Fix handling of negative offset to top for tagged types

TN: QA23-021

Author: pmderodat

gnatdbg/tagged.py

@@ -5,7 +5,8 @@
 import gdb
 
 from gnatdbg.utils import (
-    addr_to_val, encode_name, get_system_address, ptr_to_int, strip_typedefs
+    address_as_offset, addr_to_val, encode_name, get_system_address,
+    ptr_to_int, strip_typedefs
 )
 
 # The reader of this module is assumed to be familiar with how GNAT implements
@@ -130,9 +131,12 @@ def decode_tag(tag_addr):
     except IndexError:
         sig = '<invalid signature: {}>'.format(sig_int)
 
+    offset_to_top = address_as_offset(addr_to_val(offset_to_top_addr,
+                                                  system_address))
+
     return (
         sig,
-        int(addr_to_val(offset_to_top_addr, system_address)),
+        offset_to_top,
         addr_to_val(tsd_addr, system_address),
     )
 

gnatdbg/utils.py

@@ -37,6 +37,25 @@ def get_system_address():
     return gdb.lookup_type('system__address')
 
 
+def address_as_offset(addr):
+    """
+    In order to avoid requiring too much debug information, we use the
+    System.Address type (which is unsigned) to decode what is actually a
+    System.Storage_Elements.Storage_Offset (which is signed). This does the
+    conversion.
+
+    :param gdb.Value addr: Address to decode.
+    :rtype: int
+    """
+    addr_range_size = 2 ** (8 * addr.type.sizeof)
+    max_int = addr_range_size / 2 - 1
+    int_addr = int(addr)
+    if int_addr > max_int:
+        return int_addr - addr_range_size
+    else:
+        return int_addr
+
+
 def ptr_to_int(ptr_value):
     """
     Convert an access GDB value into the corresponding Python integer.