8300818: Reduce complexity of padding with DateTimeFormatter

Reviewed-by: redestad, rriggs

src/java.base/share/classes/java/time/format/DateTimeFormatterBuilder.java

@@ -2612,9 +2612,15 @@ public final class DateTimeFormatterBuilder {
                 throw new DateTimeException(
                     "Cannot print as output of " + len + " characters exceeds pad width of " + padWidth);
             }
-            for (int i = 0; i < padWidth - len; i++) {
-                buf.insert(preLen, padChar);
+            var count = padWidth - len;
+            if (count == 0) {
+                return true;
             }
+            if (count == 1) {
+                buf.insert(preLen, padChar);
+                return true;
+            }
+            buf.insert(preLen, String.valueOf(padChar).repeat(count));
             return true;
         }
 

test/micro/org/openjdk/bench/java/time/format/DateTimeFormatterWithPaddingBench.java

@@ -0,0 +1,77 @@
+/*
+ * 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.format;
+
+import org.openjdk.jmh.annotations.*;
+import org.openjdk.jmh.infra.Blackhole;
+
+import java.time.LocalDateTime;
+import java.time.format.DateTimeFormatter;
+import java.time.format.DateTimeFormatterBuilder;
+import java.time.temporal.ChronoField;
+import java.util.concurrent.TimeUnit;
+
+@BenchmarkMode(Mode.Throughput)
+@OutputTimeUnit(TimeUnit.MILLISECONDS)
+@Warmup(iterations = 5, time = 1)
+@Measurement(iterations = 5, time = 1)
+@Fork(3)
+@State(Scope.Benchmark)
+public class DateTimeFormatterWithPaddingBench {
+
+    private static final DateTimeFormatter FORMATTER_WITH_PADDING = new DateTimeFormatterBuilder()
+            .appendLiteral("Date:")
+            .padNext(20, ' ')
+            .append(DateTimeFormatter.ISO_DATE)
+            .toFormatter();
+
+    private static final DateTimeFormatter FORMATTER_WITH_PADDING_ZERO = new DateTimeFormatterBuilder()
+            .appendLiteral("Year:")
+            .padNext(4)
+            .appendValue(ChronoField.YEAR)
+            .toFormatter();
+
+    private static final DateTimeFormatter FORMATTER_WITH_PADDING_ONE = new DateTimeFormatterBuilder()
+            .appendLiteral("Year:")
+            .padNext(5)
+            .appendValue(ChronoField.YEAR)
+            .toFormatter();
+
+    private final LocalDateTime now = LocalDateTime.now();
+
+    @Benchmark
+    public String formatWithPadding() {
+        return FORMATTER_WITH_PADDING.format(now);
+    }
+
+    @Benchmark
+    public String formatWithPaddingLengthZero() {
+        return FORMATTER_WITH_PADDING_ZERO.format(now);
+    }
+
+    @Benchmark
+    public String formatWithPaddingLengthOne() {
+        return FORMATTER_WITH_PADDING_ONE.format(now);
+    }
+
+}