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To: aoliva@redhat.com, kenner@vlsi1.ultra.nyu.edu
Subject: Re: DATA_ALIGNMENT vs. DECL_USER_ALIGNMENT
Cc: gcc-patches@gcc.gnu.org, gcc@gcc.gnu.org
Message-Id: <20030421214129.C4585F28D2@nile.gnat.com>
Date: Tue, 22 Apr 2003 00:15:00 -0000
From: dewar@gnat.com (Robert Dewar)
X-SW-Source: 2003-04/txt/msg01020.txt.bz2

> And the reason is as I said: you specify alignment for a type both for
> interface and efficiency reasons, but for an object only for the latter.

Not quite. consider

   type R is array (Natural range <>) of Character;
   for R'Alignment use 1;

   X : R (1 .. 100);
   pragma Import (C, X);

now for the above, the generated code can only assume an alignment of 1.

But we might want to do

  Y : R (1 .. 1000);
  for Y'Alignment use 8;
  pragma Export (C, Y);

in a case where the C code expects an alignment for the object of 8 for
correct operation.

So even for objects, specifying an alignment different from that for the
type may be appropriately needed for functional correctness.