[RFA] block_innermost_frame tweak

[Joel Brobecker <brobecker@gnat.com>]

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I would like to make the following change:

2002-06-20  Joel Brobecker  <brobecker@gnat.com>

        * blockframe.c (block_innermost_frame): Fix a boundary condition
        bug that was causing GDB to sometimes fail to find the frame
        executing inside the given block.

I verified this change on Linux, and this introduced no regression.

We found out about this problem when we tried in a very particular Ada
program to print the value of a variable. Here are the details:

The Ada program (sorry, I tried the equivalent C program, but the
problem did not reproduce there):
<<
           procedure Try is
              procedure Inside is
              begin
                null;
              end Inside;
           begin
              declare
                Local : Integer := 18;
              begin
                Inside;
              end;
           end Try;
>>

After hitting a  breakpoint inside procedure "Inside", then going one
frame up, GDB should be able to print the value of "Local", but instead
says:

           No frame is currently executing in specified block

This is because selected_frame->pc points to the instruction following
the call to inside (ie this is more a return address than the pc), which
is right at the boundary of the block for which we are trying to find
the innermost frame... Hence the change I am suggesting.

OK to commit?

By the way, I think we have several places where we check whether a
frame is an innermost frame or not, so I think it would be useful to
create a public function in frame.h that would look like this:

     extern int frame_innermost_p (struct frame_info *);

(implemented in blockframe.c). I can submit this addition as a followup
patch if you think this is a good idea.

Thanks,
-- 
Joel

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Index: blockframe.c
===================================================================
RCS file: /cvs/src/src/gdb/blockframe.c,v
retrieving revision 1.29
diff -u -3 -p -r1.29 blockframe.c
--- blockframe.c	8 Jun 2002 18:30:14 -0000	1.29
+++ blockframe.c	20 Jun 2002 19:54:13 -0000
@@ -970,6 +970,7 @@ block_innermost_frame (struct block *blo
   struct frame_info *frame;
   register CORE_ADDR start;
   register CORE_ADDR end;
+  int innermost;
 
   if (block == NULL)
     return NULL;
@@ -983,7 +984,18 @@ block_innermost_frame (struct block *blo
       frame = get_prev_frame (frame);
       if (frame == NULL)
 	return NULL;
-      if (frame->pc >= start && frame->pc < end)
+      /* To evaluate if a given PC address is within a block range,
+         we normally check if PC is inside [block start .. block end[.
+         However, If FRAME is not the innermost frame, then frame->pc
+         normally points to the instruction *following* the call, which
+         means that the comparison with the block boundaries need to be
+         offset by one instruction. This is done by checking if PC is
+         inside ]block start .. block end] instead.  */
+      innermost = !frame->next
+                    || frame->next->signal_handler_caller
+                    || frame_in_dummy (frame->next);
+      if ((innermost && frame->pc >= start && frame->pc < end)
+          || (!innermost && frame->pc > start && frame->pc <= end))
 	return frame;
     }
 }