[gcc r11-1956] vect: Enhance condition check to use partial vectors

[Kewen Lin]

https://gcc.gnu.org/g:2a39c42a42cdea4a8962b9e140b88e0051894f38

commit r11-1956-g2a39c42a42cdea4a8962b9e140b88e0051894f38
Author: Kewen Lin <linkw@linux.ibm.com>
Date:   Wed Jul 8 22:27:41 2020 -0500

    vect: Enhance condition check to use partial vectors
    
    This patch is derived from the review of vector with length patch
    series.  The length-based partial vector approach doesn't support
    reduction so far, so we would like to disable vectorization with
    partial vectors explicitly for it in vectorizable_condition.
    Otherwise, it will cause some unexpected failures for a few cases
    like gcc.dg/vect/pr65947-2.c.
    
    But if we disable it for the cases excepting for reduction_type equal
    to EXTRACT_LAST_REDUCTION, it cause one regression failure on aarch64:
    
      gcc.target/aarch64/sve/reduc_8.c -march=armv8.2-a+sve
    
    The disabling makes the outer loop can't work with partial vectors,
    the check fails.  But the case is safe to adopt it.  As Richard S.
    pointed out in the review comments, the extra inactive lanes only
    matter for double reductions, so this patch is to permit vectorization
    with partial vectors for cases EXTRACT_LAST_REDUCTION or nested-cycle
    reduction.
    
    Bootstrapped/regtested on aarch64-linux-gnu.
    
    gcc/ChangeLog:
    
            * tree-vect-stmts.c (vectorizable_condition): Prohibit vectorization
            with partial vectors explicitly excepting for EXTRACT_LAST_REDUCTION
            or nested-cycle reduction.

Diff:
---
 gcc/tree-vect-stmts.c | 21 ++++++++++++++++-----
 1 file changed, 16 insertions(+), 5 deletions(-)

diff --git a/gcc/tree-vect-stmts.c b/gcc/tree-vect-stmts.c
index cec5c601268..5eae74e8042 100644
--- a/gcc/tree-vect-stmts.c
+++ b/gcc/tree-vect-stmts.c
@@ -9876,11 +9876,22 @@ vectorizable_condition (vec_info *vinfo,
 	  return false;
 	}
 
-      if (loop_vinfo
-	  && LOOP_VINFO_CAN_USE_PARTIAL_VECTORS_P (loop_vinfo)
-	  && reduction_type == EXTRACT_LAST_REDUCTION)
-	vect_record_loop_mask (loop_vinfo, &LOOP_VINFO_MASKS (loop_vinfo),
-			       ncopies * vec_num, vectype, NULL);
+      if (loop_vinfo && for_reduction
+	  && LOOP_VINFO_CAN_USE_PARTIAL_VECTORS_P (loop_vinfo))
+	{
+	  if (reduction_type == EXTRACT_LAST_REDUCTION)
+	    vect_record_loop_mask (loop_vinfo, &LOOP_VINFO_MASKS (loop_vinfo),
+				   ncopies * vec_num, vectype, NULL);
+	  /* Extra inactive lanes should be safe for vect_nested_cycle.  */
+	  else if (STMT_VINFO_DEF_TYPE (reduc_info) != vect_nested_cycle)
+	    {
+	      if (dump_enabled_p ())
+		dump_printf_loc (MSG_MISSED_OPTIMIZATION, vect_location,
+				 "conditional reduction prevents the use"
+				 " of partial vectors.\n");
+	      LOOP_VINFO_CAN_USE_PARTIAL_VECTORS_P (loop_vinfo) = false;
+	    }
+	}
 
       STMT_VINFO_TYPE (stmt_info) = condition_vec_info_type;
       vect_model_simple_cost (vinfo, stmt_info, ncopies, dts, ndts, slp_node,