Speedup RecyclerBytesStreamOutput.writeString (#113241)

This method is quite hot in some use-cases because it's used by
most string writing to transport messages. Overriding teh default
implementation for cases where we can write straight to the
page instead of going through an intermediary buffer speeds up
the method by more than 2x, saving lots of cycles, especially
on transport threads.

Author: original-brownbear

server/src/main/java/org/elasticsearch/common/io/stream/RecyclerBytesStreamOutput.java

@@ -177,6 +177,38 @@ public void writeWithSizePrefix(Writeable writeable) throws IOException {
         }
     }
 
+    // overridden with some code duplication the same way other write methods in this class are overridden to bypass StreamOutput's
+    // intermediary buffers
+    @Override
+    public void writeString(String str) throws IOException {
+        final int currentPageOffset = this.currentPageOffset;
+        final int charCount = str.length();
+        // maximum serialized length is 3 bytes per char + 5 bytes for the longest possible vint
+        if (charCount * 3 + 5 > (pageSize - currentPageOffset)) {
+            super.writeString(str);
+            return;
+        }
+        BytesRef currentPage = pages.get(pageIndex).v();
+        int off = currentPage.offset + currentPageOffset;
+        byte[] buffer = currentPage.bytes;
+        // mostly duplicated from StreamOutput.writeString to to get more reliable compilation of this very hot loop
+        int offset = off + putVInt(buffer, charCount, off);
+        for (int i = 0; i < charCount; i++) {
+            final int c = str.charAt(i);
+            if (c <= 0x007F) {
+                buffer[offset++] = ((byte) c);
+            } else if (c > 0x07FF) {
+                buffer[offset++] = ((byte) (0xE0 | c >> 12 & 0x0F));
+                buffer[offset++] = ((byte) (0x80 | c >> 6 & 0x3F));
+                buffer[offset++] = ((byte) (0x80 | c >> 0 & 0x3F));
+            } else {
+                buffer[offset++] = ((byte) (0xC0 | c >> 6 & 0x1F));
+                buffer[offset++] = ((byte) (0x80 | c >> 0 & 0x3F));
+            }
+        }
+        this.currentPageOffset = offset - currentPage.offset;
+    }
+
     @Override
     public void flush() {
         // nothing to do