Re: [PATCH 8/19]middle-end: updated niters analysis to handle multiple exits.

[Richard Biener <rguenther@suse.de>]

On Thu, 13 Jul 2023, Richard Biener wrote:

> On Wed, 28 Jun 2023, Tamar Christina wrote:
> 
> > Hi All,
> > 
> > For early break vectorization we have to update niters analysis to record and
> > analyze all exits of the loop, and so all conds.
> > 
> > The niters of the loop is still determined by the main/natural exit of the loop
> > as this is the O(n) bounds.  For now we don't do much with the secondary conds,
> > but their assumptions can be used to generate versioning checks later.
> > 
> > Bootstrapped Regtested on aarch64-none-linux-gnu and no issues.
> > 
> > Ok for master?
> 
> I probably confused vec_init_exit_info in the previous patch - that said,
> I'm missing a clear function that determines the natural exit of the
> original (if-converted) scalar loop.  As vec_init_exit_info seems
> to (re-)compute that I'll comment on it here.
> 
> +  /* The main IV is to be determined by the block that's the first 
> reachable
> +     block from the latch.  We cannot rely on the order the loop analysis
> +     returns and we don't have any SCEV analysis on the loop.  */
> +  auto_vec <edge> workset;
> +  workset.safe_push (loop_latch_edge (loop));
> +  hash_set <edge> visited;
> +
> +  while (!workset.is_empty ())
> +    {
> +      edge e = workset.pop ();
> +      if (visited.contains (e))
> +       continue;
> +
> +      bool found_p = false;
> +      for (edge ex : e->src->succs)
> +       {
> +         if (exits.contains (ex))
> +           {
> +             found_p = true;
> +             e = ex;
> +             break;
> +           }
> +       }
> +
> +      if (found_p)
> +       {
> +         loop->vec_loop_iv = e;
> +         for (edge ex : exits)
> +           if (e != ex)
> +             loop->vec_loop_alt_exits.safe_push (ex);
> +         return;
> +       }
> +      else
> +       {
> +         for (edge ex : e->src->preds)
> +           workset.safe_insert (0, ex);
> +       }
> +      visited.add (e);
> +    }
> 
> So this greedily follows edges from the latch and takes the first
> exit.  Why's that better than simply choosing the first?
> 
> I'd have done
> 
>  auto_vec<edge> exits = get_loop_exit_edges (loop);
>  for (e : exits)
>    {
>      if (vect_get_loop_niters (...))
>        {
>          if no assumptions use that edge, if assumptions continue
>          searching, maybe ther's an edge w/o assumptions
>        }
>    }
>  use (first) exit with assumptions
> 
> we probably want to know 'may_be_zero' as well and prefer an edge
> without that.  So eventually call number_of_iterations_exit_assumptions
> directly and look for the best niter_desc and pass that to
> vect_get_loop_niters (or re-do the work).
> 
> As said for "copying" the exit to the loop copies use the block mapping.

In case you only support treating the last exit as IV exit you can
also simply walk dominators from the latch block until you reach
the header and pick the first [niter analyzable] exit block you reach.

But I'm not yet sure this restriction exists.

Richard.

