RE: [PATCH] aarch64: Improve cost of `a ? {-,}1 : b`

["Andrew Pinski (QUIC)" <quic_apinski@quicinc.com>]

> -----Original Message-----
> From: Richard Sandiford <richard.sandiford@arm.com>
> Sent: Monday, November 27, 2023 7:35 AM
> To: Andrew Pinski (QUIC) <quic_apinski@quicinc.com>
> Cc: gcc-patches@gcc.gnu.org
> Subject: Re: [PATCH] aarch64: Improve cost of `a ? {-,}1 : b`
> 
> Andrew Pinski <quic_apinski@quicinc.com> writes:
> > While looking into PR 112454, I found the cost for `(if_then_else
> > (cmp) (const_int 1) (reg))` was being recorded as 8 (or `COSTS_N_INSNS
> > (2)`) but it should have been 4 (or `COSTS_N_INSNS (1)`).
> > This improves the cost by not adding the cost of `(const_int 1)` to
> > the total cost.
> >
> > It does not does not fix PR 112454 as that requires other changes to
> > forwprop the `(const_int 1)` earlier than combine.
> >
> > Bootstrapped and tested on aarch64-linux-gnu with no regressions.
> >
> > gcc/ChangeLog:
> >
> > 	* config/aarch64/aarch64.cc (aarch64_if_then_else_costs):
> > 	Don't add the cost of `1` or `-1`.
> >
> > Signed-off-by: Andrew Pinski <quic_apinski@quicinc.com>
> > ---
> >  gcc/config/aarch64/aarch64.cc | 13 ++++++++++---
> >  1 file changed, 10 insertions(+), 3 deletions(-)
> >
> > diff --git a/gcc/config/aarch64/aarch64.cc
> > b/gcc/config/aarch64/aarch64.cc index f6f6f94bf43..63241c5aaa5 100644
> > --- a/gcc/config/aarch64/aarch64.cc
> > +++ b/gcc/config/aarch64/aarch64.cc
> > @@ -11642,9 +11642,16 @@ aarch64_if_then_else_costs (rtx op0, rtx op1,
> rtx op2, int *cost, bool speed)
> >  	    /* CSINV/NEG with zero extend + const 0 (*csinv3_uxtw_insn3).  */
> >  	    op1 = XEXP (inner, 0);
> >  	}
> > -
> > -      *cost += rtx_cost (op1, VOIDmode, IF_THEN_ELSE, 1, speed);
> > -      *cost += rtx_cost (op2, VOIDmode, IF_THEN_ELSE, 2, speed);
> > +      if (op2 == constm1_rtx || op2 == const1_rtx)
> > +	*cost += rtx_cost (op1, VOIDmode, IF_THEN_ELSE, 1, speed);
> > +      else if (op1 == constm1_rtx || op1 == const1_rtx)
> > +	*cost += rtx_cost (op2, VOIDmode, IF_THEN_ELSE, 2, speed);
> 
> It looks like this is really an extra option on top of the previous if-else chain,
> since it only applies when OP1 and OP2 are still the operands of the
> if_then_else.  So how about:
> 
>       else if (op1 == constm1_rtx || op1 == const1_rtx)
>         {
> 	  /* Use CSINV.  */
> 	  *cost += rtx_cost (op2, VOIDmode, IF_THEN_ELSE, 2, speed);
> 	  return true;
>         }
>       else if (op2 == constm1_rtx || op2 == const1_rtx)
>         {
> 	  /* Use CSINV.  */
> 	  *cost += rtx_cost (op1, VOIDmode, IF_THEN_ELSE, 1, speed);
> 	  return true;
>         }
> 
> leaving the code to fall through to:
> 
>       *cost += rtx_cost (op1, VOIDmode, IF_THEN_ELSE, 1, speed);
>       *cost += rtx_cost (op2, VOIDmode, IF_THEN_ELSE, 2, speed);
>       return true;
> 
> as it does currently.  OK in that form if you agree.

Yes I think this is the correct way of implementing this, Let me test it and get back to you.

Thanks,
Andrew

> 
> Let me know if you don't.  But in that case:
> 
> > +      else
> > +	{
> > +	  *cost += rtx_cost (op1, VOIDmode, IF_THEN_ELSE, 1, speed);
> > +	  *cost += rtx_cost (op2, VOIDmode, IF_THEN_ELSE, 1, speed);
> 
> should be 2, speed
> 
> > +	}
> > +
> 
> Thanks,
> Richard