Re: [PATCH] vect: Use xor to invert oversized vector masks

[Richard Biener <richard.guenther@gmail.com>]

On Fri, Mar 15, 2024 at 12:24 PM Andrew Stubbs <ams@baylibre.com> wrote:
>
> On 15/03/2024 07:35, Richard Biener wrote:
> > On Fri, Mar 15, 2024 at 4:35 AM Hongtao Liu <crazylht@gmail.com> wrote:
> >>
> >> On Thu, Mar 14, 2024 at 11:42 PM Andrew Stubbs <ams@baylibre.com> wrote:
> >>>
> >>> Don't enable excess lanes when inverting vector bit-masks smaller than the
> >>> integer mode.  This is yet another case of wrong-code due to mishandling
> >>> of oversized bitmasks.
> >>>
> >>> This issue shows up in vect/tsvc/vect-tsvc-s278.c and
> >>> vect/tsvc/vect-tsvc-s279.c if I set the preferred vector size to V32
> >>> (down from V64) on amdgcn.
> >>>
> >>> OK for mainline?
> >>>
> >>> Andrew
> >>>
> >>> gcc/ChangeLog:
> >>>
> >>>          * expr.cc (expand_expr_real_2): Use xor to invert vector masks.
> >>> ---
> >>>   gcc/expr.cc | 11 +++++++++++
> >>>   1 file changed, 11 insertions(+)
> >>>
> >>> diff --git a/gcc/expr.cc b/gcc/expr.cc
> >>> index 403eeaa108e4..3540327d879e 100644
> >>> --- a/gcc/expr.cc
> >>> +++ b/gcc/expr.cc
> >>> @@ -10497,6 +10497,17 @@ expand_expr_real_2 (sepops ops, rtx target, machine_mode tmode,
> >>>                                 immed_wide_int_const (mask, int_mode),
> >>>                                 target, 1, OPTAB_LIB_WIDEN);
> >>>          }
> >>> +      /* If it's a vector mask don't enable excess bits.  */
> >>> +      else if (VECTOR_BOOLEAN_TYPE_P (type)
> >>> +              && SCALAR_INT_MODE_P (mode)
> >>> +              && maybe_ne (GET_MODE_PRECISION (mode),
> >>> +                           TYPE_VECTOR_SUBPARTS (type).to_constant ()))
> >>> +       {
> >>> +         auto nunits = TYPE_VECTOR_SUBPARTS (type).to_constant ();
> >>> +         temp = expand_binop (mode, xor_optab, op0,
> >>> +                              GEN_INT ((HOST_WIDE_INT_1U << nunits) - 1),
> >>> +                              target, true, OPTAB_WIDEN);
> >>> +       }
> >> Not review, just curious, should the issue be fixed by the commit in PR113576.
> >> Also wonder besides cbranch, excess land bits also matter?
> >> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113576#c35
> >
> > Yes, you patch BIT_NOT but we decided to patch final compares.  Is it that
> > we need to fixup every mask use in a .COND_* expansion as well?  If so
> > we should do it there.
>
> I thought that the "not" to "xor" change was nice and there was already
> code there for fixing bitfields, but OK, I take your point.
>
> The .COND_* statements are handled as internal function calls that are
> expanded directly via the optab with no special cases for different call
> types. This is because the "expand_cond_*_optab_fn" functions just map
> straight to "expand_direct_optab_fn".... would that be the right place
> to insert a special case handler to insert "and" operations?

Yes, I think in expand_fn_using_insn where we handle the "undefined" input
operands we want to handle the vector bool operands as well, masking
rhs_rtx accordingly.

Richard.

> Andrew