[Gotcha] Using functions in SystemVerilog constraints

[subbdue]

While SystemVerilog does allow using functions within constraints, you have to be careful while using it.

Consider this example from section 18.5.12 of SystemVerilog LRM 1800-2017:

class B;
  rand int x, y;
  constraint C { x <= F(y); }
  constraint D { y inside { 2, 4, 8 } ; }
endclass

Here are some things to note - once again from the LRM,

• Functions shall be called before constraints are solved, and their return values shall be treated as state variables.
• Random variables used as function arguments shall establish an implicit variable ordering or priority. Constraints that include only variables with higher priority are solved before other, lower priority constraints. Random variables solved as part of a higher priority set of constraints become state variables to the remaining set of constraints. For example, (the snippet of code above) forces y to be solved before x. Thus, constraint D is solved separately before constraint C, which uses the values of y and F(y) as state variables.
• In SystemVerilog, the behavior for variable ordering implied by function arguments differs from the behavior for ordering specified using the “solve...before...” constraint; function argument variable ordering subdivides the solution space thereby changing it. Because constraints on higher priority variables are solved without considering lower priority constraints at all, this subdivision can cause the overall constraints to fail.

So watch out while using functions in constraints, it could end up behaving differently from your expectations

Also, the Cadence simulator throws a warning like this when you use a function in your constraint:

Randomization constraint has this problem, which may cause the randomize function to generate incorrect values:
Function calc_output used in constraint is risky and has write access to non-local/persistent variable.

Here's another good article:
https://www.amiq.com/consulting/2015/03/12/gotcha-function-calls-in-systemverilog-constraints/