Re: [Gm2] Re: data alignment is now possible

[Iztok Kobal]

Gaius !

I would like to point out again that the things with packed pragma can 
be very complicated with arm architecture. I would be really happy if 
the regressions were passed on arm and some other RISC architectures 
before this stuff has been declared as really working !

I think that the old DrDobbs' article 
(http://www.drdobbs.com/architecture-and-design/184405435) pointed out 
clearly to possible issues !

As for the others - I would strongly discourage anybody to use packed 
structures for implementing e.g. communication protocols or HW accesses 
since the code in most cases can not be portable to other architectures. 
Endianness and structure field alignment issues should be solved 
conservative way - accessing bits of information within LOC (byte) 
arrays on offset basis. Also, forcing processor to work with unnatural 
alignment slows the bytecode execution radically.

If I were an advisor for GNUM2, I would never support the structure 
fields packing and clearly state that it is not supported and not to 
count on it.

Regards !


Gaius Mulley wrote:
> SiTex Graphics<address@hidden>  writes:
>
>    
> > Thanks.  That will be handy.
> >
> > I see now that my original question was poorly worded.  What I was
> > really after is a way to pack fields in a record as tightly as
> > possible.  Both XDS and StonyBrook have a global option for that.
> > Here's a simple example (source code attached).  With the following
> > type declaration:
> >
> >    Test = RECORD
> >      a : CARDINAL8;
> >      b : CARDINAL32;
> >      c : CARDINAL8;
> >    END;
> >
> > GM2 reports SIZE(Type) as 12, whereas StonyBrook and XDS report 6 with
> > packed field alignment.  With the other compilers changing the field
> > alginment does not require modificiation of the source code, and that
> > would be highly desirable. Maybe there's already a gcc-based
> > command-line option for field alignment?
> >
> > In any case the ability to specify variable alignment will be useful
> > in its own right.
> >
> > Thanks,
> > Scott
> >      
> Hi Scott,
>
> Sure I see no problem adding the __attribute__((packed)) concept that
> gcc uses into gm2.  This also provides continuity with the gcc family.
> Alas there is no command line option to specify field alignment -
> quite a few to warn if (packed) makes no difference.  I guess it is
> safer to insist on minor code changes rather than remembering which
> module was compiled with which option.  A global ISO Modula-2 comment
> directive might be another approach..
>
> regards,
> Gaius
>
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