[gcc r12-6795] tree-optimization/100089 - BB vectorization of if-converted loop bodies

[Richard Biener]

https://gcc.gnu.org/g:199cd0e0f8744ca1e61a95987b2d020a592a46d9

commit r12-6795-g199cd0e0f8744ca1e61a95987b2d020a592a46d9
Author: Richard Biener <rguenther@suse.de>
Date:   Fri Jan 21 13:29:06 2022 +0100

    tree-optimization/100089 - BB vectorization of if-converted loop bodies
    
    The PR complains that when we only partially BB vectorize an
    if-converted loop body that this can leave unvectorized code
    unconditionally executed and thus effectively slow down code.
    For -O2 we already mitigated the issue by not doing BB vectorization
    when not all if-converted stmts were covered but the issue is
    present with -O3 as well.  Thus the following simply extends the
    fix to cover all but the unlimited cost models.  It is after all
    very likely that we vectorize some stmts, if only a single
    paired store.
    
    2022-01-21  Richard Biener  <rguenther@suse.de>
    
            PR tree-optimization/100089
            * tree-vect-slp.cc (vect_slp_region): Reject BB vectorization
            of if-converted loops with unvectorized COND_EXPRs for
            all but the unlimited cost models.

Diff:
---
 gcc/tree-vect-slp.cc | 7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

diff --git a/gcc/tree-vect-slp.cc b/gcc/tree-vect-slp.cc
index 5ffb63689f2..926106c4ecd 100644
--- a/gcc/tree-vect-slp.cc
+++ b/gcc/tree-vect-slp.cc
@@ -5906,9 +5906,8 @@ vect_slp_region (vec<basic_block> bbs, vec<data_reference_p> datarefs,
 	      profitable_subgraphs.safe_push (instance);
 	    }
 
-	  /* When we're vectorizing an if-converted loop body with the
-	     very-cheap cost model make sure we vectorized all if-converted
-	     code.  */
+	  /* When we're vectorizing an if-converted loop body make sure
+	     we vectorized all if-converted code.  */
 	  if (!profitable_subgraphs.is_empty ()
 	      && orig_loop)
 	    {
@@ -5924,7 +5923,7 @@ vect_slp_region (vec<basic_block> bbs, vec<data_reference_p> datarefs,
 		      gimple_set_visited (gsi_stmt (gsi), false);
 		      continue;
 		    }
-		  if (flag_vect_cost_model != VECT_COST_MODEL_VERY_CHEAP)
+		  if (flag_vect_cost_model == VECT_COST_MODEL_UNLIMITED)
 		    continue;
 
 		  if (gassign *ass = dyn_cast <gassign *> (gsi_stmt (gsi)))