From 405383fd2f9757cebd175c5be9cfa65c48fe15f0 Mon Sep 17 00:00:00 2001
From: Andreas Kling <awesomekling@gmail.com>
Date: Mon, 3 Dec 2018 02:24:11 +0100
Subject: [PATCH] Refactor the FIFO implementation to use a DoubleBuffer as
 backing store.

This is considerably more efficient than using a CircularQueue.
---
 Kernel/FIFO.cpp | 18 +++++-------------
 Kernel/FIFO.h   |  4 ++--
 2 files changed, 7 insertions(+), 15 deletions(-)

diff --git a/Kernel/FIFO.cpp b/Kernel/FIFO.cpp
index dbf7c8a0c14..14589d4c10b 100644
--- a/Kernel/FIFO.cpp
+++ b/Kernel/FIFO.cpp
@@ -46,27 +46,22 @@ void FIFO::close(Direction direction)
 
 bool FIFO::can_read() const
 {
-    return !m_queue.is_empty() || !m_writers;
+    return !m_buffer.is_empty() || !m_writers;
 }
 
 bool FIFO::can_write() const
 {
-#ifdef FIFO_DEBUG
-    dbgprintf("can_write? size(%u) < capacity(%u) || !readers(%u)\n", m_queue.size(), m_queue.capacity(), m_readers);
-#endif
-    return m_queue.size() < m_queue.capacity() || !m_readers;
+    return true;
 }
 
 ssize_t FIFO::read(byte* buffer, size_t size)
 {
-    if (!m_writers && m_queue.is_empty())
+    if (!m_writers && m_buffer.is_empty())
         return 0;
 #ifdef FIFO_DEBUG
     dbgprintf("fifo: read(%u)\n",size);
 #endif
-    size_t nread = min(size, m_queue.size());
-    for (size_t i = 0; i < nread; ++i)
-        buffer[i] = m_queue.dequeue();
+    size_t nread = m_buffer.read(buffer, size);
 #ifdef FIFO_DEBUG
     dbgprintf("   -> read (%c) %u\n", buffer[0], nread);
 #endif
@@ -80,8 +75,5 @@ ssize_t FIFO::write(const byte* buffer, size_t size)
 #ifdef FIFO_DEBUG
     dbgprintf("fifo: write(%p, %u)\n", buffer, size);
 #endif
-    size_t nwritten = min(size, m_queue.capacity() - m_queue.size());
-    for (size_t i = 0; i < nwritten; ++i)
-        m_queue.enqueue(buffer[i]);
-    return nwritten;
+    return m_buffer.write(buffer, size);
 }
diff --git a/Kernel/FIFO.h b/Kernel/FIFO.h
index 91392cc5aa8..30a5bd33df2 100644
--- a/Kernel/FIFO.h
+++ b/Kernel/FIFO.h
@@ -1,6 +1,6 @@
 #pragma once
 
-#include <AK/CircularQueue.h>
+#include "DoubleBuffer.h"
 #include <AK/Retainable.h>
 #include <AK/RetainPtr.h>
 #include <VirtualFileSystem/UnixTypes.h>
@@ -27,5 +27,5 @@ private:
 
     unsigned m_writers { 0 };
     unsigned m_readers { 0 };
-    CircularQueue<byte, 16> m_queue;
+    DoubleBuffer m_buffer;
 };