Ada Programming/Attributes/'Restriction Set

Description

This attribute allows compile time testing of restrictions that are currently in effect. It is primarily intended for specializing code in the run-time based on restrictions that are active (e.g. don’t need to save fpt registers if restriction No_Floating_Point is known to be in effect), but can be used anywhere.

There are two forms:

System'Restriction_Set (partition_boolean_restriction_NAME)
System'Restriction_Set (No_Dependence => library_unit_NAME);

In the case of the first form, the only restriction names allowed are parameterless restrictions that are checked for consistency at bind time. For a complete list see the subtype System.Rident.Partition_Boolean_Restrictions.

The result returned is True if the restriction is known to be in effect, and False if the restriction is known not to be in effect. An important guarantee is that the value of a Restriction_Set attribute is known to be consistent throughout all the code of a partition.

This is trivially achieved if the entire partition is compiled with a consistent set of restriction pragmas. However, the compilation model does not require this. It is possible to compile one set of units with one set of pragmas, and another set of units with another set of pragmas. It is even possible to compile a spec with one set of pragmas, and then WITH the same spec with a different set of pragmas. Inconsistencies in the actual use of the restriction are checked at bind time.

In order to achieve the guarantee of consistency for the Restriction_Set pragma, we consider that a use of the pragma that yields False is equivalent to a violation of the restriction.

So for example if you write

if System'Restriction_Set (No_Floating_Point) then
   ...
else
   ...
end if;

And the result is False, so that the else branch is executed, you can assume that this restriction is not set for any unit in the partition. This is checked by considering this use of the restriction pragma to be a violation of the restriction No_Floating_Point. This means that no other unit can attempt to set this restriction (if some unit does attempt to set it, the binder will refuse to bind the partition).

Technical note: The restriction name and the unit name are intepreted entirely syntactically, as in the corresponding Restrictions pragma, they are not analyzed semantically, so they do not have a type.