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

[Lucas Redding <lucas.redding@gmail.com>]

On Monday, September 7, 2015 at 12:39:35 PM UTC+1, Lucas Redding wrote:
> Hi
> 
> I was wondering how the addressing mechanism works in GNAT Ada? Is it so vastly different to Object Ada? Is there a way I can tailor the GNAT addressing mechanism to follow that of Object Ada?
> 
> 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.
> 
> I have compiled the program is GNAT Ada but I am getting run time errors such as "misaligned address value". More worryingly, any data read from memory that relies on the addressing mechanisms described above is incoherent.
> 
> I am interested in moving to GNAT Ada because I believe the accompanying toolset is much richer and therefore more productive but I am beginning to wonder whether this is too onerous task?
> 
> Any help is greatly appreciated.
> 
> Lucas

Thanks Everybody.

Although I still have not completely resolved the issue, by analysing the output from -gnatR I can see the issue is with the alignment of underlying type and object specifications. The original code has no alignment clauses at all. 

We know that all the objects (memory input to the application) are mapped to memory so that everything is aligned at 1. 

I deduce that this means that OA by default aligns objects to 1 since the objects are being read without any alignment clauses. 

On the other hand, GNAT aligns each object to a boundary of a power of 2 and adjusts this based on the component size.

Here lies the issue.

I need to change the alignment of each underlying type mapped into memory. I am hoping the combination of explicit alignments and precise rep clauses (size etc.) should make the code portable between compilers.

Fingers crossed while I go and do the work and will report back when I finish (or if I get stuck).

Many thanks for all your help.

Much appreciated.

Lucas