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>

1 files changed, 84 insertions, 0 deletions

diff --git a/src/mem/cache/replacement_policies/lru_rp.hh b/src/mem/cache/replacement_policies/lru_rp.hh
new file mode 100644
index 000000000..3e5efe61f
--- /dev/null
+++ b/src/mem/cache/replacement_policies/lru_rp.hh
@@ -0,0 +1,84 @@
+/**
+ * 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
+ */
+
+/**
+ * @file
+ * Declaration of a Least Recently Used replacement policy.
+ * The victim is chosen using the timestamp. The timestamp may be true or
+ * pseudo, depending on the quantity of bits allocated for that.
+ */
+
+#ifndef __MEM_CACHE_REPLACEMENT_POLICIES_LRU_RP_HH__
+#define __MEM_CACHE_REPLACEMENT_POLICIES_LRU_RP_HH__
+
+#include "mem/cache/replacement_policies/base.hh"
+#include "params/LRURP.hh"
+
+class LRURP : public BaseReplacementPolicy
+{
+  public:
+    /** Convenience typedef. */
+    typedef LRURPParams Params;
+
+    /**
+     * Construct and initiliaze this replacement policy.
+     */
+    LRURP(const Params *p);
+
+    /**
+     * Destructor.
+     */
+    ~LRURP() {}
+
+    /**
+     * Touch a block to update its last touch tick.
+     *
+     * @param blk Cache block to be touched.
+     */
+    void touch(CacheBlk *blk);
+
+    /**
+     * Reset replacement data for a block. Used when a block is inserted.
+     * Sets its last touch tick as the current tick.
+     *
+     * @param blk Cache block to be reset.
+     */
+    void reset(CacheBlk *blk);
+
+    /**
+     * Find replacement victim using LRU timestamps.
+     *
+     * @param candidates Replacement candidates, selected by indexing policy.
+     * @return Cache block to be replaced.
+     */
+    CacheBlk* getVictim(const ReplacementCandidates& candidates) override;
+};
+
+#endif // __MEM_CACHE_REPLACEMENT_POLICIES_LRU_RP_HH__