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Subject: Re: [OMPI devel] RFC: add support for large counts using derived datatypes
From: Nathan Hjelm (hjelmn_at_[hidden])
Date: 2013-07-16 17:08:43


On Tue, Jul 16, 2013 at 11:03:27PM +0200, George Bosilca wrote:
>
> On Jul 16, 2013, at 22:29 , Jeff Squyres (jsquyres) <jsquyres_at_[hidden]> wrote:
>
> > On Jul 16, 2013, at 4:22 PM, George Bosilca <bosilca_at_[hidden]> wrote:
> >
> >> Btw, I have a question to you fellow MPI Forum attendees. I just can't remember why the MPI forum felt there was a need for the MPI_Type_get[_true]_extent_x? MPI_Count can't be bigger than MPI_Aint,
> >
> > Yes, it can -- it has to be the largest integer type (i.e., it even has to be able to handle an MPI_Offset).
>
> Technicalities! In the entire standard MPI_Offset is only used to access files, not to build datatypes. As such there is no way to have the extent of an datatype bigger than MPI_Aint. Thus, these accessors returning MPI_Count are a useless overkill, as they cannot offer more precision that what the version returning MPI_Aint is already offering.
>
> George.
>
> PS: I hope nobody has the idea to define the MPI_Offset as a signed type ?

Externally MPI_Offset is defines as a signed type (long long, long, or int) but internally it is treated as unsigned. I will update MPI_Count to have the same treatment (since it can be MPI_UNDEFINED which is a negative number).

-Nathan