diff --git a/ChangeLog b/ChangeLog
index 78be5fc367..e3ba79e450 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,9 @@
+Mon Sep 26 14:10:54 2016  Ary Borenszweig  <ary@esperanto.org.ar>
+
+	* string.c (lstrip_offset): add a fast path in the case of single
+	  byte optimizable strings, as well as rstrip_offset.
+	  [ruby-core:77392] [Feature #12788]
+
 Mon Sep 26 12:00:12 2016  Nobuyoshi Nakada  <nobu@ruby-lang.org>
 
 	* gems/bundled_gems: update to minitest-5.9.1.
diff --git a/string.c b/string.c
index c537bbf4ab..8a57bc9048 100644
--- a/string.c
+++ b/string.c
@@ -8041,13 +8041,19 @@ lstrip_offset(VALUE str, const char *s, const char *e, rb_encoding *enc)
     const char *const start = s;
 
     if (!s || s >= e) return 0;
-    /* remove spaces at head */
-    while (s < e) {
-	int n;
-	unsigned int cc = rb_enc_codepoint_len(s, e, &n, enc);
 
-	if (!rb_isspace(cc)) break;
-	s += n;
+    /* remove spaces at head */
+    if (single_byte_optimizable(str)) {
+	while (s < e && ascii_isspace(*s)) s++;
+    }
+    else {
+	while (s < e) {
+	    int n;
+	    unsigned int cc = rb_enc_codepoint_len(s, e, &n, enc);
+
+	    if (!rb_isspace(cc)) break;
+	    s += n;
+	}
     }
     return s - start;
 }