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NULL selected/current frame; Was: [patch/rfc] Add frame_read_signed/unsigned_register();convert h8300 to print_registers_info()



 void
+frame_read_unsigned_register (struct frame_info *frame, int regnum,
+			      ULONGEST *val)
+{
+  frame_unwind_unsigned_register (get_next_frame (frame), regnum, val);
+}

So, the register belonging to this frame.
Yes.  The old roughly equivalent function was get_saved_register.

That means the register
which would be in the hardware registers if this frame were current,
right?
To be pedantic, no. A frame's registers are ALWAYS found by unwinding get_next_frame(FRAME). It just so happens that registers unwound from get_next_frame(current_frame) come from the register cache.

The difference is subtle but important. current_frame isn't the special case, get_next_frame(current_frame) is. Unfortunatly much of the GDB code treated ``current_frame'' as special creating unnecessary complexity and ongoing confusion. Per generic_unwind_get_saved_register():

/* Reached the the bottom (youngest, inner most) of the frame chain
(youngest, inner most) frame, go direct to the hardware register
cache (do not pass go, do not try to cache the value, ...). The
unwound value would have been cached in frame->next but that
doesn't exist. This doesn't matter as the hardware register
cache is stopping any unnecessary accesses to the target. */

/* NOTE: cagney/2002-04-14: It would be nice if, instead of a
special case, there was always an inner frame dedicated to the
hardware registers. Unfortunatly, there is too much unwind code
around that looks up/down the frame chain while making the
assumption that each frame level is using the same unwind code. */

Should we allow NULL to imply the current frame?
Definitly no :-)

There is a bit of dogma here - there is always a frame. The above should not be called with NULL. Code that calls this checks that selected_frame != NULL. I'll add a ``gdb_assert (frame != NULL)'' and a comment to that effect.

As for get_next_frame(FRAME), if that returns NULL, we've fallen off the inner most frame and should get the register value from the register cache. (Oh, while get_next_frame(current_frame) will always return NULL, there may come a time when current_frame->next does not :-).

enjoy,
Andrew



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