Improve spec. reading superblock into cache (a little) by using v2 size (#4491)

* Improve spec. reading superblock into cache (a little) by using v2 size

Instead of reading the absolute minimal possible, use the likely value of
a v2+ superblock w/8-byte addresses & lengths.

src/H5Fpkg.h

@@ -131,6 +131,10 @@
     (H5F_SUPERBLOCK_FIXED_SIZE +                                                                             \
      H5F_SUPERBLOCK_VARLEN_SIZE((s)->super_vers, (s)->sizeof_addr, (s)->sizeof_size))
 
+/* Optimistic read size for superblock */
+/* Size of a v2+ superblock, w/8-byte addresses & lengths */
+#define H5F_SUPERBLOCK_SPEC_READ_SIZE (H5F_SUPERBLOCK_FIXED_SIZE + H5F_SUPERBLOCK_VARLEN_SIZE(2, 8, 8))
+
 /* For superblock version 0 & 1:
    Offset to the file consistency flags (status_flags) in the superblock (excluding H5F_SUPERBLOCK_FIXED_SIZE)
  */

src/H5Fsuper.c

@@ -416,8 +416,7 @@ H5F__super_read(H5F_t *f, H5P_genplist_t *fa_plist, bool initial_read)
         HGOTO_ERROR(H5E_FILE, H5E_BADTYPE, FAIL, "can't get property list");
 
     /* Make certain we can read the fixed-size portion of the superblock */
-    if (H5F__set_eoa(f, H5FD_MEM_SUPER,
-                     (haddr_t)(H5F_SUPERBLOCK_FIXED_SIZE + H5F_SUPERBLOCK_MINIMAL_VARLEN_SIZE)) < 0)
+    if (H5F__set_eoa(f, H5FD_MEM_SUPER, (haddr_t)H5F_SUPERBLOCK_SPEC_READ_SIZE) < 0)
         HGOTO_ERROR(H5E_FILE, H5E_CANTSET, FAIL, "set end of space allocation request failed");
 
     /* Set up the user data for cache callbacks */

src/H5Fsuper_cache.c

@@ -306,8 +306,7 @@ H5F__cache_superblock_get_initial_load_size(void H5_ATTR_UNUSED *_udata, size_t
     assert(image_len);
 
     /* Set the initial image length size */
-    *image_len = H5F_SUPERBLOCK_FIXED_SIZE + /* Fixed size of superblock */
-                 H5F_SUPERBLOCK_MINIMAL_VARLEN_SIZE;
+    *image_len = H5F_SUPERBLOCK_SPEC_READ_SIZE;
 
     FUNC_LEAVE_NOAPI(SUCCEED)
 } /* end H5F__cache_superblock_get_initial_load_size() */