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From: Brian Barrett (brbarret_at_[hidden])
Date: 2006-04-10 09:20:32

On Apr 10, 2006, at 4:31 AM, Ralf Wildenhues wrote:

> Apologies for a slightly off-topic (Autoconf-related) question.
> I believe OpenMPI deals with Fortran implementations where integer
> types contain internal padding (i.e., not all bits are used for the
> value); at least I read the configure macros this way.
> Is padding in C types a practical issue as well? Say, a 'short'
> with a
> maximum value of 32767 but 32 or 64 bits storage size?
> The only system we found that exhibits this was a Cray Y-MP from the
> cray-cyber museum (it's accessible and usable), but we don't know of
> more modern systems that do this.
> Background: there is discussion[1] over a macro to compute such
> ranges,
> fit also for cross compilation, but we're unsure about its practical
> value. I figured the readers here may have more such experience.

With very few exceptions (Fortran LOGICAL, the max value of a Fortran
handle (basically, the lower of INT_MAX and max value of Fortran
INTEGER), we try very hard to avoid looking at values in the user
buffers. Less work that way ;). George would have to say for sure,
but I think we would do the right thing for the Y-MP, as we look at
both size and alignment issues.

If I remember correctly, the oddness of some of those old Cray
machines for integers was nothing compared to their entertaining
floating point units. But I post-date all those machines, so all I
know is based on lore ;).

I think the Cray X1 does some interesting things with datatype
sizing, but nothing like the Y-MP. Not sure about the MTA machine.
OMPI is a long way from supporting either of those machines, so not a
huge deal for us. The other machines from Cray (the XT-3/Red Storm
and XD-1) are Opteron-based machines that basically only do the
standard LP64 setup. So no worries there.


   Brian Barrett
   Open MPI developer