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From: "jimmaureenrogers@worldnet.att.net" <jimmaureenrogers@worldnet.att.net>
Newsgroups: comp.lang.ada
Subject: Re: Allocated aligned arrays
Date: 18 Nov 2005 14:06:00 -0800
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Date: 2005-11-18T14:06:00-08:00
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ldb wrote:
> I'm trying to allocate two floating point (4 bytes each) arrays that
> are both address-aligned to 16. Ideally, the first element of each
> array is aligned on 16 (ie, the address mod 16 = 0), but that's not
> really necessary, as long as they are both aligned to the same mod 16
> (i don't care if the first element is 8 or 12 or whatever, as long as
> both arrays have the same).
>
> I'm trying to set up these arrays so I can use aligned SSE instructions
> in assembly.
>
> I have tried something along the lines of:
>
> type Matrix is array (INTEGER range <>, INTEGER range <>) of float;
> for Matrix'Alignment use 16;
>
>
> and then, for an input of 0..200 by 0..200
>
> sam := new Matrix(0..200,2..198)
> bob := new Matrix(2..198,2..198);
>
>
> These are the actual indicies I would use (it's an image processing
> algorithm that trims the edges). I'm not sure if the strange indices
> could be causing the problem, but I cannot imagine it is.
>
> However, the three matricies, input, sam, and bob aren't necessarily
> aligned (for certain input index ranges they are, by default, and some
> times they are not. The alignment statement I am using seems to have no
> effect).

You may specify alignment only for the first subtype of a base type.
Thus, you may want to define a new type:

type Real is new float;
for Real'Alignment use 16;

The AARM contains the following bit of wisdom:

"The Alignment of a composite object is always equal to the least
common multiple of the Alignments of its components, or a multiple
thereof."

Jim Rogers