8239787: AArch64: String.indexOf may incorrectly handle empty strings

Reviewed-by: aph, lmesnik, yan

src/hotspot/cpu/aarch64/macroAssembler_aarch64.cpp

@@ -4860,6 +4860,8 @@ void MacroAssembler::string_indexof_char(Register str1, Register cnt1,
   Register ch1 = rscratch1;
   Register result_tmp = rscratch2;
 
+  cbz(cnt1, NOMATCH);
+
   cmp(cnt1, (u1)4);
   br(LT, DO1_SHORT);
 

test/hotspot/jtreg/runtime/StringIntrinsic/StringIndexOfChar.java

@@ -0,0 +1,41 @@
+/*
+ * Copyright (c) 2020, Azul Systems, Inc. All rights reserved.
+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
+ *
+ * This code is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.
+ *
+ * This code is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ */
+
+/* @test
+ * @bug 8239787
+ * @summary String.indexOf(char) for empty string must give -1
+ * @run main/othervm -XX:-CompactStrings StringIndexOfChar
+ */
+public class StringIndexOfChar {
+    public static void main(String[] args) throws Exception {
+        String emptyString = "";
+        for (int i = 0; i < 100; i++) {
+            for(int c = 0; c < 0xFFFF; c++) {
+                int result = emptyString.indexOf((char)c, -1);
+                if (result != -1) {
+                    throw new Exception("new String(\"\").indexOf(char, -1) must be -1, but got " + result);
+                }
+            }
+        }
+    }
+}