a b

.gitignore

@@ -0,0 +1,2 @@
+# binary
+*.out

a/butterfly.cc

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+#include <mpi.h>
+#include <cassert>
+#include <cmath>
+#include <iostream>
+
+int main(int argc, char *argv[]) {
+  int rank, size;
+  int send, recv;
+  // MPI init
+  MPI_Init(&argc, &argv);
+  MPI_Comm_rank(MPI_COMM_WORLD, &rank);
+  MPI_Comm_size(MPI_COMM_WORLD, &size);
+  // check proc size is power of 2
+  float steps_f = std::log2(size);
+  int steps = (int)steps_f;
+  assert(std::abs(steps - steps_f) < 1e-9);
+  // init value
+  send = rank;
+  // sum
+  int base = 1;
+  for (int i = 0; i < steps; ++i) {
+    int group = rank / base;
+    int offset = rank % base;
+    int target = (group % 2 ? group - 1 : group + 1) * base + offset;
+    if (rank < target) {
+      MPI_Send(reinterpret_cast<void*>(&send), 1, MPI_INT, target, target, MPI_COMM_WORLD);
+      MPI_Recv(reinterpret_cast<void*>(&recv), 1, MPI_INT, target, rank, MPI_COMM_WORLD, MPI_STATUS_IGNORE);
+    } else {
+      MPI_Recv(reinterpret_cast<void*>(&recv), 1, MPI_INT, target, rank, MPI_COMM_WORLD, MPI_STATUS_IGNORE);
+      MPI_Send(reinterpret_cast<void*>(&send), 1, MPI_INT, target, target, MPI_COMM_WORLD);
+    }
+    send += recv;
+    base *= 2;
+  }
+  // print result
+  std::cout << "rank:" << rank << ", sum:" << send << std::endl;
+  MPI_Finalize();
+}

b/fox.cc

@@ -0,0 +1,7 @@
+#include <mpi.h>
+#include <iostream>
+
+int main() {
+
+  return 0;
+}