[TUHS] Introduction

[Jose R. Valverde]

On Sun, 29 Jun 2008 10:25:23 +0200
Oliver Lehmann <lehmann at ans-netz.de> wrote:
> Jose R. Valverde wrote:
> 
> > Why don't you try to split the assignment into various statements
> > to reproduce the assembly and the recombine them? Like, e.g.
> > 
> > 1:	r2 = uap->linkname;		/* ldl rr2,rr8(#4) */
> > 2:	r4 = (long) r2;			/* ldl rr4,rr2 */
> > 3:	r4 &= 0x7F00FFFF;		/* and rr4,#32512 */
> > 4:	u.u_dirp.l = (caddr_t) r4;	/* ldl _u+78, rr4 */
> 
> hm.. this won't work because the compiler starts handing out registers
> the register-declared variables with the highest register possible so
> would start with rr10 or so.

But you would still be able to see what did generate the code (barring
register number).
> 
> > 	u.u_dirp.l = (caddr_t) ((long) (((saddr_t) uap->linkname).l) & 0x7F00FFFF);
> 
> I've changed it to:
> 
> 	u.u_dirp.l = (caddr_t) ((long) (((saddr_t *) uap->linkname)->l) & 0x7F00FFFF);
> 
> otherwise it won't compile. It compiles to:
> 
>         ldl     rr2,rr8(#4)
>         ldl     rr4, at rr2
>         and     r4,#32512
>         ldl     _u+78,rr4
> 
> is it because I added a * and changed . to ->?

Yes, but it also does not reflect the correct usage. linkname is not an saddr_t* but
an saddrt_t. And you want to assign directly the value of uap->linkname not what it
points to.

typedef	union	
{
    caddr_t		l;
    struct
    {
	unsigned	left;
	unsigned	right;
    }			half;
}			saddr_t;	/* segmented address with parts */

> 
> > 	u.u_dirp = (saddr_t) (((long) uap->linkname) & 0x7F00FFFF);
> 
> this generates:
> "sys2.c":305: operands of CAST have incompatible types 
> "sys2.c":305: operands of "=" have incompatible types 
> 
> :(

My fault. That's a typical beginner's mistake I made there. I'm starting
to feel embarrassed of so many mistakes I'm making lately. BTW, I'm on a
deadline so most probably my mind is not 100% in place so do not take me
too seriously specially when dealing with complex abstract data types.

That is because an saddr_t is a union. You cannot assign directly to a
union (u.u_dirp), you must assign to a union member (u.u_dirp.l), but
the union member is not an saddr_t, it is a caddr_t: the correct text
would be 
 	u.u_dirp.l = (caddr_t) (((long) uap->linkname) & 0x7F00FFFF);

which you know does not work. That is why I suggested the extra cast to
see if the compiler would be misled into using an unneeded zero-offset
assignment instruction to an auxiliary register.

	u.u_dirp.l = (caddr_t) ((long) (((saddr_t) uap->linkname).l) & 0x7F00FFFF);

that should be tantamount to

	u.u_dirp.l = (caddrt_t) ((long) ((caddr_t) uap->linkname) & 0x7F00FFFF);

where due to the long cast the initial caddr_t cast would be redundant
reducing to

	u.u_dirp.l = (caddr_t) ((long) uap->linkname & 0x7F00FFFF);

but introducing a saddr_t cast that might fool the compiler into a
temporary assignment with a zero offset (the .l) into ldl rr4,rr2

And I still think that dividing the assignment into intermediate
instructions and looking at the assembly might shed some light into
what is going on.

> 
> -- 
>  Oliver Lehmann
>   http://www.pofo.de/
>   http://wishlist.ans-netz.de/


-- 
	These opinions are mine and only mine. Hey man, I saw them first!

			    José R. Valverde

	De nada sirve la Inteligencia Artificial cuando falta la Natural
-------------- next part --------------
A non-text attachment was scrubbed...
Name: signature.asc
Type: application/pgp-signature
Size: 189 bytes
Desc: not available
URL: <http://minnie.tuhs.org/pipermail/tuhs/attachments/20080630/e747ed01/attachment.sig>