Re: [PATCH 13/21]middle-end: Update loop form analysis to support early break

[Richard Biener <rguenther@suse.de>]

On Mon, 6 Nov 2023, Tamar Christina wrote:

> Hi All,
> 
> This sets LOOP_VINFO_EARLY_BREAKS and does some misc changes so the other
> patches are self contained.
> 
> Bootstrapped Regtested on aarch64-none-linux-gnu and no issues.
> 
> Ok for master?
> 
> Thanks,
> Tamar
> 
> gcc/ChangeLog:
> 
> 	* tree-vect-loop.cc (vect_analyze_loop_form): Analyse all exits.
> 	(vect_create_loop_vinfo): Set LOOP_VINFO_EARLY_BREAKS.
> 	(vect_transform_loop): Use it.
> 
> --- inline copy of patch -- 
> diff --git a/gcc/tree-vect-loop.cc b/gcc/tree-vect-loop.cc
> index 51a054c5b035ac80dfbbf3b5ba2f6da82fda91f6..f9483eff6e9606e835906fb991f07cd6052491d0 100644
> --- a/gcc/tree-vect-loop.cc
> +++ b/gcc/tree-vect-loop.cc
> @@ -1700,12 +1700,12 @@ vect_compute_single_scalar_iteration_cost (loop_vec_info loop_vinfo)
>    loop_vinfo->scalar_costs->finish_cost (nullptr);
>  }
>  
> -
>  /* Function vect_analyze_loop_form.
>  
>     Verify that certain CFG restrictions hold, including:
>     - the loop has a pre-header
> -   - the loop has a single entry and exit
> +   - the loop has a single entry
> +   - nested loops can have only a single exit.
>     - the loop exit condition is simple enough
>     - the number of iterations can be analyzed, i.e, a countable loop.  The
>       niter could be analyzed under some assumptions.  */
> @@ -1841,10 +1841,14 @@ vect_analyze_loop_form (class loop *loop, vect_loop_form_info *info)
>  				   "not vectorized: latch block not empty.\n");
>  
>    /* Make sure the exit is not abnormal.  */
> -  if (exit_e->flags & EDGE_ABNORMAL)
> -    return opt_result::failure_at (vect_location,
> -				   "not vectorized:"
> -				   " abnormal loop exit edge.\n");
> +  auto_vec<edge> exits = get_loop_exit_edges (loop);
> +  for (edge e : exits)

Seeing this multiple times, this isn't the most efficient way to
iterate over all exits with LOOPS_HAVE_RECORDED_EXITS.

Note to myself: fix (add to) the API.

> +    {
> +      if (e->flags & EDGE_ABNORMAL)
> +	return opt_result::failure_at (vect_location,
> +				       "not vectorized:"
> +				       " abnormal loop exit edge.\n");
> +    }
>  
>    info->conds
>      = vect_get_loop_niters (loop, exit_e, &info->assumptions,
> @@ -1920,6 +1924,10 @@ vect_create_loop_vinfo (class loop *loop, vec_info_shared *shared,
>  
>    LOOP_VINFO_IV_EXIT (loop_vinfo) = info->loop_exit;
>  
> +  /* Check to see if we're vectorizing multiple exits.  */
> +  LOOP_VINFO_EARLY_BREAKS (loop_vinfo)
> +    = !LOOP_VINFO_LOOP_CONDS (loop_vinfo).is_empty ();
> +

Seeing this, s/LOOP_VINFO_LOOP_CONDS/LOOP_VINFO_LOOP_EXIT_CONDS/g
might be good, if we in future avoid if-conversion in a separate
pass we will have other CONDs as well.

>    if (info->inner_loop_cond)
>      {
>        stmt_vec_info inner_loop_cond_info
> @@ -11577,7 +11585,7 @@ vect_transform_loop (loop_vec_info loop_vinfo, gimple *loop_vectorized_call)
>    /* Make sure there exists a single-predecessor exit bb.  Do this before 
>       versioning.   */
>    edge e = LOOP_VINFO_IV_EXIT (loop_vinfo);
> -  if (! single_pred_p (e->dest))
> +  if (! single_pred_p (e->dest) && !LOOP_VINFO_EARLY_BREAKS (loop_vinfo))
>      {
>        split_loop_exit_edge (e, true);

Note this splitting is done to fulfil versioning constraints on CFG
update.  Do you have test coverage with alias versioning and early
breaks?

Otherwise OK.

Thanks,
Richard.