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From: Robert Dewar <robert_dewar@my-deja.com>
Subject: Re: Portability questions
Date: 2000/04/25
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 <wccbt2ycni0.fsf@world.std.com>
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In article <wccbt2ycni0.fsf@world.std.com>,
  Robert A Duff <bobduff@world.std.com> wrote:

> Without pragma Import, the compiler can do what it likes
> initialize B or not.  I suspect most compilers do not.  I
> think compilers should not.

Why do you think that? This is a case where the quality and
efficiency of the code is greatly improved by making sure
that there are no unused garbage bits in the value, which can
otherwise happen with packed arrays. This is very similar to
the trade off choice of whether to clear garbage bits in records
to speed up record comparisons.

> With pragma Import on B, I would consider it a bug for a
> compiler to initialize B, although I'm not sure I can prove
> that from the RM.

Most certainly pragma Import suppresses the initialization, and
I find Bob's "not sure" here odd. This is one of the cases
in which GNAT gives a helpful warning:

Warning: default initialization of x may modify overlaid storage
Warning: packed array components will be initialized to zero
Warning: Use pragma Import for x to suppress initialization
                                                  RM B.1(24)

As you see, this is a case where GNAT includes the RM reference,
and it seems clear enough, no?

24   The declaration of an imported object shall not include an
explicit initialization expression.  Default initializations are
not performed.

> Then there's pragma Normalize_Scalars, which probably has some
> effect on all this.

NS certainly causes more default initialization :-)

Robert Dewar


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