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NAME

       shmem_double_max_to_all(3),       shmem_float_max_to_all(3),      shmem_int_max_to_all(3),
       shmem_int4_max_to_all(3),       shmem_int8_max_to_all(3),        shmem_long_max_to_all(3),
       shmem_longdouble_max_to_all(3),  shmem_longlong_max_to_all(3),  shmem_real4_max_to_all(3),
       shmem_real8_max_to_all(3),   shmem_real16_max_to_all(3),    shmem_short_max_to_all(3)    -
       Performs a maximum function reduction across a set of processing elements (PEs).

SYNOPSIS

       C or C++:
       #include <mpp/shmem.h>

       void shmem_double_max_to_all(double *target, const double *source,
         int nreduce, int PE_start, int logPE_stride, int PE_size,
         double *pWrk, long *pSync);

       void shmem_float_max_to_all(float *target, const float *source,
         int nreduce, int PE_start, int logPE_stride, int PE_size,
         float *pWrk, long *pSync);

       void shmem_int_max_to_all(int *target, const int *source,
         int nreduce, int PE_start, int logPE_stride, int PE_size,
         int *pWrk, long *pSync);

       void shmem_long_max_to_all(long *target, const long *source,
         int nreduce, int PE_start, int logPE_stride, int PE_size,
         long *pWrk, long *pSync);

       void shmem_longdouble_max_to_all(long double *target,
         const long double *source, int nreduce, int PE_start,
         int logPE_stride, int PE_size, long double *pWrk, long *pSync);

       void shmem_longlong_max_to_all(long long *target,
         const long long *source, int nreduce,  int PE_start,
         int logPE_stride, int PE_size, long long *pWrk, long *pSync);

        void shmem_short_max_to_all(short *target, const short *source,
         int nreduce, int PE_start, int logPE_stride, int PE_size,
         short *pWrk, long *pSync);

       Fortran:
       INCLUDE "mpp/shmem.fh"

       INTEGER pSync(SHMEM_REDUCE_SYNC_SIZE)

       INTEGER nreduce, PE_start, logPE_stride, PE_size

       CALL SHMEM_INT4_MAX_TO_ALL(target, source, nreduce,
       & PE_start, logPE_stride, PE_size, pWrk, pSync)

       CALL SHMEM_INT8_MAX_TO_ALL(target, source, nreduce,
       & PE_start, logPE_stride, PE_size, pWrk, pSync)

       CALL SHMEM_REAL4_MAX_TO_ALL(target, source, nreduce,
       & PE_start, logPE_stride, PE_size, pWrk, pSync)

       CALL SHMEM_REAL8_MAX_TO_ALL(target, source, nreduce,
       & PE_start, logPE_stride, PE_size, pWrk, pSync)

       CALL SHMEM_REAL16_MAX_TO_ALL(target, source, nreduce,
       & PE_start, logPE_stride, PE_size, pWrk, pSync)

DESCRIPTION

       The  shared  memory  (SHMEM)  reduction  routines  compute  one  or more reductions across
       symmetric arrays on multiple virtual PEs.  A  reduction  performs  an  associative  binary
       operation  across  a  set  of  values.  For  a list of other SHMEM reduction routines, see
       intro_shmem(3).

       As with all SHMEM collective routines, each of these routines assumes that only PEs in the
       active  set  call  the  routine.  If  a  PE not in the active set calls a SHMEM collective
       routine, undefined behavior results.

       The nreduce argument determines the number of separate reductions to perform.  The  source
       array on all PEs in the active set provides one element for each reduction. The results of
       the reductions are placed in the target array on all PEs in the active set. The active set
       is defined by the PE_start, logPE_stride, PE_size triplet.

       The  source  and  target  arrays  may  be  the same array, but they may not be overlapping
       arrays.

       The arguments are as follows:

       target A symmetric array of  length  nreduce  elements  to  receive  the  results  of  the
              reduction  operations.  The  data  type  of  target  varies with the version of the
              reduction routine being called. When calling from C, refer to the SYNOPSIS  section
              for data type information.

       When calling from Fortran, the target data types are as follows:

              shmem_comp8_max_to_all: Complex, with an element size equal to two
                     8-byte real values.

              shmem_int4_max_to_all: Integer, with an element size of 4 bytes.

              shmem_int8_max_to_all: Integer, with an element size of 8 bytes.

              shmem_real4_max_to_all: Real, with an element size of 4 bytes.

              shmem_real16_max_to_all: Real, with an element size of 16 bytes.

       source A  symmetric  array  of  length nreduce elements that contains one element for each
              separate reduction operation. The source argument must have the same data  type  as
              target.

       nreduce
              The  number  of  elements  in the target and source arrays. nreduce must be of type
              integer. If you are using Fortran, it must be a default integer value.

