[PATCH][v2] Remove --param vect-inner-loop-cost-factor

Kewen.Lin linkw@linux.ibm.com
Tue Aug 24 02:22:24 GMT 2021


Hi Richi,

on 2021/8/23 下午10:33, Richard Biener via Gcc-patches wrote:
> This removes --param vect-inner-loop-cost-factor in favor of looking
> at the estimated number of iterations of the inner loop
> when available and otherwise just assumes a single inner
> iteration which is conservative on the side of not vectorizing.
> 

I may miss something, the factor seems to be an amplifier, a single
inner iteration on the side of not vectorizing only relies on that
vector_cost < scalar_cost, if scalar_cost < vector_cost, the direction
will be flipped? ({vector,scalar}_cost is only for inner loop part).

Since we don't calculate/compare costing for inner loop independently
and early return if scalar_cost < vector_cost for inner loop, I guess
it's possible to have "scalar_cost < vector_cost" case theoretically,
especially when targets can cost something more on vector side.

> The alternative is to retain the --param for exactly that case,
> not sure if the result is better or not.  The --param is new on
> head, it was static '50' before.
> 

I think the intention of --param is to offer ports a way to tweak
it (no ports do it for now though :)).  Not sure how target costing
is sensitive to this factor, but I also prefer to make its default
value as 50 as Honza suggested to avoid more possible tweakings.

If targets want more, maybe we can extend it to:

default_hook:
  return estimated or likely_max if either is valid;
  return default value;
  
target hook:
  val = default_hook; // or from scratch
  tweak the val as it wishes;  

I guess there is no this need for now.

> Any strong opinions?
> 
> Richard.
> 
> 2021-08-23  Richard Biener  <rguenther@suse.de>
> 
> 	* doc/invoke.texi (vect-inner-loop-cost-factor): Remove
> 	documentation.
> 	* params.opt (--param vect-inner-loop-cost-factor): Remove.
> 	* tree-vect-loop.c (_loop_vec_info::_loop_vec_info):
> 	Initialize inner_loop_cost_factor to 1.
> 	(vect_analyze_loop_form): Initialize inner_loop_cost_factor
> 	from the estimated number of iterations of the inner loop.
> ---
>  gcc/doc/invoke.texi  |  5 -----
>  gcc/params.opt       |  4 ----
>  gcc/tree-vect-loop.c | 12 +++++++++++-
>  3 files changed, 11 insertions(+), 10 deletions(-)
> 
> diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
> index c057cc1e4ae..054950132f6 100644
> --- a/gcc/doc/invoke.texi
> +++ b/gcc/doc/invoke.texi
> @@ -14385,11 +14385,6 @@ code to iterate.  2 allows partial vector loads and stores in all loops.
>  The parameter only has an effect on targets that support partial
>  vector loads and stores.
>  
> -@item vect-inner-loop-cost-factor
> -The factor which the loop vectorizer applies to the cost of statements
> -in an inner loop relative to the loop being vectorized.  The default
> -value is 50.
> -
>  @item avoid-fma-max-bits
>  Maximum number of bits for which we avoid creating FMAs.
>  
> diff --git a/gcc/params.opt b/gcc/params.opt
> index f9264887b40..f7b19fa430d 100644
> --- a/gcc/params.opt
> +++ b/gcc/params.opt
> @@ -1113,8 +1113,4 @@ Bound on number of runtime checks inserted by the vectorizer's loop versioning f
>  Common Joined UInteger Var(param_vect_partial_vector_usage) Init(2) IntegerRange(0, 2) Param Optimization
>  Controls how loop vectorizer uses partial vectors.  0 means never, 1 means only for loops whose need to iterate can be removed, 2 means for all loops.  The default value is 2.
>  
> --param=vect-inner-loop-cost-factor=
> -Common Joined UInteger Var(param_vect_inner_loop_cost_factor) Init(50) IntegerRange(1, 999999) Param Optimization
> -The factor which the loop vectorizer applies to the cost of statements in an inner loop relative to the loop being vectorized.
> -
>  ; This comment is to ensure we retain the blank line above.
> diff --git a/gcc/tree-vect-loop.c b/gcc/tree-vect-loop.c
> index c521b43a47c..cb48717f20e 100644
> --- a/gcc/tree-vect-loop.c
> +++ b/gcc/tree-vect-loop.c
> @@ -841,7 +841,7 @@ _loop_vec_info::_loop_vec_info (class loop *loop_in, vec_info_shared *shared)
>      single_scalar_iteration_cost (0),
>      vec_outside_cost (0),
>      vec_inside_cost (0),
> -    inner_loop_cost_factor (param_vect_inner_loop_cost_factor),
> +    inner_loop_cost_factor (1),
>      vectorizable (false),
>      can_use_partial_vectors_p (param_vect_partial_vector_usage != 0),
>      using_partial_vectors_p (false),
> @@ -1519,6 +1519,16 @@ vect_analyze_loop_form (class loop *loop, vec_info_shared *shared)
>        stmt_vec_info inner_loop_cond_info
>  	= loop_vinfo->lookup_stmt (inner_loop_cond);
>        STMT_VINFO_TYPE (inner_loop_cond_info) = loop_exit_ctrl_vec_info_type;
> +      /* If we have an estimate on the number of iterations of the inner
> +	 loop use that as the scale for costing, otherwise conservatively
> +	 assume a single inner iteration.  */
> +      widest_int nit;
> +      if (get_estimated_loop_iterations (loop->inner, &nit))
> +	LOOP_VINFO_INNER_LOOP_COST_FACTOR (loop_vinfo)
> +	  /* Since costing is done on unsigned int cap the scale on
> +	     some large number consistent with what we'd see in
> +	     CFG counts.  */
> +	  = wi::smax (nit, REG_BR_PROB_BASE).to_uhwi ();

I noticed loop-doloop.c use _int version and likely_max, maybe you want that here?
 
  est_niter = get_estimated_loop_iterations_int (loop);
  if (est_niter == -1)
    est_niter = get_likely_max_loop_iterations_int (loop)


BR,
Kewen


More information about the Gcc-patches mailing list