Re: Selective suppression of warnings --- gnat on GNU/Linux

[Robert A Duff <bobduff@shell01.TheWorld.com>]

"Jeffrey R. Carter" <spam.jrcarter.not@spam.acm.org> writes:

> Robert A Duff wrote:
>> More generally, it would also be nice to have a way to say, "Give me a
>> number that would be appropriate as the upper bound of an array whose
>> component type is T."  The number would be guaranteed to be big enough
>> that you would get Storage_Error if you ever create an array that big.
>
> Isn't that approximated by System.Max_Int and System.Max_Binary_Modulus?

Well, the RM doesn't say that, and it's not true on at least some
implementations.  For example, GNAT on a 32-bit machine has
System.Max_Int = 2**63-1.  But if I want a possibly large
array of Integers, I want something like:

    type Index is range 1..2**31-1;
    type A is array (Index range <>) of Integer;

Or I might want 2**32-1, but 2**63-1 is way too big for a 32-bit
machine, and would be unnecessarily inefficient.

> On many modern processors and OSes, though, one could possibly have a
> packed array of Boolean with those bounds that would fit in the
> available memory. But since you can't create an integer type with more
> values, the desired effect would be impossible to achieve.

Good point.

On a 64-bit machine, you have enough virtual memory to create a packed
array of 2**63-1 Booleans (2**60 bytes), but you don't have enough
physical memory, or even paging file, to store it.

- Bob