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

[Richard Sandiford]

Andrew Pinski <quic_apin...@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_apin...@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.

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