RE: [PATCH 1/4]middle-end Vect: Add support for dot-product where the sign for the multiplicant changes.

[Tamar Christina <Tamar.Christina@arm.com>]

> -----Original Message-----
> From: Richard Sandiford <richard.sandiford@arm.com>
> Sent: Monday, July 12, 2021 10:39 AM
> To: Tamar Christina <Tamar.Christina@arm.com>
> Cc: Richard Biener <rguenther@suse.de>; nd <nd@arm.com>; gcc-
> patches@gcc.gnu.org
> Subject: Re: [PATCH 1/4]middle-end Vect: Add support for dot-product
> where the sign for the multiplicant changes.
> 
> Tamar Christina <Tamar.Christina@arm.com> writes:
> > Hi,
> >
> >> Richard Sandiford <richard.sandiford@arm.com> writes:
> >> >> @@ -992,21 +1029,27 @@ vect_recog_dot_prod_pattern (vec_info
> >> *vinfo,
> >> >>    /* FORNOW.  Can continue analyzing the def-use chain when this
> >> >> stmt in
> >> a phi
> >> >>       inside the loop (in case we are analyzing an outer-loop).  */
> >> >>    vect_unpromoted_value unprom0[2];
> >> >> +  enum optab_subtype subtype = optab_vector;
> >> >>    if (!vect_widened_op_tree (vinfo, mult_vinfo, MULT_EXPR,
> >> WIDEN_MULT_EXPR,
> >> >> -			     false, 2, unprom0, &half_type))
> >> >> +			     false, 2, unprom0, &half_type, &subtype))
> >> >> +    return NULL;
> >> >> +
> >> >> +  if (subtype == optab_vector_mixed_sign
> >> >> +      && TYPE_UNSIGNED (unprom_mult.type)
> >> >> +      && TYPE_PRECISION (half_type) * 4 > TYPE_PRECISION
> >> >> + (unprom_mult.type))
> >> >>      return NULL;
> >> >
> >> > Isn't the final condition here instead that TYPE1 is narrower than TYPE2?
> >> > I.e. we need to reject the case in which we multiply a signed and
> >> > an unsigned value to get a (logically) signed result, but then
> >> > zero-extend it (rather than sign-extend it) to the precision of the
> addition.
> >> >
> >> > That would make the test:
> >> >
> >> >   if (subtype == optab_vector_mixed_sign
> >> >       && TYPE_UNSIGNED (unprom_mult.type)
> >> >       && TYPE_PRECISION (unprom_mult.type) < TYPE_PRECISION (type))
> >> >     return NULL;
> >> >
> >> > instead.
> >>
> >> And folding that into the existing test gives:
> >>
> >>   /* If there are two widening operations, make sure they agree on the
> sign
> >>      of the extension.  The result of an optab_vector_mixed_sign operation
> >>      is signed; otherwise, the result has the same sign as the operands.  */
> >>   if (TYPE_PRECISION (unprom_mult.type) != TYPE_PRECISION (type)
> >>       && (subtype == optab_vector_mixed_sign
> >> 	  ? TYPE_UNSIGNED (unprom_mult.type)
> >> 	  : TYPE_SIGN (unprom_mult.type) != TYPE_SIGN (half_type)))
> >>     return NULL;
> >>
> >
> > I went with the first one which doesn't add the extra constraints for
> > the normal dotproduct as that makes it too restrictive. It's the type
> > of the multiplication that determines the operation so dotproduct can
> > be used a bit more than where we currently do.
> >
> > This was relaxed in an earlier patch.
> 
> I didn't mean that we should add extra constraints to the normal case though.
> The existing test I was referring to above was:
> 
>   /* If there are two widening operations, make sure they agree on
>      the sign of the extension.  */
>   if (TYPE_PRECISION (unprom_mult.type) != TYPE_PRECISION (type)
>       && TYPE_SIGN (unprom_mult.type) != TYPE_SIGN (half_type))
>     return NULL;

But as I mentioned, this restriction is unneeded and has been removed hence why it's not in my patchset's diff.
It's removed by https://gcc.gnu.org/pipermail/gcc-patches/2021-May/569851.html which Richi conditioned on
the rest of these patches being approved.

This change needlessly blocks test vect-reduc-dot-[2,3,6,7].c from being dotproducts for instance

It's also part of the deficiency between GCC codegen and Clang https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88492#c6

Regards,
Tamar

> 
> Although this existing test makes sense for the normal case, IMO testing
> TYPE_SIGN (half_type) doesn't make sense for the mixed-sign case.  I think
> we should therefore replace the existing test with:
> 
>   /* If there are two widening operations, make sure they agree on the sign
>      of the extension.  The result of an optab_vector_mixed_sign operation
>      is signed; otherwise, the result has the same sign as the operands.  */
>   if (TYPE_PRECISION (unprom_mult.type) != TYPE_PRECISION (type)
>       && (subtype == optab_vector_mixed_sign
>          ? TYPE_UNSIGNED (unprom_mult.type)
>          : TYPE_SIGN (unprom_mult.type) != TYPE_SIGN (half_type)))
>     return NULL;
> 
> rather than add a separate condition for the mixed-sign case.
> The behaviour of the normal case is the same both ways.
> 
> Thanks,
> Richard
>