RE: Data structure alignment

["Sims, Ken" <KSIMS@xxxxxxxxxxxxxxxx>]

Hi Peter, Richard, Jon, et.al. -

>How can one tell what boundary a data structure, or any other field for
that
>matter, is aligned on?

For my test programs, I put debug(*yes) in the H-spec.
I used eval [pointerfield] = %addr[field or data structure]
followed by dump to get a nice readable output with the addresses of the
fields and data structures.

For the V4R4 RPG IV compiler, my observations are: 

1. all data structures are currently 16-byte aligned, even ones containing
just a simple character field.  However as Barbara has said, you shouldn't
count on this always being the case for data structures that do not contain
pointers.

2. for multiple occurence data structures, only the first occurrence is
necessarily 16-byte aligned.  If the data structure contains pointers, all
occurrences are 16-byte aligned of course.  If a data structure contains
float or integer fields and the ALIGN keyword is not used, there are no gaps
between occurrences and so those fields can be aligned in some occurrences
and not aligned in other occurrences.  If the ALIGN keyword is used, there
will be gaps between occurrences as needed so that all of the occurrences
will start on the boundary needed to force alignment, but not necessarily on
a 16-byte boundary.

3. for individual fields, float and integer fields are aligned on the
appropriate boundary for performance as expected.  Packed and zoned fields
are not aligned in any particular way.  Interestingly, it appears that
individual character fields are always aligned to the boundary of the power
of 2 greater than or equal to the length of the field, up to a maximum of
16-byte alignment!  (Character fields in a data structure just follow each
other as expected.)  The order of fields in memory will be rearranged from
the order specified in the program to take advantage of the gaps left by
alignment.

But again, one should not count anything of the things mentioned above
(except pointer alignment) as being something that one can count on to stay
the same from release to release.

Ken
Southern Wine and Spirits of Nevada, Inc.
Opinions expressed are my own and do not necessarily represent the views of
my employer or anyone in their right mind.

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