string.c: performance improvement

* string.c (rb_str_rpartition): get rid of conversion from byte offset
  to char offset and backward, for performance improvement.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@39928 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

string.c

@@ -7522,7 +7522,6 @@ rb_str_rpartition(VALUE str, VALUE sep)
 
     if (RB_TYPE_P(sep, T_REGEXP)) {
 	pos = rb_reg_search(sep, str, pos, 1);
-	pos = rb_str_sublen(str, pos);
 	regex = TRUE;
     }
     else {
@@ -7543,9 +7542,13 @@ rb_str_rpartition(VALUE str, VALUE sep)
     if (regex) {
 	sep = rb_reg_nth_match(0, rb_backref_get());
     }
-    return rb_ary_new3(3, rb_str_substr(str, 0, pos),
+    else {
+	pos = rb_str_offset(str, pos);
+    }
+    return rb_ary_new3(3, rb_str_subseq(str, 0, pos),
 		          sep,
-			  rb_str_substr(str,pos+str_strlen(sep,STR_ENC_GET(sep)),RSTRING_LEN(str)));
+		          rb_str_subseq(str, pos+RSTRING_LEN(sep),
+					RSTRING_LEN(str)-pos-RSTRING_LEN(sep)));
 }
 
 /*