code example
shmem SGI shmem
one-sided communication
POSIX threads
AltiVec PowerPC SIMD instructions

#include <mpp/shmem.h>
#include <pthread.h>
#include <altivec.h>

float *x, *y;
#define PROC (8)
float e_vec[PROC];
int n_thread0, n_thread1;

void *thread1(void *arg) {
  int p = (int)arg;
  int n0 = n_thread0 + (p * (n_thread1-n_thread0)) / PROC;
  int n1 = n_thread0 + ((p+1) * (n_thread1-n_thread0)) / PROC;
  // do computation
  float ve[4] = {0, 0, 0, 0};
  float e = 0;
  for (int i=n0; i<n1; i+=4) {
   float* yp = &y[i+1], y0 = &y[i], ym = &y[i-1];
   vec_st(vec_madd(
    vec_splats(.5),
    vec_add(
     vec_perm(vec_ld(0,ym), vec_ld(16,ym),
      vec_lvsl(0,ym)),
     vec_perm(vec_ld(0,yp), vec_ld(16,yp),
      vec_lvsl(0,yp))),
    vec_splats(0.)),
    0, &x[i]);
   vec_st(vec_add(
    vec_ld(0,&ve[0]),vec_madd(
     vec_ld(0,y0),
     vec_ld(0,y0),
     vec_splats(0.))),
    0, &ve[0]);
  }
  e += ve[0] + ve[1] + ve[2] + ve[3];
  e_vec[p] = e;
  return (void*) 0;
}

int main(int argc, char *argv[]) {
  int n = ...;
  start_pes(0);
  int nn = (n-1) / _num_pes();
  int n_local0 = 1 + _my_pe() * nn;
  int n_local1 = 1 + (_my_pe()+1) * nn;
  // allocate only local part + ghost zone of the arrays x,y
  x = (float*)shmalloc((n_local1 - n_local0 + 2)*sizeof(float));
  y = (float*)shmalloc((n_local1 - n_local0 + 2)*sizeof(float));
  x -= (n_local0 - 1);
  y -= (n_local0 - 1);
  shmem_barrier_all();

  ... // fill x, y

  // fill ghost zone
  if (_my_pe() > 0)
   shmem_float_put(&y[n_local1], &y[n_local0], 1, _my_pe()-1);
  if (_my_pe() < _num_pes()-1)
   shmem_float_put(&y[n_local0-1], &y[n_local1-1], 1, _my_pe()+1);
  shmem_barrier_all();

  pthread_t threads[PROC];
  pthread_attr_t attr;
  pthread_attr_init(&attr);
  n_thread0 = n_local0;
  n_thread1 = n_local1;
  float e = 0;
  // start threads and wait for termination
  for (int p=0; p<PROC; ++p)
   pthread_create(&threads[p], &attr, thread1, (void *)p);
  for (int p=0; p<PROC; ++p) {
   pthread_join(threads[p], NULL);
   e += e_vec[p];
  }

  static float work[_SHMEM_REDUCE_MIN_WRKDATA_SIZE];
  static long sync[_SHMEM_REDUCE_MIN_WRKDATA_SIZE];
  static float el, es;
  el = e;
  shmem_float_sum_to_all(&es, &el, 1,
   0, 0, _num_pes(), work, sync);
  e = es;

  ... // output x, e

  x += (n_local0 - 1);
  y += (n_local0 - 1);
  shfree(x);
  shfree(y);
  return 0;
}

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