Regarding const'() cast

SystemVerilog
,
1

Hi ,
I was trying to understand working of const’() cast.
LRM 6.24.1 mentions


An expression may be changed to a constant with a const cast. 
const'(x)

Consider the following code :


class C;
  
  rand bit[2:0] a ;  
  function void pre_randomize();
   // 'a' could be assigned here
  endfunction
  constraint CONST { a == const'(a) + 1 ; }
  function void post_randomize();
   // 'a' could be assigned here
  endfunction
  
endclass  

C c1;

module test;
  
initial begin
  c1 = new();
  
  repeat(5) begin
    if( c1.randomize() )
      $display("Success with %p",c1);
    else  $display("Fails");
      // c1.a could also be assigned here prior to call randomize() again
  end
  
end  
endmodule

Consider following 3 cases where variable ‘a’ is assigned.

  1. Via pre_randomize()
  2. Via post_randomize()
  3. Via assignment to variable ‘a’ after randomize() call i.e assignment is done after post_randomize()

When we mention const’(a) which value of a should it pick ? Is it just before the constraints are solved i,e value assigned in pre_randomize() ?
There could be any possible combination of 1,2,3 i.e all 3 exist or only 1 and 2 exist or 1 and 3 exist , etc

2

In reply to hisingh:

As with any other non-random variable or expression, the constraint solver uses that values after pre_randomize completes to solve the constraints.

3

In reply to dave_59:

Thanks Dave.
So in absence of assignment within pre_randomize() , the value assigned procedurally would be used. In absence of both assignment within pre_randomize() and procedural assignment after randomize() is successful , the value assigned within post_randomize() would be used.

4

When casting an expression as a constant, the type of the expression to be cast shall pass through unchanged. The only effect is to treat the value as though it had been used to define a const variable of the type of the expression.
I see some simulators failed the compilation, based on the statements above it seems to be illegal to do constant cast to rand variable in constraint.

5

Pass through unchanged means it keeps the value it has as if defined as a const. So randomization can’t change the value.

6

Hi Dave,
If randomization can’t change the value of a, how could this constraint be satisfied in this case?
Isn’t it a conflict?