Re: [PATCH] simplify-rtx: Fix invalid simplification with paradoxical subregs [PR110206]

[Uros Bizjak <ubizjak@gmail.com>]

On Mon, Jul 10, 2023 at 11:47 AM Richard Biener
<richard.guenther@gmail.com> wrote:
>
> On Mon, Jul 10, 2023 at 11:26 AM Uros Bizjak <ubizjak@gmail.com> wrote:
> >
> > On Mon, Jul 10, 2023 at 11:17 AM Richard Biener
> > <richard.guenther@gmail.com> wrote:
> > >
> > > On Sun, Jul 9, 2023 at 10:53 AM Uros Bizjak via Gcc-patches
> > > <gcc-patches@gcc.gnu.org> wrote:
> > > >
> > > > As shown in the PR, simplify_gen_subreg call in simplify_replace_fn_rtx:
> > > >
> > > > (gdb) list
> > > > 469           if (code == SUBREG)
> > > > 470             {
> > > > 471               op0 = simplify_replace_fn_rtx (SUBREG_REG (x),
> > > > old_rtx, fn, data);
> > > > 472               if (op0 == SUBREG_REG (x))
> > > > 473                 return x;
> > > > 474               op0 = simplify_gen_subreg (GET_MODE (x), op0,
> > > > 475                                          GET_MODE (SUBREG_REG (x)),
> > > > 476                                          SUBREG_BYTE (x));
> > > > 477               return op0 ? op0 : x;
> > > > 478             }
> > > >
> > > > simplifies with following arguments:
> > > >
> > > > (gdb) p debug_rtx (op0)
> > > > (const_vector:V4QI [
> > > >         (const_int -52 [0xffffffffffffffcc]) repeated x4
> > > >     ])
> > > > (gdb) p debug_rtx (x)
> > > > (subreg:V16QI (reg:V4QI 98) 0)
> > > >
> > > > to:
> > > >
> > > > (gdb) p debug_rtx (op0)
> > > > (const_vector:V16QI [
> > > >         (const_int -52 [0xffffffffffffffcc]) repeated x16
> > > >     ])
> > > >
> > > > This simplification is invalid, it is not possible to get V16QImode vector
> > > > from V4QImode vector, even when all elements are duplicates.
>
> ^^^
>
> I think this simplification is valid.  A simplification to
>
> (const_vector:V16QI [
>          (const_int -52 [0xffffffffffffffcc]) repeated x4
>          (const_int 0 [0]) repeated x12
>      ])
>
> would be valid as well.
>
> > > > The simplification happens in simplify_context::simplify_subreg:
> > > >
> > > > (gdb) list
> > > > 7558          if (VECTOR_MODE_P (outermode)
> > > > 7559              && GET_MODE_INNER (outermode) == GET_MODE_INNER (innermode)
> > > > 7560              && vec_duplicate_p (op, &elt))
> > > > 7561            return gen_vec_duplicate (outermode, elt);
> > > >
> > > > but the above simplification is valid only for non-paradoxical registers,
> > > > where outermode <= innermode.  We should not assume that elements outside
> > > > the original register are valid, let alone all duplicates.
> > >
> > > Hmm, but looking at the audit trail the x86 backend expects them to be zero?
> > > Isn't that wrong as well?
> >
> > If you mean Comment #10, it is just an observation that
> > simplify_replace_rtx simplifies arguments from Comment #9 to:
> >
> > (gdb) p debug_rtx (src)
> > (const_vector:V8HI [
> >         (const_int 204 [0xcc]) repeated x4
> >         (const_int 0 [0]) repeated x4
> >     ])
> >
> > instead of:
> >
> > (gdb) p debug_rtx (src)
> > (const_vector:V8HI [
> >         (const_int 204 [0xcc]) repeated x8
> >     ])
> >
> > which is in line with the statement below.
> > >
> > > That is, I think putting any random value into the upper lanes when
> > > constant folding
> > > a paradoxical subreg sounds OK to me, no?
> >
> > The compiler is putting zero there as can be seen from the above new RTX.
> >
> > > Of course we might choose to not do such constant propagation for
> > > efficiency reason - at least
> > > when the resulting CONST_* would require a larger constant pool entry
> > > or more costly
> > > construction.
> >
> > This is probably a follow-up improvement, where this patch tries to
> > fix a specific invalid simplification of simplify_replace_rtx that is
> > invalid universally.
>
> How so?  What specifies the values of the paradoxical subreg for the
> bytes not covered by the subreg operand?

I don't know why 0 is generated here (and if it is valid) for
paradoxical bytes, but 0xcc is not correct, since it sets REG_EQUAL to
the wrong constant and triggers unwanted propagation later on.

Uros.