Bit slicing with variable width in SystemVerilog - system-verilog

Question

I am trying to access certain sections of an array using the +: operator however getting the infamous [variable] is not a constant error. The only problem is, the width I would like the get from the array is changing as well.
This is the loop I have:
logic [N-1:0] a;
logic [2**N-2:0] b;
for (i = 0; i < N; i++)
a[i] = b[(2**i)-1 +: 2**i] == {(2**i){1'b1}};
In other words, if N = 4, I want this loop to do this:
a[0] = b[0:0] == 1'b1;
a[1] = b[2:1] == 2'b11;
a[2] = b[6:3] == 4'b1111;
a[3] = b[14:7] == 8'b11111111;
Logically, I'm pretty certain that the loop I provided works however SystemVerilog doesn't allow non-constants to be used for setting the width (after the a:).
How can I utilize the +: operator when my starting index and width are both dependent on a non-constant variable? or is there another way of doing this considering that N can be a large number.
Thanks!
EDIT:
This can be done with shifts, here is a working code:
for (i = 0; i < N; i++)
a[i] <= ((b >> (2**i)-1) << ((2**N) - (2**i) - 1)) ==
{(2**N-1){1'b1}} << ((2**N) - (2**i) - 1);

Answer 1

You cannot use +: with variable widths. It is actually just a short-hand for shifts and masks. For example, something like the following should work in your case:
logic [N-1:0] a;
logic [2**N-2:0] b;
always_comb begin
for (int i = 0; i < N; i++) begin
logic [2**N-2:0] tmpb, tmp1;
tmpb = b >> ((2**i)+1);
tmp1 = ((2**N)'(1) << (2**i)) - 1;
a[i] = (tmpb & tmp1) == 0;
end
end
you just need to figure out exact numbers of shifts and widths.

Answer 2

You can use a combination of '+:` operator with a mask
parameter N = 8; localparam N2 = 2**(N-1);
logic [N-1:0] a;
logic [2**N-1:0] b;
initial begin
b ={8'b000001,4'b1111,2'b01,1'b1};
for (int i = 0; i < N; i++)
a[i] = (b[(2**i)-1 +: N2] | ~N2'((1 << 2**i)- 1)) == '1;
$displayb(a,,b);
end

Answer 3

You can use +: provided that the right hand side is a constant or genvar.
logic [N-1:0] a;
logic [2**N-2:0] b;
for (genvar i = 0; i < N; i++) begin : gen_a
assign a[i] = b[(2**i)-1 +: 2**i] == {(2**i){1'b1}};
end
Note that this for-loop is a generate-for-loop which is not within a procedural block (ie begin-end)