial begin
    DataFull = 1;
    #150;
    DataFull = 0;
    #150 $finish;
end

endmodule

Outputs:

full=1 0 2
full=1 0 4
full=1 0 6
full=1 0 0
full=1 0 2
full=1 0 4
full=1 0 6
full=1 0 0
full=1 0 2
full=1 0 4
full=1 0 6
full=1 0 0
full=1 0 2
full=1 0 4
full=0 0 6
full=0 7 7
full=0 6 0
full=0 5 1
full=0 4 2
full=0 3 3
full=0 2 4
full=0 1 5
full=0 0 6
full=0 7 7
full=0 6 0
full=0 5 1
full=0 4 2
full=0 3 3
full=0 2 4

The first example behaves as you'd expect, but the second example is more complicated.

In your second example, before the subtraction, DataFull will be extended to 3 bits, then it will be bitwise inverted, yielding 7 and 6. When DataFull=0, ~DataFull=7. When DataFull=1, ~DataFull=6.