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From: Robert A Duff <bobduff@shell01.TheWorld.com>
Newsgroups: comp.lang.ada
Subject: Re: Efficiency of code generated by Ada compilers
Date: Tue, 10 Aug 2010 08:39:36 -0400
Organization: The World Public Access UNIX, Brookline, MA
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Date: 2010-08-10T08:39:36-04:00
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Jacob Sparre Andersen <sparre@nbi.dk> writes:

> Robert A Duff wrote:
>> Jeffrey Carter <spam.jrcarter.not@spam.not.acm.org> writes:
>
>>> type T is range 0 .. 2 ** N - 1;
>>> for T'Size use N;
>
>> Yes.  And T'Size = N by default in Ada >= 95, so I would write:
>>
>>     pragma Assert(T'Size = N);
>>
>> instead of the Size clause.
>
> But don't you loose the compile time error in case the compiler
> cannot make T'Size = N?

No.  Compilers are required to make T'Size = N.  See 13.3(54-55).
The Assert is just a hint that we care about that fact.

Suppose N = 32, and I make a mistake, and write:

    pragma Assert (T'Size = 33);

GNAT will warn that this is False, which is what I want.
(And in -gnatwe mode, that's an error.)

If I instead wrote "for T'Size use 33;", the compiler
will not catch the error, but will instead use a strange
Size.

That's why I don't like to use rep clauses to assert that
the compiler chose the obviously-correct representation --
the compiler will complain if it's too small, but not
if it's too big.

Consider:

    type M is mod 32; -- Oops, I meant 2**32.
    for M'Size use 32; -- No error here.

I've seen this bug in real code.

- Bob