RFR: 8179527: Ineffective use of volatile hurts performance of Charset.atBugLevel()

[Kazunori Ogata]

Hi all, please review a change for JDK-8182743.

Bug report: https://bugs.openjdk.java.net/browse/JDK-8182743
Webrev: http://cr.openjdk.java.net/~horii/8179527/webrev.00/

This change removes a "volatile" qualifier of a static variable used by 
Charset.atBugLevel() because it does not improve thread-safety of the code 
and it hurts performance on PPC (and should be the same in ARM, too). 
Removing the "volatile" improved performance by 26% in a POWER8 machine 
using following micro benchmark:

------
import java.io.*;
import java.nio.ByteBuffer;
import java.nio.charset.Charset;
import java.util.ArrayList;

class ConvertTest {
    static String str;

    public static void main(String[] args) {
        byte buf[] = { 0x41 };
        Charset fromCharset = Charset.forName("iso-8859-1");

        long start = System.currentTimeMillis();

        for(long i = 0; i < 100000000; i++)
            str = new String(buf, 0, 1, fromCharset);

        long end = System.currentTimeMillis();

        System.out.println("Elapsed: "+(end-start)+" ms");
    }
}
------


Regards,
Ogata