Re: [PATCH][i386]: Update ix86_can_change_mode_class target hook to accept QImode conversions

[Hongtao Liu <crazylht@gmail.com>]

On Mon, Nov 14, 2022 at 10:14 AM Hongtao Liu <crazylht@gmail.com> wrote:
>
> On Fri, Nov 11, 2022 at 10:47 PM Tamar Christina via Gcc-patches
> <gcc-patches@gcc.gnu.org> wrote:
> >
> > Hi All,
> >
> > The current i386 implementation of the TARGET_CAN_CHANGE_MODE_CLASS is currently
> > not useful before re-alloc.
> >
> > In particular before regalloc optimization passes query the hook using ALL_REGS,
> > but because of the
> >
> >   if (MAYBE_FLOAT_CLASS_P (regclass))
> >       return false;
> >
> > The hook returns false for all modes, even integer ones because ALL_REGS
> > overlaps with floating point regs.
> >
> > The vector permute fallback cases used to unconditionally convert vector integer
> > permutes to vector QImode ones as a fallback plan.  This is incorrect and can
> > result in incorrect code if the target doesn't support this conversion.
> >
> > To fix this some more checks were added, however that ended up introducing ICEs
> > in the i386 backend because e.g. the hook would reject conversions between modes
> > like V2TImode and V32QImode.
> >
> > My understanding is that for x87 we don't want to allow floating point
> > conversions, but integers are fine.  So I have modified the check such that it
> > also checks the modes, not just the register class groups.
> >
> > The second part of the code is needed because now that integer modes aren't
> > uniformly rejected the i386 backend trigger further optimizations.  However the
> > backend lacks instructions to deal with canonical RTL representations of
> > certain instructions.  for instance the back-end seems to prefer vec_select 0
> > instead of subregs.
> >
> > So to prevent the canonicalization I reject integer modes when the sizes of to
This should not be right, we still want something like (subreg:SI (reg:V4SI))
> > and from don't match and when we would have exited with false previously.
> >
> > This fixes all the ICEs and codegen regressions, but perhaps an x86 maintainer
> > should take a deeper look at this hook implementation.
> >
> > Bootstrapped Regtested on x86_64-pc-linux-gnu and no issues.
> >
> > Ok for master?
> >
> > Thanks,
> > Tamar
> >
> > gcc/ChangeLog:
> >
> >         * config/i386/i386.cc (ix86_can_change_mode_class): Update the target
> >         hook.
> >
> > --- inline copy of patch --
> > diff --git a/gcc/config/i386/i386.cc b/gcc/config/i386/i386.cc
> > index c4d0e36e9c0a2256f5dde1f4dc021c0328aa0cba..477dd007ea80272680751b61e35cc3eec79b66c3 100644
> > --- a/gcc/config/i386/i386.cc
> > +++ b/gcc/config/i386/i386.cc
> > @@ -19682,7 +19682,15 @@ ix86_can_change_mode_class (machine_mode from, machine_mode to,
> >
> >    /* x87 registers can't do subreg at all, as all values are reformatted
> >       to extended precision.  */
> > -  if (MAYBE_FLOAT_CLASS_P (regclass))
> > +  if (MAYBE_FLOAT_CLASS_P (regclass)
> > +      && VALID_FP_MODE_P (from)
> > +      && VALID_FP_MODE_P (to))
> > +    return false;
> This change looks reasonable since only VALID_FP_MODE_P will be
> allocated to FLOAT_CLASS.
> > +
> > +  /* Reject integer modes if the sizes aren't the same.  It would have
> > +     normally exited above.  */
> > +  if (MAYBE_FLOAT_CLASS_P (regclass)
> > +      && GET_MODE_SIZE (from) != GET_MODE_SIZE (to))
> >      return false;
> Do you have a case(or a patch so I can reproduce the regression
> myself) to indicate the regression, so I can have a deep look.
This is not right, we still want something like (subreg:SI
(reg:V4SI)). I'm trying to adjust all those vec_select to subregs, and
it may take some time. We also need to adjust modes_tieable_p to
prevent high rtx_cost for subreg which will prevent insns being
combined. x86 is conservative on both can_change_mode_class and
modes_tieable_p.
I'm a bit worried that these changes are a bit big for stage3, let me
try it first.
> >
> >    if (MAYBE_SSE_CLASS_P (regclass) || MAYBE_MMX_CLASS_P (regclass))
> >
> >
> >
> >
> > --
>
>
>
> --
> BR,
> Hongtao



-- 
BR,
Hongtao