ChangeLog:

	* features/gdb-target.dtd (target): Accept optional
	<compatible> elements.
	(compatible): Define element.

	* target-descriptions.h (tdesc_compatible_p): New.
	(tdesc_add_compatible): New.
	* target-descriptions.c (arch_p): New VEC_P type.
	(struct target_desc): New member compatible.
	(free_target_description): Handle it.
	(maint_print_c_tdesc_cmd): Likewise.
	(tdesc_compatible_p): New function.
	(tdesc_add_compatible): New function.

	* xml-tdesc.c (tdesc_end_compatible): New function.
	(target_children): Handle <compatible> element.

	* arch-utils.c (choose_architecture_for_target): Accept target
	description instead of BFD architecture as input.  Query target
	description for compatible architectures.
	(gdbarch_info_fill): Update call.

	* NEWS: Mention <compatible> element of target descriptions.

doc/ChangeLog:

	* gdb.texinfo (Target Descriptions): Document <compatible> element.

1 files changed, 21 insertions, 7 deletions

diff --git a/gdb/arch-utils.c b/gdb/arch-utils.c
index f075922bf04..5cf4afd4f4c 100644
--- a/gdb/arch-utils.c
+++ b/gdb/arch-utils.c
@@ -320,15 +320,24 @@ set_endian (char *ignore_args, int from_tty, struct cmd_list_element *c)
 }
 
 /* Given SELECTED, a currently selected BFD architecture, and
-   FROM_TARGET, a BFD architecture reported by the target description,
-   return what architecture to use.  Either may be NULL; if both are
-   specified, we use the more specific.  If the two are obviously
-   incompatible, warn the user.  */
+   TARGET_DESC, the current target description, return what
+   architecture to use.
+
+   SELECTED may be NULL, in which case we return the architecture
+   associated with TARGET_DESC.  If SELECTED specifies a variant
+   of the architecture associtated with TARGET_DESC, return the
+   more specific of the two.
+
+   If SELECTED is a different architecture, but it is accepted as
+   compatible by the target, we can use the target architecture.
+
+   If SELECTED is obviously incompatible, warn the user.  */
 
 static const struct bfd_arch_info *
-choose_architecture_for_target (const struct bfd_arch_info *selected,
-				const struct bfd_arch_info *from_target)
+choose_architecture_for_target (const struct target_desc *target_desc,
+				const struct bfd_arch_info *selected)
 {
+  const struct bfd_arch_info *from_target = tdesc_architecture (target_desc);
   const struct bfd_arch_info *compat1, *compat2;
 
   if (selected == NULL)
@@ -358,6 +367,11 @@ choose_architecture_for_target (const struct bfd_arch_info *selected,
 
   if (compat1 == NULL && compat2 == NULL)
     {
+      /* BFD considers the architectures incompatible.  Check our target
+	 description whether it accepts SELECTED as compatible anyway.  */
+      if (tdesc_compatible_p (target_desc, selected))
+	return from_target;
+
       warning (_("Selected architecture %s is not compatible "
 		 "with reported target architecture %s"),
 	       selected->printable_name, from_target->printable_name);
@@ -685,7 +699,7 @@ gdbarch_info_fill (struct gdbarch_info *info)
   /* From the target.  */
   if (info->target_desc != NULL)
     info->bfd_arch_info = choose_architecture_for_target
-      (info->bfd_arch_info, tdesc_architecture (info->target_desc));
+			   (info->target_desc, info->bfd_arch_info);
   /* From the default.  */
   if (info->bfd_arch_info == NULL)
     info->bfd_arch_info = default_bfd_arch;