The return type of $bits should be `integer`, not an unsigned type.

[zachjs]

module top;
    integer x;
    localparam Y = 1'sb1;
    localparam integer Z = 1'sb1;
    initial begin
        $display("%b", $bits(x) ^ 1'sb1);
        $display("%b", $bits(x) ^ Y);
        $display("%b", $bits(x) ^ Z);
        $display("%b", integer'($bits(x)) ^ 1'sb1);
        $display("%b", integer'($bits(x)) ^ Y);
        $display("%b", integer'($bits(x)) ^ Z);
    end
endmodule

On the above test case, iverilog -g2012 test.v && ./a.out outputs:

00000000000000000000000000100001
00000000000000000000000000100001
11111111111111111111111111011111
11111111111111111111111111011111
11111111111111111111111111011111
11111111111111111111111111011111

By contrast, all commercial tools on edaplayground.com output:

11111111111111111111111111011111
11111111111111111111111111011111
11111111111111111111111111011111
11111111111111111111111111011111
11111111111111111111111111011111
11111111111111111111111111011111

Per IEEE 1800-2017 Section 20.6.2:

The $bits system function returns the number of bits required to hold an expression as a bit stream. The return type is integer.

Hence, $bits(x) and integer'($bits(x)) should be equivalent.

[martinwhitaker]

Yes, 1800-2005 didn't define the return type, so as $bits can never return a negative value, I set the return type to unsigned.

[martinwhitaker]

Fixed in the master branch.