Ensure Digest.digestAsHex() has a fixed width value.

Author: cstancu

compiler/src/jdk.graal.compiler/src/jdk/graal/compiler/util/Digest.java

@@ -123,7 +123,18 @@ private static void encodeBase62(long value, byte[] result, int resultIndex) {
     public static String digestAsHex(String value) {
         byte[] bytes = value.getBytes(StandardCharsets.UTF_8);
         LongLong hash = MurmurHash3_x64_128(bytes, 0, bytes.length, HASH_SEED);
-        return Long.toHexString(hash.l1) + Long.toHexString(hash.l2);
+        String asHex = toFixedSizeHexString(hash.l1) + toFixedSizeHexString(hash.l2);
+        GraalError.guarantee(asHex.length() == 32, "Expecting a 32 digits long hex value.");
+        return asHex;
+    }
+
+    /**
+     * Note: The string generated by {@link Long#toHexString(long)} can vary in length, and it
+     * doesn't include any leading 0s. With {@link String#format(String, Object...)} we inject
+     * leading 0s when necessary to ensure that the produced digest has a fixed size.
+     */
+    private static String toFixedSizeHexString(long l) {
+        return String.format("%016x", l);
     }
 
     public static UUID digestAsUUID(String value) {