This pragma is similar to Normalize_Scalars conceptually but has two important differences. First, there is no requirement for the pragma to be used uniformly in all units of a partition, in particular, it is fine to use this just for some or all of the application units of a partition, without needing to recompile the run-time library.
In the case where some units are compiled with the pragma, and some without, then a declaration of a variable where the type is defined in package Standard or is locally declared will always be subject to initialization, as will any declaration of a scalar variable. For composite variables, whether the variable is initialized may also depend on whether the package in which the type of the variable is declared is compiled with the pragma.
The other important difference is that you can control the value used for initializing scalar objects. At bind time, you can select several options for initialization. You can initialize with invalid values (similar to Normalize_Scalars, though for Initialize_Scalars it is not always possible to determine the invalid values in complex cases like signed component fields with non-standard sizes). You can also initialize with high or low values, or with a specified bit pattern. See the GNAT User's Guide for binder options for specifying these cases.
This means that you can compile a program, and then without having to recompile the program, you can run it with different values being used for initializing otherwise uninitialized values, to test if your program behavior depends on the choice. Of course the behavior should not change, and if it does, then most likely you have an incorrect reference to an uninitialized value.
It is even possible to change the value at execution time eliminating even the need to rebind with a different switch using an environment variable. See the GNAT User's Guide for details.
Note that pragma Initialize_Scalars is particularly useful in conjunction with the enhanced validity checking that is now provided in GNAT, which checks for invalid values under more conditions. Using this feature (see description of the `-gnatV' flag in the GNAT User's Guide) in conjunction with pragma Initialize_Scalars provides a powerful new tool to assist in the detection of problems caused by uninitialized variables.
Note: the use of Initialize_Scalars has a fairly extensive effect on the generated code. This may cause your code to be substantially larger. It may also cause an increase in the amount of stack required, so it is probably a good idea to turn on stack checking (see description of stack checking in the GNAT User's Guide) when using this pragma.