Port PR270 to 1.8 Branch: Improve performance of multiple calls to H5Sget_select_elem_pointlist

release_docs/RELEASE.txt

@@ -316,6 +316,15 @@ New Features
 
     Library
     -------
+    - Improved performance of H5Sget_select_elem_pointlist
+
+      Modified library to cache the point after the last block of points
+      retrieved by H5Sget_select_elem_pointlist, so a subsequent call to the
+      same function to retrieve the next block of points from the list can
+      proceed immediately without needing to iterate over the point list.
+
+      (NAF - 2021/01/19)
+
     - Add S3 and HDFS VFDs to HDF5 maintenance
 
       Fix windows requirements and java tests. Windows requires CMake 3.13.

src/H5Spkg.h

@@ -95,6 +95,11 @@ struct H5S_pnt_node_t {
 /* Information about point selection list */
 typedef struct {
     H5S_pnt_node_t *head; /* Pointer to head of point list */
+
+    hsize_t last_idx;     /* Index of the point after the last returned from H5S__get_select_elem_pointlist() */
+    H5S_pnt_node_t *last_idx_pnt; /* Point after the last returned from H5S__get_select_elem_pointlist().
+        * If we ever add a way to remove points or add points in the middle of
+        * the pointlist we will need to invalidate these fields. */
 } H5S_pnt_list_t;
 
 /* Information about new-style hyperslab spans */

src/H5Spoint.c

@@ -618,6 +618,10 @@ H5S_point_copy(H5S_t *dst, const H5S_t *src, hbool_t H5_ATTR_UNUSED share_select
         curr = curr->next;
     } /* end while */
 
+    /* Clear cached iteration point */
+    dst->select.sel_info.pnt_lst->last_idx = 0;
+    dst->select.sel_info.pnt_lst->last_idx_pnt = NULL;
+
 done:
     if (ret_value < 0 && dst->select.sel_info.pnt_lst) {
         /* Traverse the (incomplete?) dst list, freeing all memory */
@@ -956,6 +960,7 @@ H5S_point_deserialize(H5S_t *space, const uint8_t *buf)
 static herr_t
 H5S_get_select_elem_pointlist(H5S_t *space, hsize_t startpoint, hsize_t numpoints, hsize_t *buf)
 {
+    const hsize_t   endpoint = startpoint + numpoints; /* Index of last point in iteration */
     H5S_pnt_node_t *node; /* Point node */
     unsigned        rank; /* Dataspace rank */
 
@@ -967,14 +972,20 @@ H5S_get_select_elem_pointlist(H5S_t *space, hsize_t startpoint, hsize_t numpoint
     /* Get the dataspace extent rank */
     rank = space->extent.rank;
 
-    /* Get the head of the point list */
-    node = space->select.sel_info.pnt_lst->head;
+    /* Check for cached point at the correct index */
+    if(space->select.sel_info.pnt_lst->last_idx_pnt
+            && startpoint == space->select.sel_info.pnt_lst->last_idx)
+        node = space->select.sel_info.pnt_lst->last_idx_pnt;
+    else {
+        /* Get the head of the point list */
+        node = space->select.sel_info.pnt_lst->head;
 
-    /* Iterate to the first point to return */
-    while (node != NULL && startpoint > 0) {
-        startpoint--;
-        node = node->next;
-    } /* end while */
+        /* Iterate to the first point to return */
+        while (node != NULL && startpoint > 0) {
+            startpoint--;
+            node = node->next;
+        } /* end while */
+    } /* end else */
 
     /* Iterate through the node, copying each point's information */
     while (node != NULL && numpoints > 0) {
@@ -984,6 +995,10 @@ H5S_get_select_elem_pointlist(H5S_t *space, hsize_t startpoint, hsize_t numpoint
         node = node->next;
     } /* end while */
 
+    /* Cached next point in iteration */
+    space->select.sel_info.pnt_lst->last_idx = endpoint;
+    space->select.sel_info.pnt_lst->last_idx_pnt = node;
+
     FUNC_LEAVE_NOAPI(SUCCEED)
 } /* H5S_get_select_elem_pointlist() */
 
@@ -1494,6 +1509,10 @@ H5S_point_project_simple(const H5S_t *base_space, H5S_t *new_space, hsize_t *off
         } /* end while */
     }     /* end else */
 
+    /* Clear cached iteration point */
+    new_space->select.sel_info.pnt_lst->last_idx = 0;
+    new_space->select.sel_info.pnt_lst->last_idx_pnt = NULL;
+
     /* Number of elements selected will be the same */
     new_space->select.num_elem = base_space->select.num_elem;