diff --git a/ChangeLog b/ChangeLog
index 18f090a582..9fc27dae12 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+Mon Apr 22 22:54:00 2013  Charlie Somerville  <charlie@charliesomerville.com>
+
+	* insns.def (opt_mod): Use % operator if both operands are positive for
+	  a significant performance improvement. Thanks to @samsaffron.
+
 Mon Apr 22 17:09:37 2013  Nobuyoshi Nakada  <nobu@ruby-lang.org>
 
 	* marshal.c (r_object0): copy all instance variables not only generic
diff --git a/insns.def b/insns.def
index fc4db92d4f..b51e60018c 100644
--- a/insns.def
+++ b/insns.def
@@ -1524,13 +1524,15 @@ opt_mod
 {
     if (FIXNUM_2_P(recv, obj) &&
 	BASIC_OP_UNREDEFINED_P(BOP_MOD, FIXNUM_REDEFINED_OP_FLAG )) {
-	long x, y, mod;
+	long x, y;
 
 	x = FIX2LONG(recv);
 	y = FIX2LONG(obj);
-	{
+	if (x > 0 && y > 0) {
+	    val = LONG2FIX(x % y);
+	} else {
 	    /* copied from numeric.c#fixdivmod */
-	    long div;
+	    long div, mod;
 
 	    if (y == 0)
 		rb_num_zerodiv();
@@ -1551,8 +1553,8 @@ opt_mod
 		mod += y;
 		div -= 1;
 	    }
+	    val = LONG2FIX(mod);
 	}
-	val = LONG2FIX(mod);
     }
     else if (FLONUM_2_P(recv, obj) &&
 	     BASIC_OP_UNREDEFINED_P(BOP_MOD, FLOAT_REDEFINED_OP_FLAG)) {