From d207e9ccee411877fdeac80bb68a27900560f50f Mon Sep 17 00:00:00 2001
From: "Daniel R. Carvalho" <odanrc@yahoo.com.br>
Date: Mon, 19 Feb 2018 15:13:11 +0100
Subject: mem-cache: Split array indexing and replacement policies.

Replacement policies (LRU, Random) are currently considered as array
indexing methods, but have completely different functionalities:

- Array indexers determine the possible locations for block allocation.
  This information is used to generate replacement candidates when
  conflicts happen.
- Replacement policies determine which of the replacement candidates
  should be evicted to make room for new allocations.

For this reason, they were split into different classes. Advantages:

- Easier and more straightforward to implement other replacement
  policies (RRIP, LFU, ARC, ...)
- Allow easier future implementation of cache organization schemes

As now we can't assure the use of sets, the previous way to create a
true LRU is not viable. Now a timestamp_bits parameter controls how
many bits are dedicated for the timestamp, and a true LRU can be
achieved through an infinite number of bits (although a few bits suffice
in practice).

Change-Id: I23750db121f1474d17831137e6ff618beb2b3eda
Reviewed-on: https://gem5-review.googlesource.com/8501
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Nikos Nikoleris <nikos.nikoleris@arm.com>
---
 src/mem/cache/replacement_policies/lru_rp.cc | 87 ++++++++++++++++++++++++++++
 1 file changed, 87 insertions(+)
 create mode 100644 src/mem/cache/replacement_policies/lru_rp.cc

(limited to 'src/mem/cache/replacement_policies/lru_rp.cc')

diff --git a/src/mem/cache/replacement_policies/lru_rp.cc b/src/mem/cache/replacement_policies/lru_rp.cc
new file mode 100644
index 000000000..b2fa20b6f
--- /dev/null
+++ b/src/mem/cache/replacement_policies/lru_rp.cc
@@ -0,0 +1,87 @@
+/**
+ * Copyright (c) 2018 Inria
+ * 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: Daniel Carvalho
+ */
+
+#include "mem/cache/replacement_policies/lru_rp.hh"
+
+#include "debug/CacheRepl.hh"
+
+LRURP::LRURP(const Params *p)
+    : BaseReplacementPolicy(p)
+{
+}
+
+void
+LRURP::touch(CacheBlk *blk)
+{
+    BaseReplacementPolicy::touch(blk);
+
+    // Update last touch timestamp
+    blk->lastTouchTick = curTick();
+}
+
+void
+LRURP::reset(CacheBlk *blk)
+{
+    BaseReplacementPolicy::reset(blk);
+
+    // Set last touch timestamp
+    blk->lastTouchTick = blk->tickInserted;
+}
+
+CacheBlk*
+LRURP::getVictim(const ReplacementCandidates& candidates)
+{
+    // There must be at least one replacement candidate
+    assert(candidates.size() > 0);
+
+    // Visit all candidates to find victim
+    CacheBlk* blk = candidates[0];
+    for (const auto& candidate : candidates) {
+        // Stop iteration if found an invalid block
+        if (!candidate->isValid()) {
+            blk = candidate;
+            break;
+        // Update victim block if necessary
+        } else if (candidate->lastTouchTick < blk->lastTouchTick) {
+            blk = candidate;
+        }
+    }
+
+    DPRINTF(CacheRepl, "set %x, way %x: selecting blk for replacement\n",
+            blk->set, blk->way);
+
+    return blk;
+}
+
+LRURP*
+LRURPParams::create()
+{
+    return new LRURP(this);
+}
-- 
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