> 
> > Thanks,
> > Tamar
> > 
> > gcc/ChangeLog:
> > 
> > 	* tree-vect-loop.cc (vect_get_loop_niters): Analyze all exits and return
> > 	all gconds.
> > 	(vect_analyze_loop_form): Update code checking for conds.
> > 	(vect_create_loop_vinfo): Handle having multiple conds.
> > 	(vect_analyze_loop): Release extra loop conds structures.
> > 	* tree-vectorizer.h (LOOP_VINFO_LOOP_CONDS,
> > 	LOOP_VINFO_LOOP_IV_COND): New.
> > 	(struct vect_loop_form_info): Add conds, loop_iv_cond.
> > 
> > --- inline copy of patch -- 
> > diff --git a/gcc/tree-vect-loop.cc b/gcc/tree-vect-loop.cc
> > index 55e69a7ca0b24e0872477141db6f74dbf90b7981..9065811b3b9c2a550baf44768603172b9e26b94b 100644
> > --- a/gcc/tree-vect-loop.cc
> > +++ b/gcc/tree-vect-loop.cc
> > @@ -849,80 +849,106 @@ vect_fixup_scalar_cycles_with_patterns (loop_vec_info loop_vinfo)
> >     in NUMBER_OF_ITERATIONSM1.  Place the condition under which the
> >     niter information holds in ASSUMPTIONS.
> >  
> > -   Return the loop exit condition.  */
> > +   Return the loop exit conditions.  */
> >  
> >  
> > -static gcond *
> > +static vec<gcond *>
> >  vect_get_loop_niters (class loop *loop, tree *assumptions,
> >  		      tree *number_of_iterations, tree *number_of_iterationsm1)
> >  {
> > -  edge exit = single_exit (loop);
> > +  auto_vec<edge> exits = get_loop_exit_edges (loop);
> > +  vec<gcond *> conds;
> > +  conds.create (exits.length ());
> >    class tree_niter_desc niter_desc;
> >    tree niter_assumptions, niter, may_be_zero;
> > -  gcond *cond = get_loop_exit_condition (loop);
> >  
> >    *assumptions = boolean_true_node;
> >    *number_of_iterationsm1 = chrec_dont_know;
> >    *number_of_iterations = chrec_dont_know;
> > +
> >    DUMP_VECT_SCOPE ("get_loop_niters");
> >  
> > -  if (!exit)
> > -    return cond;
> > +  if (exits.is_empty ())
> > +    return conds;
> >  
> > -  may_be_zero = NULL_TREE;
> > -  if (!number_of_iterations_exit_assumptions (loop, exit, &niter_desc, NULL)
> > -      || chrec_contains_undetermined (niter_desc.niter))
> > -    return cond;
> > +  if (dump_enabled_p ())
> > +    dump_printf_loc (MSG_NOTE, vect_location, "Loop has %d exits.\n",
> > +		     exits.length ());
> >  
> > -  niter_assumptions = niter_desc.assumptions;
> > -  may_be_zero = niter_desc.may_be_zero;
> > -  niter = niter_desc.niter;
> > +  edge exit;
> > +  unsigned int i;
> > +  FOR_EACH_VEC_ELT (exits, i, exit)
> > +    {
> > +      gcond *cond = get_edge_condition (exit);
> > +      if (cond)
> > +	conds.safe_push (cond);
> >  
> > -  if (may_be_zero && integer_zerop (may_be_zero))
> > -    may_be_zero = NULL_TREE;
> > +      if (dump_enabled_p ())
> > +	dump_printf_loc (MSG_NOTE, vect_location, "Analyzing exit %d...\n", i);
> >  
> > -  if (may_be_zero)
> > -    {
> > -      if (COMPARISON_CLASS_P (may_be_zero))
> > +      may_be_zero = NULL_TREE;
> > +      if (!number_of_iterations_exit_assumptions (loop, exit, &niter_desc, NULL)
> > +          || chrec_contains_undetermined (niter_desc.niter))
> > +	continue;
> > +
> > +      niter_assumptions = niter_desc.assumptions;
> > +      may_be_zero = niter_desc.may_be_zero;
> > +      niter = niter_desc.niter;
> > +
> > +      if (may_be_zero && integer_zerop (may_be_zero))
> > +	may_be_zero = NULL_TREE;
> > +
> > +      if (may_be_zero)
> >  	{
> > -	  /* Try to combine may_be_zero with assumptions, this can simplify
> > -	     computation of niter expression.  */
> > -	  if (niter_assumptions && !integer_nonzerop (niter_assumptions))
> > -	    niter_assumptions = fold_build2 (TRUTH_AND_EXPR, boolean_type_node,
> > -					     niter_assumptions,
> > -					     fold_build1 (TRUTH_NOT_EXPR,
> > -							  boolean_type_node,
> > -							  may_be_zero));
> > +	  if (COMPARISON_CLASS_P (may_be_zero))
> > +	    {
> > +	      /* Try to combine may_be_zero with assumptions, this can simplify
> > +		 computation of niter expression.  */
