8319423: Improve Year.isLeap by checking divisibility by 16

Reviewed-by: naoto, rriggs

src/java.base/share/classes/java/time/Year.java

@@ -315,7 +315,10 @@ public final class Year
      * @return true if the year is leap, false otherwise
      */
     public static boolean isLeap(long year) {
-        return ((year & 3) == 0) && ((year % 100) != 0 || (year % 400) == 0);
+        // A year that is a multiple of 100, 200 and 300 is not divisible by 16, but 400 is.
+        // So for a year that's divisible by 4, checking that it's also divisible by 16
+        // is sufficient to determine it must be a leap year.
+        return (year & 15) == 0 ? (year & 3) == 0 : (year & 3) == 0 && year % 100 != 0;
     }
 
     //-----------------------------------------------------------------------

src/java.base/share/classes/java/time/chrono/IsoChronology.java

@@ -475,7 +475,7 @@ public final class IsoChronology extends AbstractChronology implements Serializa
      */
     @Override
     public boolean isLeapYear(long prolepticYear) {
-        return ((prolepticYear & 3) == 0) && ((prolepticYear % 100) != 0 || (prolepticYear % 400) == 0);
+        return Year.isLeap(prolepticYear);
     }
 
     @Override

src/java.base/share/classes/java/util/GregorianCalendar.java

@@ -826,7 +826,10 @@ public class GregorianCalendar extends Calendar {
         }
 
         if (year > gregorianCutoverYear) {
-            return (year%100 != 0) || (year%400 == 0); // Gregorian
+            // A multiple of 100, 200 and 300 is not divisible by 16, but 400 is.
+            // So for a year that's divisible by 4, checking that it's also divisible by 16
+            // is sufficient to determine it must be a leap year.
+            return (year & 15) == 0 || (year % 100 != 0); // Gregorian
         }
         if (year < gregorianCutoverYearJulian) {
             return true; // Julian
@@ -840,7 +843,7 @@ public class GregorianCalendar extends Calendar {
         } else {
             gregorian = year == gregorianCutoverYear;
         }
-        return gregorian ? (year%100 != 0) || (year%400 == 0) : true;
+        return !gregorian || (year & 15) == 0 || (year % 100 != 0);
     }
 
     /**

src/java.base/share/classes/sun/util/calendar/CalendarUtils.java

@@ -40,8 +40,12 @@ public final class CalendarUtils {
      * @see CalendarDate#isLeapYear
      */
     public static boolean isGregorianLeapYear(int gregorianYear) {
-        return (((gregorianYear % 4) == 0) && (((gregorianYear % 100) != 0)
+        // A year that is a multiple of 100, 200 and 300 is not divisible by 16, but 400 is.
-                || ((gregorianYear % 400) == 0)));
+        // So for a year that's divisible by 4, checking that it's also divisible by 16
+        // is sufficient to determine it must be a leap year.
+        return (gregorianYear & 15) == 0
+                ? (gregorianYear & 3) == 0
+                : (gregorianYear & 3) == 0 && gregorianYear % 100 != 0;
     }
 
     /**

src/java.base/share/classes/sun/util/calendar/ZoneInfoFile.java

@@ -929,7 +929,7 @@ public final class ZoneInfoFile {
         }
 
         static final boolean isLeapYear(int year) {
-            return ((year & 3) == 0) && ((year % 100) != 0 || (year % 400) == 0);
+            return CalendarUtils.isGregorianLeapYear(year);
         }
 
         static final int lengthOfMonth(int year, int month) {

test/micro/org/openjdk/bench/java/time/LeapYearBench.java

@@ -0,0 +1,108 @@
+/*
+ * Copyright (c) 2023, 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.time;
+
+import org.openjdk.jmh.annotations.Benchmark;
+import org.openjdk.jmh.annotations.BenchmarkMode;
+import org.openjdk.jmh.annotations.Fork;
+import org.openjdk.jmh.annotations.Measurement;
+import org.openjdk.jmh.annotations.Mode;
+import org.openjdk.jmh.annotations.OutputTimeUnit;
+import org.openjdk.jmh.annotations.Scope;
+import org.openjdk.jmh.annotations.Setup;
+import org.openjdk.jmh.annotations.State;
+import org.openjdk.jmh.annotations.Warmup;
+import org.openjdk.jmh.infra.Blackhole;
+
+import java.time.Duration;
+import java.time.Instant;
+import java.time.Year;
+import java.time.ZonedDateTime;
+import java.time.chrono.IsoChronology;
+import java.time.temporal.ChronoUnit;
+import java.util.GregorianCalendar;
+import java.util.Random;
+import java.util.concurrent.TimeUnit;
+import java.util.stream.IntStream;
+
+/**
+ * Examine Year.leapYear-related operations
+ */
+@BenchmarkMode(Mode.Throughput)
+@OutputTimeUnit(TimeUnit.MICROSECONDS)
+@Warmup(iterations = 5, time = 1)
+@Measurement(iterations = 5, time = 1)
+@Fork(3)
+@State(Scope.Thread)
+public class LeapYearBench {
+
+    private long[] years;
+
+    private GregorianCalendar calendar;
+
+    @Setup
+    public void createInstants() {
+        // Large enough number of years to guarantee that the distribution of
+        // leap years is reasonably realistic
+        years = new long[2048];
+        final Random random = new Random(0);
+        for (int i = 0; i < years.length; i++) {
+            years[i] = random.nextLong(2000) + 2000;
+        }
+        calendar = GregorianCalendar.from(ZonedDateTime.now());
+    }
+
+    @Benchmark
+    public void isLeapYear(Blackhole bh) {
+        for (long year : years) {
+            bh.consume(Year.isLeap(year));
+        }
+    }
+
+    @Benchmark
+    public void isLeapYearChrono(Blackhole bh) {
+        for (long year : years) {
+            bh.consume(IsoChronology.INSTANCE.isLeapYear(year));
+        }
+    }
+
+    @Benchmark
+    public void isLeapYearGregorian(Blackhole bh) {
+        for (long year : years) {
+            bh.consume(calendar.isLeapYear((int)year));
+        }
+    }
+
+    public static boolean isLeapNeriSchneider(long year) {
+        int d = year % 100 != 0 ? 4 : 16;
+        return (year & (d - 1)) == 0;
+    }
+
+    @Benchmark
+    public void isLeapYearNS(Blackhole bh) {
+        for (long year : years) {
+            bh.consume(isLeapNeriSchneider(year));
+        }
+    }
+}