[Mpiwg-large-counts] Large Count - the principles for counts, sizes, and byte and nonbyte displacements

[Jeff Squyres (jsquyres)]

On Oct 9, 2019, at 1:44 PM, Rolf Rabenseifner via mpiwg-large-counts <mpiwg-large-counts at lists.mpi-forum.org> wrote:

> I never said that that a number is a displacement.
> MPI_Aint is clearly used for 
> - addresses relative to a buffer begin,
> - absolute addresses (returned by MPI_GET_ADDRESS)
> - number of bytes.
> 
> MPI-3.1 says
> 2.5.6 Absolute Addresses and Relative Address Displacements
> Some MPI procedures use address arguments that represent an absolute address in the calling
> program, or relative displacement arguments that represent differences of two absolute
> addresses. The datatype of such arguments is MPI_Aint in C and INTEGER (KIND=
> MPI_ADDRESS_KIND) in Fortran. 

I thought the whole point of MPI_Aint was to support segmented address spaces.  Indeed, the next sentence in MPI-3.1 states:

"These types must have the same width and encode address values in the same manner such that address values in one language may be passed directly to another language without conversion."

This says to me that MPI_Aint can be encoded, which I take to mean the following:

1. This is exactly the reason why we have MPI_AINT_ADD / MPI_AINT_DIFF.
2. Presuming I have an MPI_ADDRESS_KIND value in Fortran that represents a 128 bit integer, the following is valid in C:

    typedef struct { uint64_t context; uint64_t pointer; } MPI_Aint;

Is that incorrect?

-- 
Jeff Squyres
jsquyres at cisco.com