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8263090: Avoid reading volatile fields twice in Locale.getDefault(Category) #2845

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@@ -939,29 +939,38 @@ public static Locale getDefault() {
*/
public static Locale getDefault(Locale.Category category) {
// do not synchronize this method - see 4071298
switch (category) {
case DISPLAY:
if (defaultDisplayLocale == null) {
synchronized(Locale.class) {
if (defaultDisplayLocale == null) {
defaultDisplayLocale = initDefault(category);
}
}
Objects.requireNonNull(category);
if (category == Category.DISPLAY) {
Locale loc = defaultDisplayLocale; // volatile read
if (loc == null) {
loc = getDisplayLocale();
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@mrserb mrserb Mar 6, 2021

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Just interesting how did you check that the performance difference is because of volatile read, and not because of replacing of the switch by the "if"?

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@cl4es cl4es Mar 6, 2021

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I started out with this variant, only removing the double volatile reads:

    public static Locale getDefault(Locale.Category category) {
        // do not synchronize this method - see 4071298
        Locale loc;
        switch (category) {
        case DISPLAY:
            loc = defaultDisplayLocale;
            if (loc == null) {
                synchronized(Locale.class) {
                    loc = defaultDisplayLocale;
                    if (loc == null) {
                        loc = defaultDisplayLocale = initDefault(category);
                    }
                }
            }
            return loc;
        case FORMAT:
            loc = defaultFormatLocale;
            if (loc == null) {
                synchronized(Locale.class) {
                    loc = defaultFormatLocale;
                    if (loc == null) {
                        loc = defaultFormatLocale = initDefault(category);
                    }
                }
            }
            return loc;
        default:
            assert false: "Unknown Category";
        }
        return getDefault();
    }

Scores were the same:

Benchmark                         Mode  Cnt   Score   Error  Units
LocaleDefaults.getDefault         avgt    5  10.045 ± 0.032  ns/op
LocaleDefaults.getDefaultDisplay  avgt    5  11.301 ± 0.053  ns/op
LocaleDefaults.getDefaultFormat   avgt    5  11.303 ± 0.054  ns/op

I then refactored and checked that the refactorings were performance neutral.

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@mrserb mrserb Mar 6, 2021

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And it is faster than the final version?

}
return defaultDisplayLocale;
case FORMAT:
if (defaultFormatLocale == null) {
synchronized(Locale.class) {
if (defaultFormatLocale == null) {
defaultFormatLocale = initDefault(category);
}
}
return loc;
} else {
assert category == Category.FORMAT : "Unknown category";
Locale loc = defaultFormatLocale; // volatile read
if (loc == null) {
loc = getFormatLocale();
}
return defaultFormatLocale;
default:
assert false: "Unknown Category";
return loc;
}
}

private static synchronized Locale getDisplayLocale() {
Locale loc = defaultDisplayLocale;
if (loc == null) {
loc = defaultDisplayLocale = initDefault(Category.DISPLAY);
}
return loc;
}


private static synchronized Locale getFormatLocale() {
Locale loc = defaultFormatLocale;
if (loc == null) {
loc = defaultFormatLocale = initDefault(Category.FORMAT);
}
return getDefault();
return loc;
}

private static Locale initDefault() {
@@ -0,0 +1,58 @@
/*
* Copyright (c) 2021, Oracle and/or its affiliates. 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.
*/
package org.openjdk.bench.java.util;

import org.openjdk.jmh.annotations.Benchmark;
import org.openjdk.jmh.annotations.BenchmarkMode;
import org.openjdk.jmh.annotations.Mode;
import org.openjdk.jmh.annotations.OutputTimeUnit;
import org.openjdk.jmh.annotations.Scope;
import org.openjdk.jmh.annotations.State;

import java.util.Locale;
import java.util.concurrent.TimeUnit;

/*
* This benchmark tests Locale.getDefault variants
*/
@BenchmarkMode(Mode.AverageTime)
@OutputTimeUnit(TimeUnit.NANOSECONDS)
@State(Scope.Benchmark)
public class LocaleDefaults {

@Benchmark
public Locale getDefault() {
return Locale.getDefault();
}

@Benchmark
public Locale getDefaultDisplay() {
return Locale.getDefault(Locale.Category.DISPLAY);
}

@Benchmark
public Locale getDefaultFormat() {
return Locale.getDefault(Locale.Category.FORMAT);
}
}