SHMEM_INT_XOR_TO_ALL(3)

🌈 NAME 🌈

okay bestie, welcome to Open MPI's section 3 — the land of shmem_comp4_xor_to_all(3), shmem_int_xor_to_all(3), shmem_int4_xor_to_all(3), shmem_int8_xor_to_all(3), shmem_long_xor_to_all(3), shmem_longlong_xor_to_all(3) and shmem_short_xor_to_all(3) ✨

and what do they do?? they perform a bitwise XOR operation on symmetric arrays over the active set of PEs. that's it, that's the whole vibe. 🦋

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🎀 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)

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✨ DESCRIPTION ✨

so like. the shared memory (SHMEM) reduction routines compute one or more reductions across symmetric arrays on multiple virtual PEs. a reduction is literally just performing an associative binary operation across a set of values. if you want the full roster of other SHMEM reduction routines, go peek at intro_shmem(3). 👑

and listen — as with all SHMEM collective routines, each of these routines assumes that only PEs in the active set call the routine. if a PE that is not in the active set calls a SHMEM collective routine, that's undefined behavior, no cap. 💀

the nreduce argument decides how many separate reductions you're doing. the source array on all PEs in the active set hands over one element for each 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. periodt. 🫶

the source and target arrays may be the same array — slay — but they may not be overlapping arrays. 🙈

🌸 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 the version of the reduction routine being called and the language you're using. calling from C/C++? go check the SYNOPSIS section for the data type tea. calling from Fortran? the target data types are:
  • 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, that contains one element for each separate reduction operation. source must have the same data type as target. matching fits, literally. 💅
• 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 bestie, base 2, it's giving math class but make it parallel. 🧚
• 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. pWrk 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 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 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 on the same page or it's a mess, fr fr.

before any PE calls a reduction routine, you must make sure the following conditions exist — synchronization via a barrier or some other method is often needed to make that happen:

• 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. ✨

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🔥 NOTES 🔥

the terms collective, symmetric, and cache aligned are defined in intro_shmem(3) — go read it, it's the lore. 📖

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 is 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 participate in processing of a SHMEM reduction routine. be careful of these 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 — special case alert 🚨 — 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. iconic workaround, honestly. 👑

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🌈 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.

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. the alternating thing?? that's the whole trick. 💅

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🦋 SEE ALSO 🦋

• intro_shmem(3)

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💖 Copyright 💖

2003-2026, The Open MPI Community

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and that's the whole page bestie!! 🎀 this one is from Open MPI, section 3, and the footer says Sep 10, 2026 with the page name SHMEM_INT_XOR_TO_ALL(3). go forth and XOR, you absolute legend. 💜✨🌸