> > +	      if (niter_assumptions && !integer_nonzerop (niter_assumptions))
> > +		niter_assumptions = fold_build2 (TRUTH_AND_EXPR, boolean_type_node,
> > +						 niter_assumptions,
> > +						 fold_build1 (TRUTH_NOT_EXPR,
> > +							      boolean_type_node,
> > +							      may_be_zero));
> > +	      else
> > +		niter = fold_build3 (COND_EXPR, TREE_TYPE (niter), may_be_zero,
> > +				     build_int_cst (TREE_TYPE (niter), 0),
> > +				     rewrite_to_non_trapping_overflow (niter));
> > +
> > +	      may_be_zero = NULL_TREE;
> > +	    }
> > +	  else if (integer_nonzerop (may_be_zero) && exit == loop->vec_loop_iv)
> > +	    {
> > +	      *number_of_iterationsm1 = build_int_cst (TREE_TYPE (niter), 0);
> > +	      *number_of_iterations = build_int_cst (TREE_TYPE (niter), 1);
> > +	      continue;
> > +	    }
> >  	  else
> > -	    niter = fold_build3 (COND_EXPR, TREE_TYPE (niter), may_be_zero,
> > -				 build_int_cst (TREE_TYPE (niter), 0),
> > -				 rewrite_to_non_trapping_overflow (niter));
> > +	    continue;
> > +       }
> >  
> > -	  may_be_zero = NULL_TREE;
> > -	}
> > -      else if (integer_nonzerop (may_be_zero))
> > +      /* Loop assumptions are based off the normal exit.  */
> > +      if (exit == loop->vec_loop_iv)
> >  	{
> > -	  *number_of_iterationsm1 = build_int_cst (TREE_TYPE (niter), 0);
> > -	  *number_of_iterations = build_int_cst (TREE_TYPE (niter), 1);
> > -	  return cond;
> > +	  *assumptions = niter_assumptions;
> > +	  *number_of_iterationsm1 = niter;
> > +
> > +	  /* We want the number of loop header executions which is the number
> > +	     of latch executions plus one.
> > +	     ???  For UINT_MAX latch executions this number overflows to zero
> > +	     for loops like do { n++; } while (n != 0);  */
> > +	  if (niter && !chrec_contains_undetermined (niter))
> > +	    niter = fold_build2 (PLUS_EXPR, TREE_TYPE (niter),
> > +				 unshare_expr (niter),
> > +				 build_int_cst (TREE_TYPE (niter), 1));
> > +	  *number_of_iterations = niter;
> >  	}
> > -      else
> > -	return cond;
> >      }
> >  
> > -  *assumptions = niter_assumptions;
> > -  *number_of_iterationsm1 = niter;
> > -
> > -  /* We want the number of loop header executions which is the number
> > -     of latch executions plus one.
> > -     ???  For UINT_MAX latch executions this number overflows to zero
> > -     for loops like do { n++; } while (n != 0);  */
> > -  if (niter && !chrec_contains_undetermined (niter))
> > -    niter = fold_build2 (PLUS_EXPR, TREE_TYPE (niter), unshare_expr (niter),
> > -			  build_int_cst (TREE_TYPE (niter), 1));
> > -  *number_of_iterations = niter;
> > +  if (dump_enabled_p ())
> > +    dump_printf_loc (MSG_NOTE, vect_location, "All loop exits successfully analyzed.\n");
> >  
> > -  return cond;
> > +  return conds;
> >  }
> >  
> >  /* Function bb_in_loop_p
> > @@ -1768,15 +1794,26 @@ vect_analyze_loop_form (class loop *loop, vect_loop_form_info *info)
> >  				   "not vectorized:"
> >  				   " abnormal loop exit edge.\n");
> >  
> > -  info->loop_cond
> > +  info->conds
> >      = vect_get_loop_niters (loop, &info->assumptions,
> >  			    &info->number_of_iterations,
> >  			    &info->number_of_iterationsm1);
> > -  if (!info->loop_cond)
> > +
> > +  if (info->conds.is_empty ())
> >      return opt_result::failure_at
> >        (vect_location,
> >         "not vectorized: complicated exit condition.\n");
> >  
> > +  /* Determine what the primary and alternate exit conds are.  */
> > +  info->alt_loop_conds.create (info->conds.length () - 1);
> > +  for (gcond *cond : info->conds)
> > +    {
> > +      if (loop->vec_loop_iv->src != gimple_bb (cond))
> > +	info->alt_loop_conds.quick_push (cond);
> > +      else
> > +	info->loop_cond = cond;
> > +    }
> 
> Do you really need those explicitely?  ->conds and ->alt_loop_conds
> looks redundant at least.
> 
> > +
> >    if (integer_zerop (info->assumptions)
