Port PR270 to 1.12 Branch: Improve performance of multiple calls to H5Sget_select_elem_pointlist

release_docs/RELEASE.txt

@@ -178,6 +178,15 @@ 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)
+
     - Added H5VL_VERSION macro that indicates the version of the VOL framework
         implemented by a version of the library.  Currently, compatibility
         checking enforces that the 'version' field in the H5VL_class_t for

src/H5Spkg.h

@@ -137,6 +137,11 @@ struct H5S_pnt_list_t {
 
     H5S_pnt_node_t *head; /* Pointer to head of point list */
     H5S_pnt_node_t *tail; /* Pointer to tail 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. */
 };
 
 /* Information about hyperslab spans */

src/H5Spoint.c

@@ -842,6 +842,10 @@ H5S__copy_pnt_list(const H5S_pnt_list_t *src, unsigned rank)
     H5MM_memcpy(dst->high_bounds, src->high_bounds, (rank * sizeof(hsize_t)));
     H5MM_memcpy(dst->low_bounds, src->low_bounds, (rank * sizeof(hsize_t)));
 
+    /* Clear cached iteration point */
+    dst->last_idx = 0;
+    dst->last_idx_pnt = NULL;
+
     /* Set return value */
     ret_value = dst;
 
@@ -1510,6 +1514,7 @@ H5S__point_deserialize(H5S_t **space, const uint8_t **p)
 static herr_t
 H5S__get_select_elem_pointlist(const 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 */
 
@@ -1521,14 +1526,20 @@ H5S__get_select_elem_pointlist(const H5S_t *space, hsize_t startpoint, hsize_t n
     /* 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) {
@@ -1538,6 +1549,10 @@ H5S__get_select_elem_pointlist(const H5S_t *space, hsize_t startpoint, hsize_t n
         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)
 } /* end H5S__get_select_elem_pointlist() */
 
@@ -2329,6 +2344,10 @@ H5S__point_project_simple(const H5S_t *base_space, H5S_t *new_space, hsize_t *of
         } /* end for */
     }     /* 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;