gdb: Show address of the row::_cells tree (or "empty" mark)

Currently clang optimizes-out lots of critical stuff from
compact radix tree. Untill we find out the way to walk the
tree in gdb, it's better to at least show where it is in
memory.

Signed-off-by: Pavel Emelyanov <xemul@scylladb.com>

scylla-gdb.py

@@ -151,6 +151,19 @@ class intrusive_set:
             yield n
 
 
+class compact_radix_tree:
+    def __init__(self, ref):
+        self.root = ref['_root']['_v']
+
+    def to_string(self):
+        if self.root['_base_layout'] == 0:
+            return '<empty>'
+
+        # Compiler optimizes-away lots of critical stuff, so
+        # for now just show where the tree is
+        return 'compact radix tree @ 0x%x' % self.root
+
+
 class intrusive_btree:
     def __init__(self, ref):
         container_type = ref.type.strip_typedefs()
@@ -657,7 +670,7 @@ class row_printer(gdb.printing.PrettyPrinter):
     def __init__(self, val):
         self.val = val
 
-    def to_string(self):
+    def __to_string_legacy(self):
         if self.val['_type'] == gdb.parse_and_eval('row::storage_type::vector'):
             cells = str(self.val['_storage']['vector'])
         elif self.val['_type'] == gdb.parse_and_eval('row::storage_type::set'):
@@ -666,6 +679,12 @@ class row_printer(gdb.printing.PrettyPrinter):
             raise Exception('Unsupported storage type: ' + self.val['_type'])
         return '{type=%s, cells=%s}' % (self.val['_type'], cells)
 
+    def to_string(self):
+        try:
+            return '{cells=[%s]}' % compact_radix_tree(self.val['_cells']).to_string()
+        except gdb.error:
+            return self.__to_string_legacy()
+
     def display_hint(self):
         return 'row'