> >        || !info->number_of_iterations
> >        || chrec_contains_undetermined (info->number_of_iterations))
> > @@ -1821,8 +1858,14 @@ vect_create_loop_vinfo (class loop *loop, vec_info_shared *shared,
> >    if (!integer_onep (info->assumptions) && !main_loop_info)
> >      LOOP_VINFO_NITERS_ASSUMPTIONS (loop_vinfo) = info->assumptions;
> >  
> > -  stmt_vec_info loop_cond_info = loop_vinfo->lookup_stmt (info->loop_cond);
> > -  STMT_VINFO_TYPE (loop_cond_info) = loop_exit_ctrl_vec_info_type;
> > +  for (gcond *cond : info->alt_loop_conds)
> > +    {
> > +      stmt_vec_info loop_cond_info = loop_vinfo->lookup_stmt (cond);
> > +      STMT_VINFO_TYPE (loop_cond_info) = loop_exit_ctrl_vec_info_type;
> > +    }
> > +  LOOP_VINFO_LOOP_CONDS (loop_vinfo).safe_splice (info->alt_loop_conds);
> > +  LOOP_VINFO_LOOP_IV_COND (loop_vinfo) = info->loop_cond;
> > +
> >    if (info->inner_loop_cond)
> >      {
> >        stmt_vec_info inner_loop_cond_info
> > @@ -3520,6 +3563,9 @@ vect_analyze_loop (class loop *loop, vec_info_shared *shared)
> >  		     "***** Choosing vector mode %s\n",
> >  		     GET_MODE_NAME (first_loop_vinfo->vector_mode));
> >  
> > +  loop_form_info.conds.release ();
> > +  loop_form_info.alt_loop_conds.release ();
> > +
> >    /* Only vectorize epilogues if PARAM_VECT_EPILOGUES_NOMASK is
> >       enabled, SIMDUID is not set, it is the innermost loop and we have
> >       either already found the loop's SIMDLEN or there was no SIMDLEN to
> > @@ -3631,6 +3677,9 @@ vect_analyze_loop (class loop *loop, vec_info_shared *shared)
> >  			   (first_loop_vinfo->epilogue_vinfos[0]->vector_mode));
> >      }
> >  
> > +  loop_form_info.conds.release ();
> > +  loop_form_info.alt_loop_conds.release ();
> > +
> >    return first_loop_vinfo;
> >  }
> >  
> > diff --git a/gcc/tree-vectorizer.h b/gcc/tree-vectorizer.h
> > index bd5eceb5da7a45ef036cd14609ebe091799320bf..1cc003c12e2447eca878f56cb019236f56e96f85 100644
> > --- a/gcc/tree-vectorizer.h
> > +++ b/gcc/tree-vectorizer.h
> > @@ -876,6 +876,12 @@ public:
> >       we need to peel off iterations at the end to form an epilogue loop.  */
> >    bool peeling_for_niter;
> >  
> > +  /* List of loop additional IV conditionals found in the loop.  */
> 
> drop "IV"
> 
> > +  auto_vec<gcond *> conds;
> > +
> > +  /* Main loop IV cond.  */
> > +  gcond* loop_iv_cond;
> > +
> 
> I guess I have to look at the followup patches to see how often we
> have to access loop_iv_cond/conds.
> 
> >    /* True if there are no loop carried data dependencies in the loop.
> >       If loop->safelen <= 1, then this is always true, either the loop
> >       didn't have any loop carried data dependencies, or the loop is being
> > @@ -966,6 +972,8 @@ public:
> >  #define LOOP_VINFO_REDUCTION_CHAINS(L)     (L)->reduction_chains
> >  #define LOOP_VINFO_PEELING_FOR_GAPS(L)     (L)->peeling_for_gaps
> >  #define LOOP_VINFO_PEELING_FOR_NITER(L)    (L)->peeling_for_niter
> > +#define LOOP_VINFO_LOOP_CONDS(L)           (L)->conds
> > +#define LOOP_VINFO_LOOP_IV_COND(L)         (L)->loop_iv_cond
> >  #define LOOP_VINFO_NO_DATA_DEPENDENCIES(L) (L)->no_data_dependencies
> >  #define LOOP_VINFO_SCALAR_LOOP(L)	   (L)->scalar_loop
> >  #define LOOP_VINFO_SCALAR_LOOP_SCALING(L)  (L)->scalar_loop_scaling
> > @@ -2353,7 +2361,9 @@ struct vect_loop_form_info
> >    tree number_of_iterations;
> >    tree number_of_iterationsm1;
> >    tree assumptions;
> > +  vec<gcond *> conds;
> >    gcond *loop_cond;
> > +  vec<gcond *> alt_loop_conds;
> >    gcond *inner_loop_cond;
> >  };
> >  extern opt_result vect_analyze_loop_form (class loop *, vect_loop_form_info *);
> > 
> > 
> > 
> > 
> > 
> 
> 

-- 
Richard Biener <rguenther@suse.de>
SUSE Software Solutions Germany GmbH, Frankenstrasse 146, 90461 Nuernberg,
Germany; GF: Ivo Totev, Andrew Myers, Andrew McDonald, Boudien Moerman;
HRB 36809 (AG Nuernberg)