[PATCH] vect: Enhance condition check to use partial vectors in vectorizable_condition

[Kewen.Lin via Gcc-patches]

Hi,

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.

Testing is ongoing, is it ok for trunk if the testing goes well?

BR,
Kewen
-----
gcc/ChangeLog:

        * tree-vect-stmts.c (vectorizable_condition): Prohibit vectorization
        with partial vectors explicitly excepting for EXTRACT_LAST_REDUCTION
        or nested-cycle reduction.

diff --git a/gcc/tree-vect-stmts.c b/gcc/tree-vect-stmts.c
index 40e2664f93b..c23520aceab 100644
--- a/gcc/tree-vect-stmts.c
+++ b/gcc/tree-vect-stmts.c
@@ -9968,11 +9968,16 @@ 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)
+           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,