Re: [PATCH] middle-end: Fix up constant handling in emit_conditional_move [PR111260]

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

On Thu, Dec 14, 2023 at 6:42 PM Andrew Pinski (QUIC)
<quic_apinski@quicinc.com> wrote:
>
>
> > -----Original Message-----
> > From: Richard Biener <richard.guenther@gmail.com>
> > Sent: Thursday, December 14, 2023 5:23 AM
> > To: Andrew Pinski (QUIC) <quic_apinski@quicinc.com>
> > Cc: gcc-patches@gcc.gnu.org
> > Subject: Re: [PATCH] middle-end: Fix up constant handling in
> > emit_conditional_move [PR111260]
> >
> > On Wed, Dec 13, 2023 at 5:51 PM Andrew Pinski
> > <quic_apinski@quicinc.com> wrote:
> > >
> > > After r14-2667-gceae1400cf24f329393e96dd9720, we force a constant to
> > a
> > > register if it is shared with one of the other operands. The problem
> > > is used the comparison mode for the register but that could be
> > > different from the operand mode. This causes some issues on some targets.
> > > To fix it, we either need to have the modes match or if it is an
> > > integer mode, then we can use the lower part for the smaller mode.
> > >
> > > Bootstrapped and tested on both aarch64-linux-gnu and x86_64-linux.
> >
> > I think to fulfil the original purpose requiring matching modes is enough, the
> > x86 backend checks for equality here, a subreg wouldn't be enough.
> > In fact the whole point preserving equality doesn't work then.
> >
> > So please just check the modes are equal (I also see I used 'mode' for the cost
> > check - I've really split out the check done by prepare_cmp_insn here btw).
> > This seemed to be the simplest solution at the time, rather than for example
> > trying to postpone legitimizing the constants (so rtx_equal_p could continue
> > to be lazy with slight mode mismatches).
>
> I was originally deciding between that patch and what this one.  I decided to go with this
> one in the end as I was trying to keep the extra optimization.

Yeah, it's just that with one reg wrapped with subreg or lowpart will
defeat that.
Maybe if you'd have gone to force the smaller mode and a zero/sign
extend recovering
the value-equivalence could be possible, but with a truncation all bets are off.

> Anyways I test and commit
> the one with just the checking of the modes to be equal.

Good.  Any actual improvement could be done as followup if it makes sense.

