restore old qsort() based code and add preprocessor directives to switch

-DLMP_USE_LIBC_QSORT will use qsort() from libc to sort (requires static/global variables).
-DLMP_USE_MERGE_SORT will use a plain merge sort. slightly slower for expensive comparisons.
-DLMP_USE_HYBRID_SORT will use hybrid merge sort. faster than merge sort (no static/global variables)

src/irregular.cpp

@@ -21,13 +21,22 @@
 #include "comm.h"
 #include "memory.h"
 
-#include "mergesort.h"
-
 using namespace LAMMPS_NS;
 
+#if defined(LMP_USE_LIBC_QSORT)
+// allocate space for static class variable
 // prototype for non-class function
 
+int *Irregular::proc_recv_copy;
+static int compare_standalone(const void *, const void *);
+
+#else
+
+#include "mergesort.h"
+
+// prototype for non-class function
 static int compare_standalone(const int, const int, void *);
+#endif
 
 enum{LAYOUT_UNIFORM,LAYOUT_NONUNIFORM,LAYOUT_TILED};    // several files
 
@@ -423,7 +432,13 @@ int Irregular::create_atom(int n, int *sizes, int *proclist, int sortflag)
     int *length_recv_ordered = new int[nrecv_proc];
 
     for (i = 0; i < nrecv_proc; i++) order[i] = i;
+
+#if defined(LMP_USE_LIBC_QSORT)
+    proc_recv_copy = proc_recv;
+    qsort(order,nrecv_proc,sizeof(int),compare_standalone);
+#else
     merge_sort(order,nrecv_proc,(void *)proc_recv,compare_standalone);
+#endif
 
     int j;
     for (i = 0; i < nrecv_proc; i++) {
@@ -449,6 +464,25 @@ int Irregular::create_atom(int n, int *sizes, int *proclist, int sortflag)
   return nrecvsize;
 }
 
+#if defined(LMP_USE_LIBC_QSORT)
+
+/* ----------------------------------------------------------------------
+   comparison function invoked by qsort()
+   accesses static class member proc_recv_copy, set before call to qsort()
+------------------------------------------------------------------------- */
+
+int compare_standalone(const void *iptr, const void *jptr)
+{
+  int i = *((int *) iptr);
+  int j = *((int *) jptr);
+  int *proc_recv = Irregular::proc_recv_copy;
+  if (proc_recv[i] < proc_recv[j]) return -1;
+  if (proc_recv[i] > proc_recv[j]) return 1;
+  return 0;
+}
+
+#else
+
 /* ----------------------------------------------------------------------
    comparison function invoked by merge_sort()
    void pointer contains proc_recv list;
@@ -462,6 +496,8 @@ int compare_standalone(const int i, const int j, void *ptr)
   return 0;
 }
 
+#endif
+
 /* ----------------------------------------------------------------------
    communicate atoms via PlanAtom
    sendbuf = list of atoms to send
@@ -668,8 +704,13 @@ int Irregular::create_data(int n, int *proclist, int sortflag)
     int *num_recv_ordered = new int[nrecv_proc];
 
     for (i = 0; i < nrecv_proc; i++) order[i] = i;
-    merge_sort(order,nrecv_proc,(void *)proc_recv,compare_standalone);
 
+#if defined(LMP_USE_LIBC_QSORT)
+    proc_recv_copy = proc_recv;
+    qsort(order,nrecv_proc,sizeof(int),compare_standalone);
+#else
+    merge_sort(order,nrecv_proc,(void *)proc_recv,compare_standalone);
+#endif
     int j;
     for (i = 0; i < nrecv_proc; i++) {
       j = order[i];