[rs6000] Adjust vectorization cost for scalar COND_EXPR

We found that the vectorization cost modeling on scalar COND_EXPR is a bit off
on rs6000.  One typical case is 548.exchange2_r, -Ofast -mcpu=power9 -mrecip
-fvect-cost-model=unlimited is better than -Ofast -mcpu=power9 -mrecip (the
default is -fvect-cost-model=dynamic) by 1.94%.  Scalar COND_EXPR is expanded
into compare + branch or compare + isel normally, either of them should be
priced more than the simple FXU operation.  This patch is to add additional
vectorization cost onto scalar COND_EXPR on top of builtin_vectorization_cost.
The idea to use additional cost value 2 instead of the others: 1) try various
possible value candidates from 1 to 5, 2 is the best measured on Power9.  2) 
from latency view, compare takes 3 cycles and isel takes 2 on Power9, it's 
2.5 times of simple FXU instruction which takes cost 1 in the current
modeling, it's close.  3) get fine SPEC2017 ratio on Power8 as well.

gcc/ChangeLog

    * config/rs6000/rs6000.c (adjust_vectorization_cost): New function.
    (rs6000_add_stmt_cost): Call adjust_vectorization_cost and update
    stmt_cost.

From-SVN: r279336

gcc/ChangeLog

+2019-12-13  Kewen Lin  <linkw@gcc.gnu.org>
+
+	* config/rs6000/rs6000.c (adjust_vectorization_cost): New function.
+	(rs6000_add_stmt_cost): Call adjust_vectorization_cost and update
+	stmt_cost.
+
 2019-12-12  Jakub Jelinek  <jakub@redhat.com>
 
 	PR target/92904

gcc/config/rs6000/rs6000.c

@@ -4997,6 +4997,29 @@ rs6000_init_cost (struct loop *loop_info)
   return data;
 }
 
+/* Adjust vectorization cost after calling rs6000_builtin_vectorization_cost.
+   For some statement, we would like to further fine-grain tweak the cost on
+   top of rs6000_builtin_vectorization_cost handling which doesn't have any
+   information on statement operation codes etc.  One typical case here is
+   COND_EXPR, it takes the same cost to simple FXU instruction when evaluating
+   for scalar cost, but it should be priced more whatever transformed to either
+   compare + branch or compare + isel instructions.  */
+
+static unsigned
+adjust_vectorization_cost (enum vect_cost_for_stmt kind,
+			   struct _stmt_vec_info *stmt_info)
+{
+  if (kind == scalar_stmt && stmt_info && stmt_info->stmt
+      && gimple_code (stmt_info->stmt) == GIMPLE_ASSIGN)
+    {
+      tree_code subcode = gimple_assign_rhs_code (stmt_info->stmt);
+      if (subcode == COND_EXPR)
+	return 2;
+    }
+
+  return 0;
+}
+
 /* Implement targetm.vectorize.add_stmt_cost.  */
 
 static unsigned
@@ -5012,6 +5035,7 @@ rs6000_add_stmt_cost (void *data, int count, enum vect_cost_for_stmt kind,
       tree vectype = stmt_info ? stmt_vectype (stmt_info) : NULL_TREE;
       int stmt_cost = rs6000_builtin_vectorization_cost (kind, vectype,
 							 misalign);
+      stmt_cost += adjust_vectorization_cost (kind, stmt_info);
       /* Statements in an inner loop relative to the loop being
 	 vectorized are weighted more heavily.  The value here is
 	 arbitrary and could potentially be improved with analysis.  */