Enhance TestTrampoline for Windows UTF-16 handling

test/hotspot/jtreg/compiler/c2/aarch64/TestTrampoline.java

@@ -24,6 +24,7 @@
 
 package compiler.c2.aarch64;
 
+import java.nio.charset.Charset;
 import java.util.ArrayList;
 import java.util.Iterator;
 import jdk.test.lib.process.OutputAnalyzer;
@@ -96,6 +97,26 @@ private static void test(String s, int i) {
 
         public static void main(String[] args) {
             String s = "Returns the char value at the specified index.";
+
+            boolean isWindows = System.getProperty("os.name").startsWith("Windows");
+            if (isWindows) {
+                /*
+                Depending on the Windows code page, the UTF-16 path in the String.charAt method
+                may be executed during JVM startup. However, it may not execute often enough to
+                cause it to be inlined by C2. In this case, C2 may not fully inline the
+                String.charAt method, resulting in the unexpected generation of trampolines.
+                Therefore, we execute the UTF-16 path in the String.charAt method in a loop
+                enough times to ensure that it will be inlined by C2.
+                */
+                String jnuEncoding = System.getProperty("sun.jnu.encoding");
+                if (jnuEncoding != null && Charset.isSupported(jnuEncoding)) {
+                    byte[] bytes = s.getBytes(Charset.forName(jnuEncoding));
+
+                    for (int i = 0; i < ITERATIONS_TO_HEAT_LOOP; ++i) {
+                        String encoded = new String(bytes, Charset.forName(jnuEncoding));
+                    }
+                }
+            }
             for (int i = 0; i < ITERATIONS_TO_HEAT_LOOP; ++i) {
                 test(s, i % s.length());
             }