Re: RFA: static cast from base class to derived class

[Daniel Jacobowitz <drow@mvista.com>]

On Wed, Aug 21, 2002 at 01:00:52AM -0500, Jim Blandy wrote:
> 
> Once when my grandfather was a boy, he was riding his bicycle along a
> gravel path, and going pretty fast, when he hit a stone, went flying
> over the handlebars, and landed on his face.  The gravel was ground so
> deeply into his face that, even in his forties, he'd still
> occasionally encounter, while shaving, bits of rock that had finally
> worked their way out to the surface.
> 
> Here's a patch for a bit of 1998 HP Merge gravel that has finally
> found its way to the surface.
> 
> I didn't see any regressions with this patch using either STABS or
> Dwarf 2.  I'll put together a regression test for the original bug
> tomorrow.
> 
> 2002-08-20  Jim Blandy  <jimb@redhat.com>
> 
> 	* valops.c (value_cast): Simplify and correct logic for doing a
> 	static cast from a pointer to a base class to a pointer to a
> 	derived class.
> 
> Index: gdb/valops.c
> ===================================================================
> RCS file: /cvs/src/src/gdb/valops.c,v
> retrieving revision 1.67
> diff -c -r1.67 valops.c
> *** gdb/valops.c	19 Aug 2002 23:19:53 -0000	1.67
> --- gdb/valops.c	21 Aug 2002 02:42:21 -0000
> ***************
> *** 361,378 ****
>   				       value_zero (t1, not_lval), 0, t1, 1);
>   		  if (v)
>   		    {
> ! 		      struct value *v2 = value_ind (arg2);
> ! 		      VALUE_ADDRESS (v2) -= VALUE_ADDRESS (v)
> ! 			+ VALUE_OFFSET (v);
> ! 
> !                       /* JYG: adjust the new pointer value and
> ! 			 embedded offset. */
> !                       v2->aligner.contents[0] -=  VALUE_EMBEDDED_OFFSET (v);
> !                       VALUE_EMBEDDED_OFFSET (v2) = 0;
> ! 
> ! 		      v2 = value_addr (v2);
> ! 		      VALUE_TYPE (v2) = type;
> ! 		      return v2;
>   		    }
>   		}
>   	    }
> --- 361,371 ----
>   				       value_zero (t1, not_lval), 0, t1, 1);
>   		  if (v)
>   		    {
> !                       CORE_ADDR addr2 = value_as_address (arg2);
> !                       addr2 -= (VALUE_ADDRESS (v)
> !                                 + VALUE_OFFSET (v)
> !                                 + VALUE_EMBEDDED_OFFSET (v));
> !                       return value_from_pointer (type, addr2);
>   		    }
>   		}
>   	    }

I've got a question... what does VALUE_ADDRESS mean in this context? 
If it means what it normally means (ought to mean?), then taking an
address, subtracting an address, and using it as a pointer doesn't make
a lot of sense.

Answering my own question - it's dereferencing a pointer to a struct at
0.  So we'll actually get an address relative to zero, which makes
everything work out.  This looks good to me.

-- 
Daniel Jacobowitz
MontaVista Software                         Debian GNU/Linux Developer