HttpObjectDecoder performance improvements

Motivation:
The HttpObjectDecoder is on the hot code path for the http codec. There are a few hot methods which can be modified to improve performance.

Modifications:
- Modify AppendableCharSequence to provide unsafe methods which don't need to re-check bounds for every call.
- Update HttpObjectDecoder methods to take advantage of new AppendableCharSequence methods.

Result:
Peformance boost for decoding http objects.

codec-http/src/main/java/io/netty/handler/codec/http/HttpObjectDecoder.java

@@ -666,9 +666,9 @@ public abstract class HttpObjectDecoder extends ByteToMessageDecoder {
         cEnd = findEndOfString(sb);
 
         return new String[] {
-                sb.substring(aStart, aEnd),
-                sb.substring(bStart, bEnd),
-                cStart < cEnd? sb.substring(cStart, cEnd) : "" };
+                sb.subStringUnsafe(aStart, aEnd),
+                sb.subStringUnsafe(bStart, bEnd),
+                cStart < cEnd? sb.subStringUnsafe(cStart, cEnd) : "" };
     }
 
     private void splitHeader(AppendableCharSequence sb) {
@@ -694,44 +694,41 @@ public abstract class HttpObjectDecoder extends ByteToMessageDecoder {
             }
         }
 
-        name = sb.substring(nameStart, nameEnd);
+        name = sb.subStringUnsafe(nameStart, nameEnd);
         valueStart = findNonWhitespace(sb, colonEnd);
         if (valueStart == length) {
             value = EMPTY_VALUE;
         } else {
             valueEnd = findEndOfString(sb);
-            value = sb.substring(valueStart, valueEnd);
+            value = sb.subStringUnsafe(valueStart, valueEnd);
         }
     }
 
-    private static int findNonWhitespace(CharSequence sb, int offset) {
-        int result;
-        for (result = offset; result < sb.length(); result ++) {
-            if (!Character.isWhitespace(sb.charAt(result))) {
-                break;
+    private static int findNonWhitespace(AppendableCharSequence sb, int offset) {
+        for (int result = offset; result < sb.length(); ++result) {
+            if (!Character.isWhitespace(sb.charAtUnsafe(result))) {
+                return result;
             }
         }
-        return result;
+        return sb.length();
     }
 
-    private static int findWhitespace(CharSequence sb, int offset) {
-        int result;
-        for (result = offset; result < sb.length(); result ++) {
-            if (Character.isWhitespace(sb.charAt(result))) {
-                break;
+    private static int findWhitespace(AppendableCharSequence sb, int offset) {
+        for (int result = offset; result < sb.length(); ++result) {
+            if (Character.isWhitespace(sb.charAtUnsafe(result))) {
+                return result;
             }
         }
-        return result;
+        return sb.length();
     }
 
-    private static int findEndOfString(CharSequence sb) {
-        int result;
-        for (result = sb.length(); result > 0; result --) {
-            if (!Character.isWhitespace(sb.charAt(result - 1))) {
-                break;
+    private static int findEndOfString(AppendableCharSequence sb) {
+        for (int result = sb.length() - 1; result > 0; --result) {
+            if (!Character.isWhitespace(sb.charAtUnsafe(result))) {
+                return result + 1;
             }
         }
-        return result;
+        return 0;
     }
 
     private static class HeaderParser implements ByteProcessor {

common/src/main/java/io/netty/util/internal/AppendableCharSequence.java

@@ -30,6 +30,9 @@ public final class AppendableCharSequence implements CharSequence, Appendable {
     }
 
     private AppendableCharSequence(char[] chars) {
+        if (chars.length < 1) {
+            throw new IllegalArgumentException("length: " + chars.length + " (length: >= 1)");
+        }
         this.chars = chars;
         pos = chars.length;
     }
@@ -47,6 +50,17 @@ public final class AppendableCharSequence implements CharSequence, Appendable {
         return chars[index];
     }
 
+    /**
+     * Access a value in this {@link CharSequence}.
+     * This method is considered unsafe as index values are assumed to be legitimate.
+     * Only underlying array bounds checking is done.
+     * @param index The index to access the underlying array at.
+     * @return The value at {@code index}.
+     */
+    public char charAtUnsafe(int index) {
+        return chars[index];
+    }
+
     @Override
     public AppendableCharSequence subSequence(int start, int end) {
         return new AppendableCharSequence(Arrays.copyOfRange(chars, start, end));
@@ -54,17 +68,12 @@ public final class AppendableCharSequence implements CharSequence, Appendable {
 
