[RFA] block_innermost_frame tweak

Joel Brobecker
2002-06-20T20:14:43Z

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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;
}
}