pragma Pure_Function ([Entity =>] function_LOCAL_NAME);
This pragma appears in the same declarative part as a function declaration (or a set of function declarations if more than one overloaded declaration exists, in which case the pragma applies to all entities). It specifies that the function Entity is to be considered pure for the purposes of code generation. This means that the compiler can assume that there are no side effects, and in particular that two calls with identical arguments produce the same result. It also means that the function can be used in an address clause.
Note that, quite deliberately, there are no static checks to try to ensure that this promise is met, so Pure_Function can be used with functions that are conceptually pure, even if they do modify global variables. For example, a square root function that is instrumented to count the number of times it is called is still conceptually pure, and can still be optimized, even though it modifies a global variable (the count). Memo functions are another example (where a table of previous calls is kept and consulted to avoid re-computation).
Note also that the normal rules excluding optimization of subprograms in pure units (when parameter types are descended from System.Address, or when the full view of a parameter type is limited), do not apply for the Pure_Function case. If you explicitly specify Pure_Function, the compiler may optimize away calls with identical arguments, and if that results in unexpected behavior, the proper action is not to use the pragma for subprograms that are not (conceptually) pure.
Note: Most functions in a Pure package are automatically pure, and there is no need to use pragma Pure_Function for such functions. One exception is any function that has at least one formal of type System.Address or a type derived from it. Such functions are not considered pure by default, since the compiler assumes that the Address parameter may be functioning as a pointer and that the referenced data may change even if the address value does not. Similarly, imported functions are not considered to be pure by default, since there is no way of checking that they are in fact pure. The use of pragma Pure_Function for such a function will override these default assumption, and cause the compiler to treat a designated subprogram as pure in these cases.
Note: If pragma Pure_Function is applied to a renamed function, it applies to the underlying renamed function. This can be used to disambiguate cases of overloading where some but not all functions in a set of overloaded functions are to be designated as pure.
If pragma Pure_Function is applied to a library level function, the function is also considered pure from an optimization point of view, but the unit is not a Pure unit in the categorization sense. So for example, a function thus marked is free to with non-pure units.