       PE_start
              The lowest virtual PE number of the active set of PEs. PE_start  must  be  of  type
              integer. If you are using Fortran, it must be a default integer value.

       logPE_stride
              The log (base 2) of the stride between consecutive virtual PE numbers in the active
              set. logPE_stride must be of type integer. If you are using Fortran, it must  be  a
              default integer value.

       PE_size
              The  number  of  PEs in the active set. PE_size must be of type integer. If you are
              using Fortran, it must be a default integer value.

       pWrk   A symmetric work array. The pWrk argument must have the same data type  as  target.
              In   C/C++,   this  contains  max(nreduce/2  +  1,  _SHMEM_REDUCE_MIN_WRKDATA_SIZE)
              elements.     In     Fortran,     this     contains     max(nreduce/2     +      1,
              SHMEM_REDUCE_MIN_WRKDATA_SIZE) elements.

       pSync  A   symmetric   work   array.   In   C/C++,   pSync   is  of  type  long  and  size
              _SHMEM_REDUCE_SYNC_SIZE.  In  Fortran,  pSync  is  of   type   integer   and   size
              SHMEM_REDUCE_SYNC_SIZE.  If  you  are  using  Fortran, it must be a default integer
              value.   Every  element  of  this  array  must  be  initialized  with   the   value
              _SHMEM_SYNC_VALUE (in C/C++) or SHMEM_SYNC_VALUE (in Fortran) before any of the PEs
              in the active set enter the reduction routine.

       The values of arguments nreduce, PE_start, logPE_stride, and PE_size must be equal on  all
       PEs in the active set. The same target and source arrays, and the same pWrk and pSync work
       arrays, must be passed to all PEs in the active set.

       Before any PE calls a reduction routine, you must ensure  that  the  following  conditions
       exist (synchronization via a barrier or some other method is often needed to ensure this):
       The pWrk and pSync arrays on all PEs in the active set are not still in use from  a  prior
       call to a collective SHMEM routine. The target array on all PEs in the active set is ready
       to accept the results of the reduction.

       Upon return from a reduction routine, the following are true for the local PE: The  target
       array is updated. The values in the pSync array are restored to the original values.

NOTES

       The  terms  collective,  symmetric,  and cache aligned are defined in intro_shmem(3).  All
       SHMEM reduction routines reset the values in pSync before they  return,  so  a  particular
       pSync buffer need only be initialized the first time it is used.

       You  must  ensure  that  the  pSync array is not being updated on any PE in the active set
       while any of the PEs participate in processing of a SHMEM reduction routine. Be careful of
       the  following  situations:  If  the  pSync array is initialized at run time, some type of
       synchronization is needed to ensure that all PEs in the working set have initialized pSync
       before  any  of  them enter a SHMEM routine called with the pSync synchronization array. A
       pSync or pWrk array can be reused in a subsequent reduction routine call only if  none  of
       the  PEs  in  the active set are still processing a prior reduction routine call that used
       the same pSync or pWrk arrays.

       In general, this can be assured only by doing some type of  synchronization.  However,  in
       the  special  case  of  reduction  routines being called with the same active set, you can
       allocate two pSync and pWrk arrays and alternate between them on successive calls.

EXAMPLES

       Example 1: This Fortran example statically initializes  the  pSync  array  and  finds  the
       maximum value of real variable FOO across all even PEs.
       INCLUDE "mpp/shmem.fh"

       INTEGER PSYNC(SHMEM_REDUCE_SYNC_SIZE)
       DATA PSYNC /SHMEM_REDUCE_SYNC_SIZE*SHMEM_SYNC_VALUE/
       PARAMETER (NR=1)
       REAL FOO, FOOMAX, PWRK(MAX(NR/2+1,SHMEM_REDUCE_MIN_WRKDATA_SIZE))
       COMMON /COM/ FOO, FOOMAX, PWRK
       INTRINSIC MY_PE

       IF ( MOD(MY_PE(),2) .EQ. 0) THEN
         CALL SHMEM_REAL8_MAX_TO_ALL(FOOMAX, FOO, NR, 0, 1, N$PES/2,
         & PWRK, PSYNC)
         PRINT *, 'Result on PE ', MY_PE(), ' is ', FOOMAX
       ENDIF

       Example 2: Consider the following C/C++ call:
       shmem_int_max_to_all( target, source, 3, 0, 0, 8, pwrk, psync );

       The  preceding  call is more efficient, but semantically equivalent to, the combination of
       the following calls:
       shmem_int_max_to_all(&(target[0]), &(source[0]), 1, 0, 0, 8,
         pwrk1, psync1);
       shmem_int_max_to_all(&(target[1]), &(source[1]), 1, 0, 0, 8,
         pwrk2, psync2);
       shmem_int_max_to_all(&(target[2]), &(source[2]), 1, 0, 0, 8,
         pwrk1, psync1);

       Note  that  two  sets  of  pWrk  and  pSync  arrays  are  used  alternately   because   no
       synchronization is done between calls.

SEE ALSO

       intro_shmem(3)