8336741: Optimize LocalTime.toString with StringBuilder.repeat

Reviewed-by: liach, rriggs, naoto

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

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2012, 2023, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2012, 2024, 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
@@ -92,6 +92,8 @@ import java.time.temporal.UnsupportedTemporalTypeException;
 import java.time.temporal.ValueRange;
 import java.util.Objects;
 
+import jdk.internal.util.DecimalDigits;
+
 /**
  * A time without a time-zone in the ISO-8601 calendar system,
  * such as {@code 10:15:30}.
@@ -1640,13 +1642,19 @@ public final class LocalTime
             buf.append(secondValue < 10 ? ":0" : ":").append(secondValue);
             if (nanoValue > 0) {
                 buf.append('.');
-                if (nanoValue % 1000_000 == 0) {
-                    buf.append(Integer.toString((nanoValue / 1000_000) + 1000).substring(1));
-                } else if (nanoValue % 1000 == 0) {
-                    buf.append(Integer.toString((nanoValue / 1000) + 1000_000).substring(1));
-                } else {
-                    buf.append(Integer.toString((nanoValue) + 1000_000_000).substring(1));
+                int zeros = 9 - DecimalDigits.stringSize(nanoValue);
+                if (zeros > 0) {
+                    buf.repeat('0', zeros);
                 }
+                int digits;
+                if (nanoValue % 1_000_000 == 0) {
+                    digits = nanoValue / 1_000_000;
+                } else if (nanoValue % 1000 == 0) {
+                    digits = nanoValue / 1000;
+                } else {
+                    digits = nanoValue;
+                }
+                buf.append(digits);
             }
         }
         return buf.toString();

src/java.base/share/classes/jdk/internal/util/DecimalDigits.java

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2023, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2024, 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
@@ -84,4 +84,30 @@ public final class DecimalDigits {
     public static short digitPair(int i) {
         return DIGITS[i];
     }
+
+    /**
+     * Returns the string representation size for a given int value.
+     *
+     * @param x int value
+     * @return string size
+     *
+     * @implNote There are other ways to compute this: e.g. binary search,
+     * but values are biased heavily towards zero, and therefore linear search
+     * wins. The iteration results are also routinely inlined in the generated
+     * code after loop unrolling.
+     */
+    public static int stringSize(int x) {
+        int d = 1;
+        if (x >= 0) {
+            d = 0;
+            x = -x;
+        }
+        int p = -10;
+        for (int i = 1; i < 10; i++) {
+            if (x > p)
+                return i + d;
+            p = 10 * p;
+        }
+        return 10 + d;
+    }
 }

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

@@ -0,0 +1,124 @@
+/*
+ * Copyright (c) 2024, Alibaba Group Holding Limited. 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 java.time.Duration;
+import java.time.Instant;
+import java.time.LocalDate;
+import java.time.LocalDateTime;
+import java.time.LocalTime;
+import java.time.ZonedDateTime;
+import java.time.ZoneOffset;
+import java.time.format.DateTimeFormatter;
+import java.time.temporal.ChronoUnit;
+
+import java.util.Locale;
+import java.util.Random;
+import java.util.concurrent.TimeUnit;
+import java.util.stream.IntStream;
+import java.util.stream.Stream;
+
+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.Param;
+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;
+
+@BenchmarkMode(Mode.Throughput)
+@OutputTimeUnit(TimeUnit.MILLISECONDS)
+@Warmup(iterations = 5, time = 1)
+@Measurement(iterations = 5, time = 1)
+@Fork(3)
+@State(Scope.Thread)
+public class ToStringBench {
+    private static final Instant[] INSTANTS;
+    private static final ZonedDateTime[] ZONED_DATE_TIMES;
+    private static final LocalDateTime[] LOCAL_DATE_TIMES;
+    private static final LocalDate[] LOCAL_DATES;
+    private static final LocalTime[] LOCAL_TIMES;
+
+    static {
+        Instant loInstant = Instant.EPOCH.plus(Duration.ofDays(365*50)); // 2020-01-01
+        Instant hiInstant = loInstant.plus(Duration.ofDays(1));
+        long maxOffsetNanos = Duration.between(loInstant, hiInstant).toNanos();
+        Random random = new Random(0);
+        INSTANTS = IntStream
+                .range(0, 1_000)
+                .mapToObj(ignored -> {
+                    final long offsetNanos = (long) Math.floor(random.nextDouble() * maxOffsetNanos);
+                    return loInstant.plus(offsetNanos, ChronoUnit.NANOS);
+                })
+                .toArray(Instant[]::new);
+
+        ZONED_DATE_TIMES = Stream.of(INSTANTS)
+                .map(instant -> ZonedDateTime.ofInstant(instant, ZoneOffset.UTC))
+                .toArray(ZonedDateTime[]::new);
+
+        LOCAL_DATE_TIMES = Stream.of(ZONED_DATE_TIMES)
+                .map(zdt -> zdt.toLocalDateTime())
+                .toArray(LocalDateTime[]::new);
+
+        LOCAL_DATES = Stream.of(LOCAL_DATE_TIMES)
+                .map(ldt -> ldt.toLocalDate())
+                .toArray(LocalDate[]::new);
+
+        LOCAL_TIMES = Stream.of(LOCAL_DATE_TIMES)
+                .map(ldt -> ldt.toLocalTime())
+                .toArray(LocalTime[]::new);
+    }
+
+    @Benchmark
+    public void zonedDateTimeToString(Blackhole bh) {
+        for (final ZonedDateTime zonedDateTime : ZONED_DATE_TIMES) {
+            bh.consume(zonedDateTime.toString());
+        }
+    }
+
+    @Benchmark
+    public void localDateTimeToString(Blackhole bh) {
+        for (LocalDateTime localDateTime : LOCAL_DATE_TIMES) {
+            bh.consume(localDateTime.toString());
+        }
+    }
+
+    @Benchmark
+    public void localDateToString(Blackhole bh) {
+        for (LocalDate localDate : LOCAL_DATES) {
+            bh.consume(localDate.toString());
+        }
+    }
+
+    @Benchmark
+    public void localTimeToString(Blackhole bh) {
+        for (LocalTime localTime : LOCAL_TIMES) {
+            bh.consume(localTime.toString());
+        }
+    }
+}