Re: Compiler default initialization of array types

[Kilgallen@SpamCop.net (Larry Kilgallen)]

In article <wcc3cr4vcti.fsf@shell01.TheWorld.com>, Robert A Duff <bobduff@shell01.TheWorld.com> writes:

> I have written code where there were lots of giant arrays of pointers,
> inside a record with another component indicating how many are valid.
> Something like:
> 
>     type T is
>         record
>             Last: Natural := 0;
>             Pointers: Pointer_Array(1..Big_Number);
> 
> Typically, objects of type T would use just a few pointers -- like Last
> might be 3 or 4, but very unusual for it to be 1000 or 1_000_000.
> 
> It turned out that initializing all those unused pointers to null was a
> serious performance problem.  So I cheated: I declared the array as an
> array of integers (where the integer type was chosen to be the same size
> as a pointer), and used unchecked_conversion.

Efficiency will depend on the compiler and potentially the operating
system.  For package-level data on VMS, compilers that need a lot of
zeroed memory will typically emit linker instructions to "fake it"
with manipulation of the memory management system.

>        It would also help if the OP explained why default-init is
> undesirable.  Efficiency?  Interfacing to objects initialized in some
> other language, or by hardware?