Re: [PATCH] Only allow (copysign x, NEG_CONST) -> (fneg (fabs x)) simplification for constant folding [PR112483]

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

On Tue, Nov 14, 2023 at 11:04 AM Xi Ruoyao <xry111@xry111.site> wrote:
>
> From: Andrew Pinski <pinskia@gmail.com>
>
> On targets with native copysign instructions, (copysign x, -1) is
> usually more efficient than (fneg (fabs x)).  Since r14-5284, in the
> middle end we always optimize (fneg (fabs x)) to (copysign x, -1), not
> vice versa.  If the target does not support native fcopysign,
> expand_COPYSIGN will expand it as (fneg (fabs x)) anyway.
>
> gcc/ChangeLog:
>
>         PR rtl-optimization/112483
>         * simplify-rtx.cc (simplify_binary_operation_1) <case COPYSIGN>:
>         Call simplify_unary_operation for NEG instead of
>         simplify_gen_unary.
> ---
>
> [xry111]:
>
> Following Andrew's suggestion, I bootstrapped and regtested this
> patch on loongarch64-linux-gnu.  Now with
>
>         float t(float x)
>         {
>           return __builtin_copysignf(x, -0.1234);
>         }
>
> It correctly generates
>
>         pcalau12i       $r12,%pc_hi20(.LC0)
>         fld.s   $f1,$r12,%pc_lo12(.LC0)
>         fcopysign.s     $f0,$f0,$f1
>         jr      $r1
>
> instead of the de-optimized fabs.s and fneg.s
>
> Ok for trunk?
>
>  gcc/simplify-rtx.cc | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/gcc/simplify-rtx.cc b/gcc/simplify-rtx.cc
> index 2d2e5a3c1ca..f3745d86aea 100644
> --- a/gcc/simplify-rtx.cc
> +++ b/gcc/simplify-rtx.cc
> @@ -4392,7 +4392,7 @@ simplify_ashift:
>           real_convert (&f1, mode, CONST_DOUBLE_REAL_VALUE (trueop1));
>           rtx tmp = simplify_gen_unary (ABS, mode, op0, mode);
>           if (REAL_VALUE_NEGATIVE (f1))
> -           tmp = simplify_gen_unary (NEG, mode, tmp, mode);
> +           tmp = simplify_unary_operation (NEG, mode, tmp, mode);

shouldn't that be when either the ABS or the NEG simplify?  And I wonder
when that happens - I suppose when op0 is CONST_DOUBLE only?

>           return tmp;
>         }
>        if (GET_CODE (op0) == NEG || GET_CODE (op0) == ABS)
> --
> 2.42.1
>