Jose R. Valverde
jrvalverde at cnb.csic.es
Mon Jun 30 19:30:28 AEST 2008
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
> > 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
} 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
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
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
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
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