[PATCH] AArch64: Tune case-values-threshold

[Richard Sandiford]

Wilco Dijkstra <Wilco.Dijkstra@arm.com> writes:
> Tune the case-values-threshold setting for modern cores.  A value of 11 improves
> SPECINT2017 by 0.2% and reduces codesize by 0.04%.  With -Os use value 8 which
> reduces codesize by 0.07%.
>
> Passes regress, OK for commit?
>
> ChangeLog:
>
> 2021-10-18  Wilco Dijkstra  <wdijkstr@arm.com>
>
>         * config/aarch64/aarch64.c (aarch64_case_values_threshold):
>         Change to 8 with -Os, 11 otherwise.
>
> ---
>
> diff --git a/gcc/config/aarch64/aarch64.c b/gcc/config/aarch64/aarch64.c
> index f5b25a7f7041645921e6ad85714efda73b993492..adc5256c5ccc1182710d87cc6a1091083d888663 100644
> --- a/gcc/config/aarch64/aarch64.c
> +++ b/gcc/config/aarch64/aarch64.c
> @@ -9360,8 +9360,8 @@ aarch64_cannot_force_const_mem (machine_mode mode ATTRIBUTE_UNUSED, rtx x)
>     The expansion for a table switch is quite expensive due to the number
>     of instructions, the table lookup and hard to predict indirect jump.
>     When optimizing for speed, and -O3 enabled, use the per-core tuning if
> -   set, otherwise use tables for > 16 cases as a tradeoff between size and
> -   performance.  When optimizing for size, use the default setting.  */
> +   set, otherwise use tables for >= 11 cases as a tradeoff between size and
> +   performance.  When optimizing for size, use 8 for smallest codesize.  */

I'm just concerned that here we're using the same explanation but with
different numbers.  Why are the new numbers more right than the old ones
(especially when it comes to code size, where the trade-off hasn't
really changed)?

It would be good to have more discussion of why certain numbers are
too small or too high, and why 8 is the right pivot point for -Os.

Thanks,
Richard

>
>  static unsigned int
>  aarch64_case_values_threshold (void)
> @@ -9372,7 +9372,7 @@ aarch64_case_values_threshold (void)
>        && selected_cpu->tune->max_case_values != 0)
>      return selected_cpu->tune->max_case_values;
>    else
> -    return optimize_size ? default_case_values_threshold () : 17;
> +    return optimize_size ? 8 : 11;
>  }
>
>  /* Return true if register REGNO is a valid index register.