Re: Addressing in Object Ada v/s GNAT (2013) showing Vast Differences

["Jeffrey R. Carter" <spam.jrcarter.not@spam.not.acm.org>]

On 09/07/2015 04:39 AM, Lucas Redding wrote:
> 
> I have a program that runs perfectly in Object Ada. The program relies
> heavily on "USE AT" clauses together with 'SIZE on underlying type
> definition. This is in order to map directly and deterministically to memory
> with bit precision. The data is independently produced so this is very
> important.

"Use at" is the Ada-83 mechanism for specifying an object's address; since Ada
95 the form

for V'Address use A;

is preferred, and "use at" is considered obsolete. I would guess, then, that
this program has been around for quite a while, having originally been Ada 83,
and that there has been on-going effort to port it to new hardware, OS versions,
and compilers to keep it available.

Addresses are something that are platform and compiler dependent. It probably
took a fair amount of effort to understand how Object Ada lays things out in
memory to get to the version you currently have. A similar amount of effort
should be expected to understand GNAT's approach to these things and to modify
the representation clauses to achieve the same representation as OA. You've
already found that GNAT wants to align things differently from OA, so specifying
non-default alignments may be necessary.

-- 
Jeff Carter
"Why, the Mayflower was full of Fireflies, and a few
horseflies, too. The Fireflies were on the upper deck,
and the horseflies were on the Fireflies."
Duck Soup
95