Initial support for variant parts

This adds some initial support for variant parts to gdbtypes.h.  A
variant part is represented as a union.  The union has a flag
indicating that it has a discriminant, and information about the
discriminant is attached using the dynamic property system.

2018-02-26  Tom Tromey  <tom@tromey.com>

	* value.h (value_union_variant): Declare.
	* valops.c (value_union_variant): New function.
	* gdbtypes.h (TYPE_FLAG_DISCRIMINATED_UNION): New macro.
	(struct discriminant_info): New.
	(enum dynamic_prop_node_kind) <DYN_PROP_DISCRIMINATED>: New
	enumerator.
	(struct main_type) <flag_discriminated_union>: New field.

1 files changed, 44 insertions, 0 deletions

diff --git a/gdb/valops.c b/gdb/valops.c
index e038c04..bf028fd 100644
--- a/gdb/valops.c
+++ b/gdb/valops.c
@@ -2257,6 +2257,50 @@ value_struct_elt_bitpos (struct value **argp, int bitpos, struct type *ftype,
   return NULL;
 }
 
+/* See value.h.  */
+
+int
+value_union_variant (struct type *union_type, const gdb_byte *contents)
+{
+  gdb_assert (TYPE_CODE (union_type) == TYPE_CODE_UNION
+	      && TYPE_FLAG_DISCRIMINATED_UNION (union_type));
+
+  struct dynamic_prop *discriminant_prop
+    = get_dyn_prop (DYN_PROP_DISCRIMINATED, union_type);
+  gdb_assert (discriminant_prop != nullptr);
+
+  struct discriminant_info *info
+    = (struct discriminant_info *) discriminant_prop->data.baton;
+  gdb_assert (info != nullptr);
+
+  /* If this is a univariant union, just return the sole field.  */
+  if (TYPE_NFIELDS (union_type) == 1)
+    return 0;
+  /* This should only happen for univariants, which we already dealt
+     with.  */
+  gdb_assert (info->discriminant_index != -1);
+
+  /* Compute the discriminant.  Note that unpack_field_as_long handles
+     sign extension when necessary, as does the DWARF reader -- so
+     signed discriminants will be handled correctly despite the use of
+     an unsigned type here.  */
+  ULONGEST discriminant = unpack_field_as_long (union_type, contents,
+						info->discriminant_index);
+
+  for (int i = 0; i < TYPE_NFIELDS (union_type); ++i)
+    {
+      if (i != info->default_index
+	  && i != info->discriminant_index
+	  && discriminant == info->discriminants[i])
+	return i;
+    }
+
+  if (info->default_index == -1)
+    error (_("Could not find variant corresponding to discriminant %s"),
+	   pulongest (discriminant));
+  return info->default_index;
+}
+
 /* Search through the methods of an object (and its bases) to find a
    specified method.  Return the pointer to the fn_field list FN_LIST of
    overloaded instances defined in the source language.  If available