On Wed, May 20, 2026 at 2:55 PM Uros Bizjak <[email protected]> wrote:
>
> On Fri, May 15, 2026 at 10:40 AM Uros Bizjak <[email protected]> wrote:
> >
> > On Fri, May 15, 2026 at 3:46 AM Liu, Hongtao <[email protected]> wrote:
> > >
> > >
> > >
> > > > -----Original Message-----
> > > > From: Roger Sayle <[email protected]>
> > > > Sent: Friday, May 15, 2026 5:23 AM
> > > > To: 'GCC Patches' <[email protected]>
> > > > Cc: 'Hongtao Liu' <[email protected]>; Liu, Hongtao
> > > > <[email protected]>; 'Uros Bizjak' <[email protected]>
> > > > Subject: [PATCH] Improve vector increment/decrement on x86.
> > > >
> > > >
> > > > This patch improves the code generated by the i386 backend for
> > > > incrementing
> > > > (adding one to) and decrementing (subtracting one from) a vector. With
> > > > SSE
> > > > materializing the vector -1 is more efficient than materializing the
> > > > vector +1,
> > > > hence x + 1 (increment) is better expressed as x - (-1), and x - 1
> > > > (decrement) is
> > > > better expressed as x + (-1). Conveniently the relevant additions and
> > > > subtractions are specified as a single pattern, using a plusminus
> > > > iterator, in the
> > > > machine description.
> > >
> > > Can we add pre_reload define_insn_and_split for them,
> > >
> > > (set (reg:V16QI 100 [ _2 ])
> > > (minus:V16QI (reg:V16QI 107 [ x ])
> > > (const_vector:V16QI [
> > > (const_int 1 [0x1]) repeated x16
> > > ])))
> > >
> > > Theoretically, it should be able to capture more optimization
> > > opportunities (if vector +/-1 is only exposed through RTL optimization)
>
> Another issue is with insn canonicalization [1], where
>
> (minus x (const_int n)) is converted to (plus x (const_int -n)).
>
> According to the above, x - (-1) is not a canonical form and
> optimizers will convert it to the canonical form anyway.
In recog.cc
case MINUS:
if (CONST_SCALAR_INT_P (XEXP (x, 1)))
validate_change (object, loc,
simplify_gen_binary
(PLUS, GET_MODE (x), XEXP (x, 0),
simplify_gen_unary (NEG,
GET_MODE (x), XEXP (x, 1),
GET_MODE (x))), 1);
break;
Canonicalization is restricted to CONST_SCALAR_INT_P, that's why it's
not trigger for vector.
>
> [1] https://gcc.gnu.org/onlinedocs/gccint/Insn-Canonicalizations.html
>
> Uros.
--
BR,
Hongtao
