Don't assign a cost to vectorizable_assignment

vectorizable_assignment handles true SSA-to-SSA copies (which hopefully
we don't see in practice) and no-op conversions that are required
to maintain correct gimple, such as changes between signed and
unsigned types.  These cases shouldn't generate any code and so
shouldn't count against either the scalar or vector costs.

Later patches test this, but it seemed worth splitting out.

2019-11-13  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* tree-vectorizer.h (vect_nop_conversion_p): Declare.
	* tree-vect-stmts.c (vect_nop_conversion_p): New function.
	(vectorizable_assignment): Don't add a cost for nop conversions.
	* tree-vect-loop.c (vect_compute_single_scalar_iteration_cost):
	Likewise.
	* tree-vect-slp.c (vect_bb_slp_scalar_cost): Likewise.

From-SVN: r278122

gcc/ChangeLog

 2019-11-13  Richard Sandiford  <richard.sandiford@arm.com>
 
+	* tree-vectorizer.h (vect_nop_conversion_p): Declare.
+	* tree-vect-stmts.c (vect_nop_conversion_p): New function.
+	(vectorizable_assignment): Don't add a cost for nop conversions.
+	* tree-vect-loop.c (vect_compute_single_scalar_iteration_cost):
+	Likewise.
+	* tree-vect-slp.c (vect_bb_slp_scalar_cost): Likewise.
+
+2019-11-13  Richard Sandiford  <richard.sandiford@arm.com>
+
 	* tree-vect-stmts.c (vect_model_promotion_demotion_cost): Take the
 	number of ncopies as an additional argument.
 	(vectorizable_conversion): Update call accordingly.  Use "modifier"

gcc/tree-vect-loop.c

@@ -1125,7 +1125,9 @@ vect_compute_single_scalar_iteration_cost (loop_vec_info loop_vinfo)
              else
                kind = scalar_store;
             }
-          else
+	  else if (vect_nop_conversion_p (stmt_info))
+	    continue;
+	  else
             kind = scalar_stmt;
 
 	  record_stmt_cost (&LOOP_VINFO_SCALAR_ITERATION_COST (loop_vinfo),

gcc/tree-vect-slp.c

@@ -2893,6 +2893,8 @@ vect_bb_slp_scalar_cost (basic_block bb,
           else
 	    kind = scalar_store;
         }
+      else if (vect_nop_conversion_p (stmt_info))
+	continue;
       else
 	kind = scalar_stmt;
       record_stmt_cost (cost_vec, 1, kind, stmt_info, 0, vect_body);

gcc/tree-vect-stmts.c

@@ -5281,6 +5281,29 @@ vectorizable_conversion (stmt_vec_info stmt_info, gimple_stmt_iterator *gsi,
   return true;
 }
 
+/* Return true if we can assume from the scalar form of STMT_INFO that
+   neither the scalar nor the vector forms will generate code.  STMT_INFO
+   is known not to involve a data reference.  */
+
+bool
+vect_nop_conversion_p (stmt_vec_info stmt_info)
+{
+  gassign *stmt = dyn_cast <gassign *> (stmt_info->stmt);
+  if (!stmt)
+    return false;
+
+  tree lhs = gimple_assign_lhs (stmt);
+  tree_code code = gimple_assign_rhs_code (stmt);
+  tree rhs = gimple_assign_rhs1 (stmt);
+
+  if (code == SSA_NAME || code == VIEW_CONVERT_EXPR)
+    return true;
+
+  if (CONVERT_EXPR_CODE_P (code))
+    return tree_nop_conversion_p (TREE_TYPE (lhs), TREE_TYPE (rhs));
+
+  return false;
+}
 
 /* Function vectorizable_assignment.
 
@@ -5396,7 +5419,9 @@ vectorizable_assignment (stmt_vec_info stmt_info, gimple_stmt_iterator *gsi,
     {
       STMT_VINFO_TYPE (stmt_info) = assignment_vec_info_type;
       DUMP_VECT_SCOPE ("vectorizable_assignment");
-      vect_model_simple_cost (stmt_info, ncopies, dt, ndts, slp_node, cost_vec);
+      if (!vect_nop_conversion_p (stmt_info))
+	vect_model_simple_cost (stmt_info, ncopies, dt, ndts, slp_node,
+				cost_vec);
       return true;
     }
 

gcc/tree-vectorizer.h

@@ -1650,6 +1650,7 @@ extern tree vect_get_vec_def_for_stmt_copy (vec_info *, tree);
 extern bool vect_transform_stmt (stmt_vec_info, gimple_stmt_iterator *,
 				 slp_tree, slp_instance);
 extern void vect_remove_stores (stmt_vec_info);
+extern bool vect_nop_conversion_p (stmt_vec_info);
 extern opt_result vect_analyze_stmt (stmt_vec_info, bool *, slp_tree,
 				     slp_instance, stmt_vector_for_cost *);
 extern void vect_get_load_cost (stmt_vec_info, int, bool,