     @Override
     public AppendableCharSequence append(char c) {
-        if (pos == chars.length) {
-            char[] old = chars;
-            // double it
-            int len = old.length << 1;
-            if (len < 0) {
-                throw new IllegalStateException();
-            }
-            chars = new char[len];
-            System.arraycopy(old, 0, chars, 0, old.length);
+        try {
+            chars[pos++] = c;
+        } catch (IndexOutOfBoundsException e) {
+            expand();
+            chars[pos - 1] = c;
         }
-        chars[pos++] = c;
         return this;
     }
 
@@ -121,6 +130,26 @@ public final class AppendableCharSequence implements CharSequence, Appendable {
         return new String(chars, start, length);
     }
 
+    /**
+     * Create a new {@link String} from the given start to end.
+     * This method is considered unsafe as index values are assumed to be legitimate.
+     * Only underlying array bounds checking is done.
+     */
+    public String subStringUnsafe(int start, int end) {
+        return new String(chars, start, end - start);
+    }
+
+    private void expand() {
+        char[] old = chars;
+        // double it
+        int len = old.length << 1;
+        if (len < 0) {
+            throw new IllegalStateException();
+        }
+        chars = new char[len];
+        System.arraycopy(old, 0, chars, 0, old.length);
+    }
+
     private static char[] expand(char[] array, int neededSpace, int size) {
         int newCapacity = array.length;
         do {

microbench/src/main/java/io/netty/microbenchmark/common/AppendableCharSequenceBenchmark.java

@@ -0,0 +1,87 @@
+/*
+ * Copyright 2015 The Netty Project
+ *
+ * The Netty Project licenses this file to you under the Apache License, version 2.0 (the
+ * "License"); you may not use this file except in compliance with the License. You may obtain a
+ * copy of the License at:
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software distributed under the License
+ * is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express
+ * or implied. See the License for the specific language governing permissions and limitations under
+ * the License.
+ */
+package io.netty.microbenchmark.common;
+
+import java.util.Random;
+
+import org.openjdk.jmh.annotations.Benchmark;
+import org.openjdk.jmh.annotations.Level;
+import org.openjdk.jmh.annotations.Measurement;
+import org.openjdk.jmh.annotations.Param;
+import org.openjdk.jmh.annotations.Setup;
+import org.openjdk.jmh.annotations.Threads;
+import org.openjdk.jmh.annotations.Warmup;
+
+import io.netty.microbench.util.AbstractMicrobenchmark;
+
+@Threads(1)
+@Warmup(iterations = 5)
+@Measurement(iterations = 5)
+public class AppendableCharSequenceBenchmark extends AbstractMicrobenchmark {
+    @Param({ "32", "64", "128", "256" })
+    private int charsInitSize;
+
+    @Param({ "10", "100", "10000", "1000000" })
+    private int simulatedDataSize;
+
+    private static final Random rand = new Random();
+    private char[] chars;
+    private char simulatedData;
+    private int pos;
+
+    @Setup(Level.Trial)
+    public void setup() {
+        chars = new char[charsInitSize];
+        simulatedData = (char) rand.nextInt();
+    }
+
+    @Benchmark
+    public void appendCheckBeforeCopy() {
+        checkReset();
+        if (pos == chars.length) {
+            expand();
+        }
+        chars[pos++] = simulatedData;
+    }
+
+    @Benchmark
+    public void appendCatchExceptionAfter() {
+        checkReset();
+        try {
+            chars[pos++] = simulatedData;
+        } catch (IndexOutOfBoundsException e) {
+            expand();
+            chars[pos - 1] = simulatedData;
+        }
+    }
+
+    private void checkReset() {
+        if (pos == simulatedDataSize) {
+            pos = 0;
+            chars = new char[charsInitSize];
+        }
+    }
+
+    private void expand() {
+        char[] old = chars;
+        // double it
+        int len = old.length << 1;
+        if (len < 0) {
+            throw new IllegalStateException();
+        }
+        chars = new char[len];
+        System.arraycopy(old, 0, chars, 0, old.length);
+    }
+}