Add comment in dwarf2read.c::read_subrange_type

This comment explains why we sometimes sign-extend the range type
bounds when we normally shouldn't have to.

gdb/ChangeLog:

        * dwarf2read.c (read_subrange_type): Add comment.

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index 8f2f0dc..49d8113 100644 (file)
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,5 +1,9 @@
 2014-02-27  Joel Brobecker  <brobecker@adacore.com>
 
+       * dwarf2read.c (read_subrange_type): Add comment.
+
+2014-02-27  Joel Brobecker  <brobecker@adacore.com>
+
        * dwarf2read.c (update_enumeration_type_from_children): New
        function, mostly extracted from process_structure_scope.
        (read_enumeration_type): Call update_enumeration_type_from_children.

diff --git a/gdb/dwarf2read.c b/gdb/dwarf2read.c
index 00bba47..52208d6 100644 (file)
--- a/gdb/dwarf2read.c
+++ b/gdb/dwarf2read.c
@@ -14522,6 +14522,13 @@ read_subrange_type (struct die_info *die, struct dwarf2_cu *cu)
        }
     }
 
+  /* Normally, the DWARF producers are expected to use a signed
+     constant form (Eg. DW_FORM_sdata) to express negative bounds.
+     But this is unfortunately not always the case, as witnessed
+     with GCC, for instance, where the ambiguous DW_FORM_dataN form
+     is used instead.  To work around that ambiguity, we treat
+     the bounds as signed, and thus sign-extend their values, when
+     the base type is signed.  */
   negative_mask =
     (LONGEST) -1 << (TYPE_LENGTH (base_type) * TARGET_CHAR_BIT - 1);
   if (!TYPE_UNSIGNED (base_type) && (low & negative_mask))