Re: [RFA] block_innermost_frame tweak

Jim Blandy
2002-06-20T21:09:40Z

This is similar to the code in get_frame_block that decrements the
PC before looking up its block.

It seems to me that we generally want to use the return address - 1
(well, ideally, the address of the `call' instruction, but we don't
have that) to decide which block any frame but the youngest is "in".

Would it make sense to create a new function --- `frame_calling_pc',
perhaps --- which encapsulates the logic in get_frame_block, and then
use that instead of `frame->pc' in get_frame_block and
block_innermost_frame?

Joel Brobecker <brobecker@gnat.com> writes:

> 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
>
> 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;
> }
> }