Using constant value from an interface #575
Description
I'm trying to get a constant value out of an interface to use at the next level up in the hierarchy. The reason for this is I have a module instantiating memory, the memory has different read delay depending on its size, and the next level up needs to know the delay without violating encapsulation.
I understand that constant values are supposed to be extractable from interfaces (?), although this seems more likely to work using $bits(). I have come up with the following simple test case showing a failure, a bug, and a workaround:
//
// Want to get the important calculated value A out of the interface.
//
interface subm #( parameter X=0, parameter Y=0 ) ();
parameter A = X+Y;
parameter [A:1] B = 0;
reg [A:1] C;
endinterface
module top();
subm #( .X(1), .Y(1000) ) sub ();
localparam CT1 = -1; // sub.A; // sub.A fails here at compile time with Icarus 11.0
localparam CT2 = $bits(sub.B);
localparam CT3 = $bits(sub.C);
initial $display("\n\n\nCompile-time value of A is %0d, %0d, %0d", CT1, CT2, CT3);
initial $display("Run-time value of A is %0d, %0d, %0d\n\n", sub.A, $bits(sub.B), $bits(sub.C));
endmodule // top
In Icarus Verilog 11.0 this prints:
Compile-time value of A is -1, 1001, 0
Run-time value of A is 1001, 1001, 1001
It seems to me that all of these statements should compile and that all of them should give the same results.
That may be arguable, but it's certainly a bug for $bits(sub.C) to return 0 to a localparam -- I believe it should return a correct value, but if not then it should return a compile-time error rather than returning a bad value.
Also, it seems that Icarus has everything in place to allow sub.A to be treated as a constant and extracted from the interface, since $bits(sub.B) effectively does this. So perhaps it's not too hard to make sub.A work in a future version?
In any case, $bits(sub.B) seems to be a workaround to my problem, although it's ugly. This may help others with a similar problem. Hopefully it synthesizes!