diff --git a/ChangeLog b/ChangeLog
index 82d45dad090d9d..b0367f4ba9a37a 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,7 @@
+Sat Jan 21 11:43:45 2012  Nobuyoshi Nakada  <nobu@ruby-lang.org>
+
+	* dir.c (dir_chdir, check_dirname): get rid of optimization-out.
+
 Fri Jan 20 20:47:37 2012  Kenta Murata  <mrkn@cookpad.com>
 
 	* lib/cgi/util.rb (CGI.escape): support a string with invalid byte
diff --git a/dir.c b/dir.c
index 02b03512221f64..f9de4a36680253 100644
--- a/dir.c
+++ b/dir.c
@@ -749,7 +749,6 @@ dir_close(VALUE dir)
 static void
 dir_chdir(VALUE path)
 {
-    path = rb_str_encode_ospath(path);
     if (chdir(RSTRING_PTR(path)) < 0)
 	rb_sys_fail(RSTRING_PTR(path));
 }
@@ -832,6 +831,7 @@ dir_s_chdir(int argc, VALUE *argv, VALUE obj)
     rb_secure(2);
     if (rb_scan_args(argc, argv, "01", &path) == 1) {
 	FilePathValue(path);
+	path = rb_str_encode_ospath(path);
     }
     else {
 	const char *dist = getenv("HOME");
@@ -849,9 +849,8 @@ dir_s_chdir(int argc, VALUE *argv, VALUE obj)
 
     if (rb_block_given_p()) {
 	struct chdir_data args;
-	char *cwd = my_getcwd();
 
-	args.old_path = rb_tainted_str_new2(cwd); xfree(cwd);
+	args.old_path = rb_str_encode_ospath(rb_dir_getwd());
 	args.new_path = path;
 	args.done = FALSE;
 	return rb_ensure(chdir_yield, (VALUE)&args, chdir_restore, (VALUE)&args);
@@ -896,14 +895,16 @@ dir_s_getwd(VALUE dir)
 static void
 check_dirname(volatile VALUE *dir)
 {
+    VALUE d = *dir;
     char *path, *pend;
 
     rb_secure(2);
-    FilePathValue(*dir);
-    path = RSTRING_PTR(*dir);
+    FilePathValue(d);
+    path = RSTRING_PTR(d);
     if (path && *(pend = rb_path_end(rb_path_skip_prefix(path)))) {
-	*dir = rb_str_new(path, pend - path);
+	d = rb_str_subseq(d, 0, pend - path);
     }
+    *dir = rb_str_encode_ospath(d);
 }
 
 #if defined(HAVE_CHROOT)
@@ -920,8 +921,6 @@ static VALUE
 dir_s_chroot(VALUE dir, VALUE path)
 {
     check_dirname(&path);
-
-    path = rb_str_encode_ospath(path);
     if (chroot(RSTRING_PTR(path)) == -1)
 	rb_sys_fail(RSTRING_PTR(path));
 
@@ -960,7 +959,6 @@ dir_s_mkdir(int argc, VALUE *argv, VALUE obj)
     }
 
     check_dirname(&path);
-    path = rb_str_encode_ospath(path);
     if (mkdir(RSTRING_PTR(path), mode) == -1)
 	rb_sys_fail(RSTRING_PTR(path));
 
@@ -980,7 +978,6 @@ static VALUE
 dir_s_rmdir(VALUE obj, VALUE dir)
 {
     check_dirname(&dir);
-    dir = rb_str_encode_ospath(dir);
     if (rmdir(RSTRING_PTR(dir)) < 0)
 	rb_sys_fail(RSTRING_PTR(dir));