[Bug exp/26875] Incorrect value printed for address of first element of zero-length array

["vries at gcc dot gnu.org" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=26875

--- Comment #5 from Tom de Vries <vries at gcc dot gnu.org> ---
(In reply to Tom de Vries from comment #4)
> This passed with gdb 9.
> 
> The first bad commit is either:
> - commit 7c6f271296319576fa00587928e5ff52ced9c1bb (could not build)
>   gdb: make get_discrete_bounds check for non-constant range bounds, or
> - commit 8c2e4e0689ea244d0ed979171a3d09c9176b8175
>   gdb: add accessors to struct dynamic_prop

Hmm, before these commits, we handle this type in get_discrete_bounds:
...
(gdb) p recursive_dump_type (type, 0)
type node 0x2283020
name '<NULL>' (0x0)
code 0xc (TYPE_CODE_RANGE)
length 8
objfile 0x1fced60
target_type 0x223c440
  type node 0x223c440
  name 'long unsigned int' (0x225be83)
  code 0x8 (TYPE_CODE_INT)
  length 8
  objfile 0x1fced60
  target_type 0x0
  pointer_type 0x0
  reference_type 0x0
  type_chain 0x223c440
  instance_flags 0x0
  flags TYPE_UNSIGNED
  nfields 0 0x0
pointer_type 0x0
reference_type 0x0
type_chain 0x2283020
instance_flags 0x0
flags TYPE_UNSIGNED
nfields 0 0x22830a0
low 0  high 0 (undefined)
$12 = void
...
and ignored the TYPE_HIGH_BOUND_UNDEFINED, and just return 1, with lowerbound
== 0 and upperbound == 0.  The setting of lowerbound == 0 had the effect that
we printed the right value.

After the commits, get_discrete_bounds returns -1, and we have both lowerbound
and upperbound uninitialized.  We don't check the return status though in
value_subscript, and proceed with the uninitialized values:
...
(gdb) p lowerbound
$2 = 36429408
(gdb) p upperbound
$3 = 11281392
...

And after this:
...
(gdb) 
171           index -= lowerbound;
...
we have:
...
(gdb) p index
$4 = -36429408
...

So we end up here:
...
    return value_ind (value_ptradd (array, index));
...
constructing a value "*(array + -36429408)", and we end up printing 
&ubound.a[-36429408].

This is an ad-hoc patch that sets lowerbound, even if upperbound is undefined:
...
diff --git a/gdb/gdbtypes.c b/gdb/gdbtypes.c
index 686edafcf64..dfc3de870c1 100644
--- a/gdb/gdbtypes.c
+++ b/gdb/gdbtypes.c
@@ -1049,11 +1049,16 @@ get_discrete_bounds (struct type *type, LONGEST *lowp,
LONGEST
 *highp)
     case TYPE_CODE_RANGE:
       /* This function currently only works for ranges with two defined,
         constant bounds.  */
-      if (type->bounds ()->low.kind () != PROP_CONST
-         || type->bounds ()->high.kind () != PROP_CONST)
+      if (type->bounds ()->low.kind () != PROP_CONST)
        return -1;
-
       *lowp = type->bounds ()->low.const_val ();
+
+      if (type->bounds ()->high.kind () != PROP_CONST)
+       {
+         *highp = *lowp - 1;
+         return 1;
+       }
+
       *highp = type->bounds ()->high.const_val ();

       if (TYPE_TARGET_TYPE (type)->code () == TYPE_CODE_ENUM)
...
and handles the undefined upper bound by returning an empty range.

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