[sv-bc] Are $signed/$unsigned constant expressions?

From: Surya Pratik Saha <spsaha_at_.....>
Date: Wed Jul 12 2006 - 22:09:23 PDT

Hi,
Verilog 2K LRM does not consider a system function as constant. But Verilog 2005 enhanced it and considered some system functions as constant expressions provided their arguments are also constants.

Verilog 2005: (Section 5. Expressions)
Constant system function calls are calls to certain built-in system functions where the arguments are constant
expressions. When used in constant expressions, these function calls shall be evaluated at elaboration time.
The system functions which may be used in constant system function calls are "pure" functions, i.e. those
whose value depends only on their input arguments and which have no side effects. Specifically, the system
functions allowed in constant expressions are the conversion system functions listed in 17.8 and the
mathematical system functions listed in 17.11.

17.8 Conversion functions listed following system functions:
$rtoi converts real values to integers by truncating the real value (for example, 123.45 becomes 123)
$itor converts integers to real values (for example, 123 becomes 123.0)
$realtobits passes bit patterns across module ports; converts from a real number to the 64-bit
representation (vector) of that real number
$bitstoreal is the reverse of $realtobits; converts from the bit pattern to a real number.

17.11 Math functions includes only  $clog2 (yet to be implemented in Cheetah).

SV 1800 LRM further enhances the list so that $bits and array query functions also can be considered as constant
expressions.

So are $signed/$unsigned constant expressions?
Is the following case valid?

   module test();
      dut dut();
   endmodule // test

   module dut(i, o1, o2, o3, o4);
      input [3:0] i;
      output [7:0] o1, o2, o3, o4;
      reg [7:0]    o2, o3, o4;
      parameter    P = 2;
      localparam LP1 = P + 1;
      localparam signed [2:0] LP2 = LP1 - 1;
      localparam signed [7:0] LP3 = $signed(LP2[1:0]); // Is it constant or not
      assign o1 = LP2 * i + LP3;
      always @(i) begin : block
         localparam LQ1 = LP3 * LP3;
         o2 = LQ1;
         begin : block2
            localparam LQ2 = 3'sd1;
            o3 = LQ2 + $signed(o2);
         end
         o4 = foo(i) + foo(1);

      end

      function foo;
         localparam LR1 = P+P;
         input [2:0] fi;
         foo = fi * LR1;
      endfunction // foo

   endmodule // dut

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-- 
Regards
Surya.

  Received on Wed Jul 12 22:41:43 2006