Re: [PATCH] x86: Increase generic tune branch misprediction cost

[Hongtao Liu]

On Fri, May 15, 2026 at 3:02 PM Lili Cui <lili....@intel.com> wrote:
>
> Hi,
>
> This patch increases the branch misprediction scale for generic tuning to 
> better reflect the cost on modern CPUs with deeper pipelines.
>
> Bootstrapped and regression tested on x86_64-pc-linux-gnu. OK for trunk?

Any comments from AMD folks?

>
> Thanks,
> Lili.
>
>
> Increase the branch misprediction scale for generic tuning from
> COSTS_N_INSNS (2) to COSTS_N_INSNS (2) + 3.
>
> Modern CPUs have deeper pipelines, making branch mispredictions more
> expensive. Increasing this cost encourages if-conversion, avoiding
> pipeline stalls from mispredicted branches.
>
> This improves 544.nab_r (-O2) by 12.7% on GNR and 12.1% on Znver5 with
> single-copy.
>
> gcc/ChangeLog:
>
>         * config/i386/x86-tune-costs.h (generic_cost): Increase branch
>         mispredict scale from COSTS_N_INSNS (2) to COSTS_N_INSNS (2) + 3.
> ---
>  gcc/config/i386/x86-tune-costs.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/gcc/config/i386/x86-tune-costs.h 
> b/gcc/config/i386/x86-tune-costs.h
> index 7819fdf7c02..0c687a11a74 100644
> --- a/gcc/config/i386/x86-tune-costs.h
> +++ b/gcc/config/i386/x86-tune-costs.h
> @@ -4274,7 +4274,7 @@ struct processor_costs generic_cost = {
>    "16",                                        /* Func alignment.  */
>    4,                                   /* Small unroll limit.  */
>    2,                                   /* Small unroll factor.  */
> -  COSTS_N_INSNS (2),                   /* Branch mispredict scale.  */
> +  COSTS_N_INSNS (2) + 3,               /* Branch mispredict scale.  */
>  };
>
>  /* core_cost should produce code tuned for Core familly of CPUs.  */
> --
> 2.34.1
>


-- 
BR,
Hongtao