On Wed, Jul 12, 2023 at 12:58 PM Uros Bizjak <ubiz...@gmail.com> wrote:
>
> On Wed, Jul 12, 2023 at 12:23 PM Richard Sandiford
> <richard.sandif...@arm.com> wrote:
> >
> > Richard Biener via Gcc-patches <gcc-patches@gcc.gnu.org> writes:
> > > On Mon, Jul 10, 2023 at 1:01 PM Uros Bizjak <ubiz...@gmail.com> wrote:
> > >>
> > >> On Mon, Jul 10, 2023 at 11:47 AM Richard Biener
> > >> <richard.guent...@gmail.com> wrote:
> > >> >
> > >> > On Mon, Jul 10, 2023 at 11:26 AM Uros Bizjak <ubiz...@gmail.com> wrote:
> > >> > >
> > >> > > On Mon, Jul 10, 2023 at 11:17 AM Richard Biener
> > >> > > <richard.guent...@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.
> > >
> > > Quoting what I wrote in the PR below. I think pragmatically the fix is
> > > good - we might miss some opportunistic folding this way but we for
> > > sure may not optimistically register an equality via REG_EQUAL without
> > > enforcing it (removing the producer and replacing it with the optimistic
> > > constant).
> > >
> > > So consider the patch approved if no other RTL maintainer chimes in
> > > within 48h.
> >
> > Sorry, can you hold off a bit longer? Wanted to have a look but the
> > deadline is about to expire.
>
> No problem, I will wait for you.
Please also note Comment #14 in the PR. With the patch, the compiler sets
(const_vector:V8HI [
(const_int 204 [0xcc]) repeated x4
(const_int 0 [0]) repeated x4
])
as a new REG_EQUAL note. This also does not look OK to me. IMO, the
compiler should not emit REG_EQUAL note when some of the elements are
derived from undefined values outside the original register.
Uros.
