The values appear to be designed to be decoded quickly using e.g. bitmasks. �Comparing a general character string to "char" is expensive relative to a bitmask. Just write write a C code to that compares strncmp/strcmp to |/& and you'll see this clearly. One will observe that information appears to be coded in those values. You see that the third-to-last hex-digit indicates the number of bytes in the type, with the exception of long double, which uses c=12 when the type is actually 16 bytes. I don't know why this is, but it doesn't really matter since there is only one built-in type of this size. Inspecting the first- and second-to-list hex-digits indicates that signed and unsigned types often (but not always) differ by only one bit. I didn't write that code but it is clear that the author was concerned about performance, not human readability, although the scheme is clearly not random, so I suspect it is quite readable to the MPICH2 developers. You can look at OpenMPI for a different perspective on implementing these types: #define OMPI_PREDEFINED_GLOBAL(type, global) ((type) ((void *) &(global))) OMPI_DECLSPEC extern struct ompi_predefined_datatype_t ompi_mpi_char; #define MPI_CHAR OMPI_PREDEFINED_GLOBAL(MPI_Datatype, ompi_mpi_char) You'll have to look at the source if you want more information. I know very little about the internal design of OpenMPI. Best, Jeff On Sun, May 27, 2012 at 9:42 AM, ���� �� <[email protected]> wrote:

when I read mpich2-1.4.1 code, I saw code as follow:

typedef int MPI_Datatype; #define MPI_CHAR���������� ((MPI_Datatype)0x4c000101) #define MPI_SIGNED_CHAR��� ((MPI_Datatype)0x4c000118) #define MPI_UNSIGNED_CHAR� ((MPI_Datatype)0x4c000102) #define MPI_BYTE���������� ((MPI_Datatype)0x4c00010d) #define MPI_WCHAR��������� ((MPI_Datatype)0x4c00040e) #define MPI_SHORT��������� ((MPI_Datatype)0x4c000203) #define MPI_UNSIGNED_SHORT ((MPI_Datatype)0x4c000204) #define MPI_INT����������� ((MPI_Datatype)0x4c000405) #define MPI_UNSIGNED������ ((MPI_Datatype)0x4c000406) #define MPI_LONG���������� ((MPI_Datatype)0x4c000407) #define MPI_UNSIGNED_LONG� ((MPI_Datatype)0x4c000408) #define MPI_FLOAT��������� ((MPI_Datatype)0x4c00040a) #define MPI_DOUBLE�������� ((MPI_Datatype)0x4c00080b) #define MPI_LONG_DOUBLE��� ((MPI_Datatype)0x4c000c0c) #define MPI_LONG_LONG_INT� ((MPI_Datatype)0x4c000809) #define MPI_UNSIGNED_LONG_LONG ((MPI_Datatype)0x4c000819) #define MPI_LONG_LONG����� MPI_LONG_LONG_INT

Anyone can tell me what is the meaning of those hex values? and why does the author use them? why do they define "MPI_CHAR" as ((MPI_Datatype)0x4c000101) instead of "char"? thx

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-- Jeff Hammond Argonne Leadership Computing Facility University of Chicago Computation Institute [email protected] / (630) 252-5381 http://www.linkedin.com/in/jeffhammond https://wiki.alcf.anl.gov/parts/index.php/User:Jhammond