guideline

Use strong typing with carefully selected constraints to reduce run-time constraint checking when measured performance indicates.

In this example, two potential constraint checks are eliminated. If the function Get_Response returns String, then the initialization of the variable Input would require constraint checking. If the variable Last is type Positive, then the assignment inside the loop would require constraint checking:

   ...
   subtype Name_Index is Positive range 1 .. 32;
   subtype Name       is String (Name_Index);
   ...
   function Get_Response return Name is separate;
   ...
begin
   ...
   Find_Last_Period:
      declare
         -- No Constraint Checking needed for initialization
         Input       : constant Name       := Get_Response;
         Last_Period :          Name_Index := 1;
      begin  -- Find_Last_Period
         for I in Input'Range loop
            if Input(I) = '.' then
               -- No Constraint Checking needed in  this `tight' loop
               Last_Period := I;
            end if;
         end loop;
         ...
      end Find_Last_Period;

rationale

Because run-time constraint checking is associated with slow performance, it is not intuitive that the addition of constrained subtypes could actually improve performance. However, the need for constraint checking appears in many places regardless of the use of constrained subtypes. Even assignments to variables that use the predefined subtypes may need constraint checks. By consistently using constrained subtypes, many of the unnecessary run-time checking can be eliminated. Instead, the checking is usually moved to less frequently executed code involved in system input. In the example, the function Get_Response may need to check the length of a user-supplied string and raise an exception.