SHMEM_LONG_XOR_TO_ALL(3)

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💖 SHMEM_LONG_XOR_TO_ALL 💖 (3)

okay bestie, welcome to the Open MPI corner of the manual, and specifically to SHMEM_LONG_XOR_TO_ALL(3). this whole little family of functions is out here doing a bitwise XOR operation on symmetric arrays across the active set of PEs, and honestly? slay. ✨

✨ NAME ✨

Meet the squad 🫶 — they all pull the exact same move, just with different data types:

…and what do they do? Performs a bitwise XOR operation on symmetric arrays over the active set of PEs. periodt. 💅

🌸 SYNOPSIS 🌸

C or C++ 🎀

#include <mpp/shmem.h>

void shmem_int_xor_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_xor_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_xor_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_xor_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_COMP4_XOR_TO_ALL(target, source, nreduce,
& PE_start, logPE_stride, PE_size, pWrk, pSync)

CALL SHMEM_INT4_XOR_TO_ALL(target, source, nreduce,
& PE_start, logPE_stride, PE_size, pWrk, pSync)

CALL SHMEM_INT8_XOR_TO_ALL(target, source, nreduce,
& PE_start, logPE_stride, PE_size, pWrk, pSync)

💜 DESCRIPTION 💜

okay so. the SHMEM reduction routines compute one or more reductions across symmetric arrays on multiple virtual PEs. a reduction is just an associative binary operation applied across a set of values, no biggie. want the full roster of other SHMEM reduction routines? go peek at intro_shmem(3). 👑

like literally every SHMEM collective routine, each of these assumes that only the PEs in the active set call the routine. if a PE that is not in the active set calls a SHMEM collective routine → undefined behavior, and not the fun kind. 💀

the nreduce argument tells you how many separate reductions to perform. the source array on every PE in the active set hands over one element per reduction. the results land 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.

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

the arguments, one by one 🎀

  • target — a symmetric array of length nreduce elements that receives the results of the reduction operations. the data type of target varies with which version of the reduction routine you called and which language you're in. calling from C/C++? check SYNOPSIS for the data types. calling from Fortran? here's the tea:
    • shmem_comp8_xor_to_all — Complex, with an element size equal to two 8-byte real values
    • shmem_comp4_xor_to_all — Complex, with an element size equal to two 4-byte real values
    • shmem_int8_xor_to_all — Integer, with an element size of 8 bytes
    • shmem_int4_xor_to_all — Integer, with an element size of 4 bytes
    • shmem_real8_xor_to_all — Real, with an element size of 8 bytes
    • shmem_real4_xor_to_all — Real, with an element size of 4 bytes
  • source — a symmetric array of length nreduce elements, holding one element for each separate reduction operation. it must have the same data type as target, no switching up. 💅
  • nreduce — the number of elements in the target and source arrays. must be of type integer. if you're in Fortran, it must be a default integer value.
  • PE_start — the lowest virtual PE number of the active set of PEs. must be of type integer. if you're in 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. must be of type integer. if you're in Fortran, it must be a default integer value.
  • PE_size — the number of PEs in the active set. must be of type integer. if you're in Fortran, it must be a default integer value.
  • pWrk — a symmetric work array. must have the same data type as target. in C/C++ it holds max(nreduce/2 + 1, _SHMEM_REDUCE_MIN_WRKDATA_SIZE) elements. in Fortran it holds 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 in Fortran, it must be a default integer value. every single element of this array must be initialized with the value _SHMEM_SYNC_VALUE (C/C++) or SHMEM_SYNC_VALUE (Fortran) before any of the PEs in the active set enter the reduction routine. no skipping this step bestie, i'm serious. 🫶

the values of 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. everybody matching, everybody synced, iconic behavior only. ✨

before any PE calls a reduction routine, you need to make sure these conditions exist (synchronization via a barrier or some other method is often needed to guarantee it — fr fr, don't skip the barrier):

  • 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

and upon return from a reduction routine, these 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 all defined in intro_shmem(3), go look. 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. cute, right? 🎀

🚨 you must ensure 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 of the following situations, bestie: - 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 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. the double-buffer hack, we love her. 💅

🌈 EXAMPLES 🌈

Example 1 🌸

this Fortran example statically initializes the pSync array and computes the exclusive OR of variable FOO across all even PEs. main character energy tbh.

INCLUDE "mpp/shmem.fh"

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

IF ( MOD(MY_PE(),2) .EQ. 0) THEN
  CALL SHMEM_REAL8_XOR_TO_ALL(FOOXOR, FOO, NR, 0, 1, N$PES/2,
  & PWRK, PSYNC)
  PRINT *, 'Result on PE ', MY_PE(), ' is ', FOOXOR
ENDIF

Example 2 🦋

consider the following C/C++ call:

shmem_short_xor_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_short_xor_to_all(&(target[0]), &(source[0]), 1, 0, 0, 8,
  pwrk1, psync1);
shmem_short_xor_to_all(&(target[1]), &(source[1]), 1, 0, 0, 8,
  pwrk2, psync2);
shmem_short_xor_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. that's the whole trick, iykyk. 😭

👑 See also 👑

2003-2026, The Open MPI Community.

and that's the whole page bestie, you ate it 💅✨ this diva of a manpage dropped on Sep 10, 2026, brought to you by Open MPI and the SHMEM_LONG_XOR_TO_ALL(3) gang — go forth and XOR responsibly, and always init your pSync before you show up. 🫶💖🌸

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