a.diff

diff --git a/core/src/main/java/org/jruby/util/ConvertBytes.java b/core/src/main/java/org/jruby/util/ConvertBytes.java
index 644d46da13..a2d4a8709d 100644
--- a/core/src/main/java/org/jruby/util/ConvertBytes.java
+++ b/core/src/main/java/org/jruby/util/ConvertBytes.java
@@ -176,10 +176,13 @@ public class ConvertBytes {
         if (neg) newSize++;
         string.resize(newSize);
         ByteList byteList = string.getByteList();
+        byte[] bytes = byteList.getUnsafeBytes();
+        int beg = byteList.begin();
 
-        do {
-            byteList.set(--newSize, LOWER_DIGITS[(int) (i % 10)]);
-        } while ((i /= 10) > 0);
+        for (int n = newSize - 1; i > 0 && n >= newSize - len; n--) {
+            bytes[beg + n] = LOWER_DIGITS[(int) (i % 10)];
+            i /= 10;
+        }
         if (neg) byteList.set(--newSize, '-');
     }