SHMEM_LONG_OR_TO_ALL(3)

🎀 NAME 🎀

shmem_int_or_to_all, shmem_int4_or_to_all, shmem_int8_or_to_all, shmem_long_or_to_all, shmem_longlong_or_to_all, shmem_short_or_to_all — bestie, these babies do a bitwise OR function reduction across a set of processing elements (PEs). that's the whole job. iconic behavior honestly 💅

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🌸 SYNOPSIS 🌸

C or C++

#include <mpp/shmem.h>

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

void shmem_longlong_or_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_or_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_OR_TO_ALL(target, source, nreduce, PE_start,
& logPE_stride, PE_size, pWrk, pSync)

CALL SHMEM_INT8_OR_TO_ALL(target, source, nreduce, PE_start,
& logPE_stride, PE_size, pWrk, pSync)

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💖 DESCRIPTION 💖

okay so SHMEM reduction routines compute one or more reductions across symmetric arrays on multiple virtual PEs. a reduction is just an associative binary operation getting applied across a set of values, gorgeous, no notes. and if you want the whole roster of other SHMEM reduction routines, go read intro_shmem(3) 🫶

and now for the part where I need you to lock in 🔒 — as with all SHMEM collective routines, each of these routines assumes that only the PEs in the active set call it. if a PE that is not in the active set calls a SHMEM collective routine, undefined behavior results. that is not a vibe. do not do that fr fr 😭

the nreduce argument determines the number of separate reductions to perform. the source array on all PEs in the active set hands over one element for each reduction. the results of the reductions get placed in the target array on all PEs in the active set. and the active set itself is defined by the PE_start, logPE_stride, PE_size triplet. trio. the holy trinity of "who's actually invited to this party" ✨

the source and target arrays may be the same array, but they may not be overlapping arrays. same = slay. overlapping = absolutely not 🚫

The arguments are as follows:

• target — a symmetric array of length nreduce elements that receives the results of the reduction operations 💝 the data type of target varies with whichever version of the reduction routine you're calling. when you're calling from C/C++, go look at the SYNOPSIS section for the data type tea. when you're calling from Fortran, the target data types are:
  • shmem_int8_or_to_all — Integer, with an element size of 8 bytes.
  • shmem_int4_or_to_all — Integer, with an element size of 4 bytes.
• source — a symmetric array, of length nreduce elements, holding one element for each separate reduction operation. the source argument must have the same data type as target. matching fits only, bestie 👯
• nreduce — the number of elements in the target and source arrays. nreduce must be of type integer. if you're 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're 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're using Fortran, it must be a default integer value. yes it's a log base 2, yes that's on purpose 💜
• PE_size — the number of PEs in the active set. PE_size must be of type integer. if you're 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're using Fortran, it must be a default integer value. every single element of this array has to 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 👑 don't skip this. not even a little.

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the values of the arguments nreduce, PE_start, logPE_stride, and PE_size must be equal on all PEs in the active set. and the same target and source arrays, plus the same pWrk and pSync work arrays, must be passed to all PEs in the active set. everyone agrees or nobody eats 🍓

before any PE calls a reduction routine, you have to make sure the following conditions exist (synchronization via a barrier or some other method is often needed to make this true, so like… plan for it):

• 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:

• the target array is updated ✅
• the values in the pSync array are restored to the original values ✅

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🧚 NOTES 🧚

the terms collective, symmetric, and cache aligned are defined over in intro_shmem(3), go check it out. all SHMEM reduction routines reset the values in pSync before they return, so a particular pSync buffer only needs to be initialized the first time it's used — casual little time-saver, love that for us 💅

also you must make sure that the pSync array is not being updated on any PE in the active set while any of the PEs are participating in processing of a SHMEM reduction routine. be careful and dodge these situations:

• if the pSync array is initialized at run time, some type of synchronization is needed to make sure all PEs in the working set have initialized pSync before any of them enter a SHMEM routine called with that 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. two sets, tag-teaming. we love a workaround 🌈

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🎀 EXAMPLES 🎀

✨ Example 1

this Fortran example statically initializes the pSync array and finds the logical OR of the integer 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 PWRK(MAX(NR/2+1,SHMEM_REDUCE_MIN_WRKDATA_SIZE))
INTEGER FOO, FOOOR
COMMON /COM/ FOO, FOOOR, PWRK
INTRINSIC MY_PE

IF ( MOD(MY_PE(),2) .EQ. 0) THEN
  CALL SHMEM_INT8_OR_TO_ALL(FOOOR, FOO, NR, 0, 1, N$PES/2,
  & PWRK, PSYNC)
  PRINT *,'Result on PE ',MY_PE(),' is ',FOOOR
ENDIF

✨ Example 2

consider the following C/C++ call:

shmem_int_or_to_all( target, source, 3, 0, 0, 8, pwrk, psync );

that preceding call is more efficient than — but semantically equivalent to — the combination of the following calls:

shmem_int_or_to_all(&(target[0]), &(source[0]), 1, 0, 0, 8,
  pwrk1, psync1);
shmem_int_or_to_all(&(target[1]), &(source[1]), 1, 0, 0, 8,
  pwrk2, psync2);
shmem_int_or_to_all(&(target[2]), &(source[2]), 1, 0, 0, 8,
  pwrk1, psync1);

and note that two sets of pWrk and pSync arrays are used alternately because no synchronization is done between calls. see? told you the two-array tag-team was real 😌

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👑 SEE ALSO 👑

• intro_shmem(3)

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💜 COPYRIGHT 💜

Copyright 2003-2026, The Open MPI Community.

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and that's the whole page bestie 🫶 this was shipped in the Open MPI manual as SHMEM_LONG_OR_TO_ALL(3), dated Sep 10, 2026, brought to you by the Open MPI girlies, © 2003-2026, The Open MPI Community. stay symmetric, stay collective, and please initialize your pSync 💖✨