Re: [RFC/RFA] gdb.cp/classes.exp: Don't try to print local variable out of scope

[Daniel Jacobowitz <drow@false.org>]

On Tue, Mar 09, 2004 at 08:51:05PM -0500, Andrew Cagney wrote:
> 
> >BTW, I think the NORMAL_FRAME check is wrong too:
> >
> >    {
> >      int i;
> >      stuff (i);
> >->  }
> >
> >get signal
> 
> Er, hold on.  The intent of address-in-block is:
> 
> /* An address (not necessarily alligned to an instruction boundary)
>    that falls within THIS frame's code block.
> 
>    When a function call is the last statement in a block, the return
>    address for the call may land at the start of the next block.
>    Similarly, if a no-return function call is the last statement in
>    the function, the return address may end up pointing beyond the
>    function, and possibly at the start of the next function.

> The only way to get a PC pointing at the first instruction of a function 
> is for that function to have been interrupted just as that first 
> instruction was about to be executed -- thats the very case where the 
> existing address_in_block correctly leaves the PC as is.
> 
> In the example in question:
> 
> >
> >     {
> >       int i;
> >       stuff (i);
> > ->  }
> 
> the existing code correctly puts the PC at the instruction about to 
> destroy the prologue.

Think about this for a moment.  I'm going to give addresses so that I
can be more precise.

0x10 <stuff>: ret		stuff(int) { }
0x20 <main>: push		main() {
0x21 <main+1>: push			{
0x22 <main+2>: move arg1, i			stuff(i)
0x23 <main+3>: call stuff			  "
0x24 <main+4>: pop			}
0x25 <main+5>: pop		}
0x26 <main+6>: ret		"

The inner scope is probably <main+2> to <main+3> inclusive.

Suppose PC == 0x10.  We backtrace.  Look at main; saved PC is 0x24.  We
want an address in the block.  We subtract 1.  OK, saved addr-in-block
is 0x23.  'i' is in scope.

Suppose PC == 0x24.  Shouldn't this be the same?  For the purposes of
looking at local variables, aren't we still in the the block?

Suppose PC was 0x24 and we got a signal.  Ditto.

Suppose PC == 0x20 and we get a signal.  Obviously we don't want to
change the behavior of this.

-- 
Daniel Jacobowitz
MontaVista Software                         Debian GNU/Linux Developer