> Also it is not that rtx_equal_p is lazy on mode mismatches but rather all CONST_INT
> don't have a mode (all have VOIDmode); this is why you have to pass around the cmpmode
> and the outer mode of the conditional.
>
> Thanks,
> Andrew
>
> >
> > Richard.
> >
> > >         PR middle-end/111260
> > >
> > > gcc/ChangeLog:
> > >
> > >         * optabs.cc (emit_conditional_move): Fix up mode handling for
> > >         forcing the constant to a register.
> > >
> > > gcc/testsuite/ChangeLog:
> > >
> > >         * gcc.c-torture/compile/condmove-1.c: New test.
> > >
> > > Signed-off-by: Andrew Pinski <quic_apinski@quicinc.com>
> > > ---
> > >  gcc/optabs.cc                                 | 40 +++++++++++++++++--
> > >  .../gcc.c-torture/compile/condmove-1.c        |  9 +++++
> > >  2 files changed, 45 insertions(+), 4 deletions(-)  create mode 100644
> > > gcc/testsuite/gcc.c-torture/compile/condmove-1.c
> > >
> > > diff --git a/gcc/optabs.cc b/gcc/optabs.cc index
> > > f0a048a6bdb..573cf22760e 100644
> > > --- a/gcc/optabs.cc
> > > +++ b/gcc/optabs.cc
> > > @@ -5131,26 +5131,58 @@ emit_conditional_move (rtx target, struct
> > rtx_comparison comp,
> > >           /* If we are optimizing, force expensive constants into a register
> > >              but preserve an eventual equality with op2/op3.  */
> > >           if (CONSTANT_P (orig_op0) && optimize
> > > +             && (cmpmode == mode
> > > +                 || (GET_MODE_CLASS (cmpmode) == MODE_INT
> > > +                     && GET_MODE_CLASS (mode) == MODE_INT))
> > >               && (rtx_cost (orig_op0, mode, COMPARE, 0,
> > >                             optimize_insn_for_speed_p ())
> > >                   > COSTS_N_INSNS (1))
> > >               && can_create_pseudo_p ())
> > >             {
> > > +             machine_mode new_mode;
> > > +             if (known_le (GET_MODE_PRECISION (cmpmode),
> > GET_MODE_PRECISION (mode)))
> > > +               new_mode = mode;
> > > +             else
> > > +               new_mode = cmpmode;
> > >               if (rtx_equal_p (orig_op0, op2))
> > > -               op2p = XEXP (comparison, 0) = force_reg (cmpmode, orig_op0);
> > > +               {
> > > +                 rtx r = force_reg (new_mode, orig_op0);
> > > +                 op2p = gen_lowpart (mode, r);
> > > +                 XEXP (comparison, 0) = gen_lowpart (cmpmode, r);
> > > +               }
> > >               else if (rtx_equal_p (orig_op0, op3))
> > > -               op3p = XEXP (comparison, 0) = force_reg (cmpmode, orig_op0);
> > > +               {
> > > +                 rtx r = force_reg (new_mode, orig_op0);
> > > +                 op3p = gen_lowpart (mode, r);
> > > +                 XEXP (comparison, 0) = gen_lowpart (cmpmode, r);
> > > +               }
> > >             }
> > >           if (CONSTANT_P (orig_op1) && optimize
> > > +             && (cmpmode == mode
> > > +                 || (GET_MODE_CLASS (cmpmode) == MODE_INT
> > > +                     && GET_MODE_CLASS (mode) == MODE_INT))
> > >               && (rtx_cost (orig_op1, mode, COMPARE, 0,
> > >                             optimize_insn_for_speed_p ())
> > >                   > COSTS_N_INSNS (1))
> > >               && can_create_pseudo_p ())
> > >             {
> > > +             machine_mode new_mode;
> > > +             if (known_le (GET_MODE_PRECISION (cmpmode),
> > GET_MODE_PRECISION (mode)))
> > > +               new_mode = mode;
> > > +             else
> > > +               new_mode = cmpmode;
> > >               if (rtx_equal_p (orig_op1, op2))
> > > -               op2p = XEXP (comparison, 1) = force_reg (cmpmode, orig_op1);
> > > +               {
> > > +                 rtx r = force_reg (new_mode, orig_op1);
> > > +                 op2p = gen_lowpart (mode, r);
> > > +                 XEXP (comparison, 1) = gen_lowpart (cmpmode, r);
> > > +               }
> > >               else if (rtx_equal_p (orig_op1, op3))
> > > -               op3p = XEXP (comparison, 1) = force_reg (cmpmode, orig_op1);
> > > +               {
> > > +                 rtx r = force_reg (new_mode, orig_op1);
> > > +                 op3p = gen_lowpart (mode, r);
> > > +                 XEXP (comparison, 1) = gen_lowpart (cmpmode, r);
> > > +               }
> > >             }
> > >           prepare_cmp_insn (XEXP (comparison, 0), XEXP (comparison, 1),
> > >                             GET_CODE (comparison), NULL_RTX,
> > > unsignedp, diff --git
> > > a/gcc/testsuite/gcc.c-torture/compile/condmove-1.c
> > > b/gcc/testsuite/gcc.c-torture/compile/condmove-1.c
> > > new file mode 100644
> > > index 00000000000..3fcc591af00
> > > --- /dev/null
> > > +++ b/gcc/testsuite/gcc.c-torture/compile/condmove-1.c
> > > @@ -0,0 +1,9 @@
> > > +/* PR middle-end/111260 */
> > > +
> > > +/* Used to ICE while expansion of the `(a == b) ? b : 0;` */ int
> > > +f1(long long a) {
> > > +  int b = 822920;
> > > +  int t = a == b;
> > > +  return t * (int)b;
> > > +}
> > > --
> > > 2.39.3
> > >