[Mpi-22] New proposal: Support for large message counts

[Richard Treumann]

Are there systems today  that use a 128 bit long long?  Are there virtual
memory systems or filesystems coming in the reasonable future that use a
128 bit address range?

On IBM Power we support both 32 bit and 64 bit executables.  For both 32
bit and 64 bit executables, the C compilers treat int as 32 bits and long
long as 64 bit.   C long differs so  is 32 bits in a 32 bit executable  and
64 bits in a 64 bit executable (I.E. big enough to hold an
address/pointer).

The jump from 32 bits to 64 bits is a factor of 4 billion so it may be
goodness in the abstract to think about what comes after 64 bits gives way
to 128 bits, should we consider this a real problem?

I do not see much reason to worry about the fact that multiplying 2
arbitrary 64 bit values can overflow a 64 bit result unless we think some
real situation will call for 128 bit results.

Dick Treumann  -  MPI Team
IBM Systems & Technology Group
Dept X2ZA / MS P963 -- 2455 South Road -- Poughkeepsie, NY 12601
Tele (845) 433-7846         Fax (845) 433-8363

Alexander wrote
<snip>
> Another thing is that if we multiply to long values, the result may
> potentially overflow a long. So, for a library that allows long
> counts and long datatype extents, the internals of the library will
> have to be long long. long long (128 bit) arithmetic may be rather
> expensive on some CPUs. So, the 64-bit interface should probably be
optional.
<snip>





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