mem: Create a separate class for the cache write buffer

This patch breaks out the cache write buffer into a separate class,
without affecting any stats. The goal of the patch is to avoid
encumbering the much-simpler write queue with the complex MSHR
handling. In a follow on patch this simplification allows us to
implement write combining.

The WriteQueue gets its own class, but shares a common ancestor, the
generic Queue, with the MSHRQueue.

src/mem/cache/write_queue.cc

@@ -0,0 +1,91 @@
+/*
+ * Copyright (c) 2012-2013, 2015-2016 ARM Limited
+ * All rights reserved.
+ *
+ * The license below extends only to copyright in the software and shall
+ * not be construed as granting a license to any other intellectual
+ * property including but not limited to intellectual property relating
+ * to a hardware implementation of the functionality of the software
+ * licensed hereunder.  You may use the software subject to the license
+ * terms below provided that you ensure that this notice is replicated
+ * unmodified and in its entirety in all distributions of the software,
+ * modified or unmodified, in source code or in binary form.
+ *
+ * Copyright (c) 2003-2005 The Regents of The University of Michigan
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are
+ * met: redistributions of source code must retain the above copyright
+ * notice, this list of conditions and the following disclaimer;
+ * redistributions in binary form must reproduce the above copyright
+ * notice, this list of conditions and the following disclaimer in the
+ * documentation and/or other materials provided with the distribution;
+ * neither the name of the copyright holders nor the names of its
+ * contributors may be used to endorse or promote products derived from
+ * this software without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+ * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+ * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+ * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+ * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+ * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+ * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ *
+ * Authors: Erik Hallnor
+ *          Andreas Sandberg
+ *          Andreas Hansson
+ */
+
+/** @file
+ * Definition of WriteQueue class functions.
+ */
+
+#include "mem/cache/write_queue.hh"
+
+using namespace std;
+
+WriteQueue::WriteQueue(const std::string &_label,
+                       int num_entries, int reserve)
+    : Queue<WriteQueueEntry>(_label, num_entries, reserve)
+{}
+
+WriteQueueEntry *
+WriteQueue::allocate(Addr blk_addr, unsigned blk_size, PacketPtr pkt,
+                    Tick when_ready, Counter order)
+{
+    assert(!freeList.empty());
+    WriteQueueEntry *entry = freeList.front();
+    assert(entry->getNumTargets() == 0);
+    freeList.pop_front();
+
+    entry->allocate(blk_addr, blk_size, pkt, when_ready, order);
+    entry->allocIter = allocatedList.insert(allocatedList.end(), entry);
+    entry->readyIter = addToReadyList(entry);
+
+    allocated += 1;
+    return entry;
+}
+
+void
+WriteQueue::markInService(WriteQueueEntry *entry)
+{
+    if (!entry->isUncacheable()) {
+        // a normal eviction, such as a writeback or a clean evict, no
+        // more to do as we are done from the perspective of this
+        // cache
+        entry->popTarget();
+        deallocate(entry);
+    } else {
+        // uncacheable write, and we will eventually receive a
+        // response
+        entry->markInService();
+        readyList.erase(entry->readyIter);
+        _numInService += 1;
+    }
+}