There are scenarios in which programs may use low level techniques to modify variables that otherwise might be considered to be unassigned. For example, a variable can be passed to a procedure by reference, which takes the address of the parameter and uses the address to modify the variable's value, even though it is passed as an IN parameter. Consider the following example:
procedure P is Max_Length : constant Natural := 16; type Char_Ptr is access all Character; procedure Get_String(Buffer: Char_Ptr; Size : Integer); pragma Import (C, Get_String, "get_string"); Name : aliased String (1 .. Max_Length) := (others => ' '); Temp : Char_Ptr; function Addr (S : String) return Char_Ptr is function To_Char_Ptr is new Ada.Unchecked_Conversion (System.Address, Char_Ptr); begin return To_Char_Ptr (S (S'First)'Address); end; begin Temp := Addr (Name); Get_String (Temp, Max_Length); end;
where Get_String is a C function that uses the address in Temp to modify the variable Name. This code is dubious, and arguably erroneous, and the compiler would be entitled to assume that Name is never modified, and generate code accordingly.
However, in practice, this would cause some existing code that seems to work with no optimization to start failing at high levels of optimzization.
What the compiler does for such cases is to assume that marking a variable as aliased indicates that some "funny business" may be going on. The optimizer recognizes the aliased keyword and inhibits optimizations that assume the value cannot be assigned. This means that the above example will in fact "work" reliably, that is, it will produce the expected results.