Distinguish sentinel frame from null frame.

This patch replaces the `current_frame' static global in frame.c with
`sentinel_frame'.  It also makes the sentinel frame id unique and
different from the null frame.

By itself, there is not much point to this patch, but it makes
the code cleaner for the VALUE_FRAME_ID changes in another patch.
Since we now allow "navigation" to the sentinel frame, it removes
the necessity of adding special cases to other parts of GDB.

Note that a new function, get_next_frame_sentinel_okay, is introduced
in this patch.  It will be used by the VALUE_FRAME_ID changes that
I've made.

Thanks to Pedro Alves for this suggestion.

gdb/ChangeLog:

    	* frame.h (enum frame_id_stack_status): Add FID_STACK_SENTINEL.
    	(struct frame_id): Increase number of bits required for storing
    	stack status to 3 from 2.
    	(sentinel_frame_id): New declaration.
    	(get_next_frame_sentinel_okay): Declare.
    	(frame_find_by_id_sentinel_okay): Declare.
    	* frame.c (current_frame): Rename this static global to...
    	(sentinel_frame): ...this static global, which has also been
    	moved an earlier location in the file.
    	(fprint_frame_id): Add case for sentinel frame id.
    	(get_frame_id): Return early for sentinel frame.
    	(sentinel_frame_id): Define.
    	(frame_find_by_id): Add case for sentinel_frame_id.
    	(create_sentinel_frame): Use sentinel_frame_id for this_id.value
    	instead of null_frame_id.
    	(get_current_frame): Add local declaration for `current_frame'.
    	Remove local declaration for `sentinel_frame.'
    	(get_next_frame_sentinel_okay): New function.
    	(reinit_frame_cache): Use `sentinel_frame' in place of
    	`current_frame'.

1 files changed, 11 insertions, 1 deletions

diff --git a/gdb/frame.h b/gdb/frame.h
index a05ac82f90..dc25ce96a2 100644
--- a/gdb/frame.h
+++ b/gdb/frame.h
@@ -90,6 +90,9 @@ enum frame_id_stack_status
   /* Stack address is valid, and is found in the stack_addr field.  */
   FID_STACK_VALID = 1,
 
+  /* Sentinel frame.  */
+  FID_STACK_SENTINEL = 2,
+
   /* Stack address is unavailable.  I.e., there's a valid stack, but
      we don't know where it is (because memory or registers we'd
      compute it from were not collected).  */
@@ -150,7 +153,7 @@ struct frame_id
   CORE_ADDR special_addr;
 
   /* Flags to indicate the above fields have valid contents.  */
-  ENUM_BITFIELD(frame_id_stack_status) stack_status : 2;
+  ENUM_BITFIELD(frame_id_stack_status) stack_status : 3;
   unsigned int code_addr_p : 1;
   unsigned int special_addr_p : 1;
 
@@ -166,6 +169,9 @@ struct frame_id
 /* For convenience.  All fields are zero.  This means "there is no frame".  */
 extern const struct frame_id null_frame_id;
 
+/* Sentinel frame.  */
+extern const struct frame_id sentinel_frame_id;
+
 /* This means "there is no frame ID, but there is a frame".  It should be
    replaced by best-effort frame IDs for the outermost frame, somehow.
    The implementation is only special_addr_p set.  */
@@ -310,6 +316,10 @@ extern void select_frame (struct frame_info *);
 extern struct frame_info *get_prev_frame (struct frame_info *);
 extern struct frame_info *get_next_frame (struct frame_info *);
 
+/* Like get_next_frame(), but allows return of the sentinel frame.  NULL
+   is never returned.  */
+extern struct frame_info *get_next_frame_sentinel_okay (struct frame_info *);
+
 /* Return a "struct frame_info" corresponding to the frame that called
    THIS_FRAME.  Returns NULL if there is no such frame.