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Re: [PATCH V3] rs6000: Refine small loop unroll in loop_unroll_adjust hook


Hi!

On Mon, Nov 04, 2019 at 02:31:43PM +0800, Jiufu Guo wrote:
> In this patch, loop unroll adjust hook is introduced for powerpc.  We can do
> target related hueristic adjustment in this hook. In this patch, small loops
> is unrolled 2 times for O2 and O3 by default.  With this patch, we can see
> some improvement for spec2017.  This patch enhanced a little for [Patch V2] to
> enable small loops unroll for O3 by default like O2.

> 	* gcc/config/rs6000/rs6000.opt (unroll_small_loops): New internal flag.

That's the declaration of a variable.  A command line flag is something
like -munroll-small-loops.  Do we want a command line option like that?
It makes testing simpler.

> -      /* unroll very small loops 2 time if no -funroll-loops.  */
> +      /* If funroll-loops is not enabled explicitly, then enable small loops
> +	 unrolling for -O2, and do not turn fweb or frename-registers on.  */
>        if (!global_options_set.x_flag_unroll_loops
>  	  && !global_options_set.x_flag_unroll_all_loops)
>  	{
> -	  maybe_set_param_value (PARAM_MAX_UNROLL_TIMES, 2,
> -				 global_options.x_param_values,
> -				 global_options_set.x_param_values);
> -
> -	  maybe_set_param_value (PARAM_MAX_UNROLLED_INSNS, 20,
> -				 global_options.x_param_values,
> -				 global_options_set.x_param_values);
> +	  unroll_small_loops = optimize >= 2 ? 1 : 0;

That includes -Os?

I think you shouldn't always set it to some value, only enable it where
you want to enable it.  If you make a command line option for it this is
especially simple (the table in common/config/rs6000/rs6000-common.c).

> +static unsigned
> +rs6000_loop_unroll_adjust (unsigned nunroll, struct loop * loop)
> +{
> +  if (unroll_small_loops)
> +    {
> +      /* TODO: This is hardcoded to 10 right now.  It can be refined, for
> +	 example we may want to unroll very small loops more times (4 perhaps).
> +	 We also should use a PARAM for this.  */
> +      if (loop->ninsns <= 10)
> +	return MIN (2, nunroll);
> +      else
> +	return 0;
> +    }

(Add an empty line here?)

> +  return nunroll;
> +}

Great :-)

> @@ -23472,6 +23488,7 @@ rs6000_function_specific_save (struct cl_target_option *ptr,
>  {
>    ptr->x_rs6000_isa_flags = opts->x_rs6000_isa_flags;
>    ptr->x_rs6000_isa_flags_explicit = opts->x_rs6000_isa_flags_explicit;
> +  ptr->x_unroll_small_loops = opts->x_unroll_small_loops;
>  }

Yeah we shouldn't need to add that, this should all be automatic.


Segher


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