mem: cache: tags: Introduce NMRU Cache Replacement Policy

Introduce NMRU Cache Replacement Policy

Reference implementation is here
http://pages.cs.wisc.edu/~david/courses/cs752/Fall2015/gem5-tutorial/part2/simobject.html

Note that the reference implementation is outdated and does not
build/work with the current gem5 branch. This commit modifies the
above example to make it work with the current gem5 branch.

src/mem/cache/tags/SConscript

@@ -37,3 +37,4 @@ Source('base_set_assoc.cc')
 Source('lru.cc')
 Source('random_repl.cc')
 Source('fa_lru.cc')
+Source('nmru.cc')

src/mem/cache/tags/Tags.py

@@ -80,3 +80,8 @@ class FALRU(BaseTags):
     type = 'FALRU'
     cxx_class = 'FALRU'
     cxx_header = "mem/cache/tags/fa_lru.hh"
+
+class NMRU(BaseSetAssoc):
+    type = 'NMRU'
+    cxx_class = 'NMRU'
+    cxx_header = "mem/cache/tags/nmru.hh"

src/mem/cache/tags/nmru.cc

@@ -0,0 +1,79 @@
+/**
+ * @file
+ * Definitions of a NMRU tag store.
+ */
+
+#include "mem/cache/tags/nmru.hh"
+
+#include "base/random.hh"
+#include "debug/CacheRepl.hh"
+#include "mem/cache/base.hh"
+
+NMRU::NMRU(const Params *p) : BaseSetAssoc(p)
+{
+
+}
+
+BaseSetAssoc::BlkType*
+NMRU::accessBlock(Addr addr, bool is_secure, Cycles &lat, int master_id)
+{
+    // Accesses are based on parent class, no need to do anything special
+    BlkType *blk = BaseSetAssoc::accessBlock(addr, is_secure, lat, master_id);
+
+    if (blk != NULL) {
+        // move this block to head of the MRU list
+        sets[blk->set].moveToHead(blk);
+        DPRINTF(CacheRepl, "set %x: moving blk %x (%s) to MRU\n",
+        blk->set, regenerateBlkAddr(blk->tag, blk->set),
+        is_secure ? "s" : "ns");
+    }
+
+    return blk;
+}
+
+BaseSetAssoc::BlkType*
+NMRU::findVictim(Addr addr)
+{
+    BlkType *blk = BaseSetAssoc::findVictim(addr);
+
+    // if all blocks are valid, pick a replacement that is not MRU at random
+    if (blk->isValid()) {
+        // find a random index within the bounds of the set
+        int idx = random_mt.random<int>(1, assoc - 1);
+        assert(idx < assoc);
+        assert(idx >= 0);
+        blk = sets[extractSet(addr)].blks[idx];
+
+        DPRINTF(CacheRepl,
+                "set %x: selecting blk %x for replacement\n",
+                blk->set,
+                regenerateBlkAddr(blk->tag, blk->set));
+    }
+
+    return blk;
+}
+
+void
+NMRU::insertBlock(PacketPtr pkt, BlkType *blk)
+{
+    BaseSetAssoc::insertBlock(pkt, blk);
+
+    int set = extractSet(pkt->getAddr());
+    sets[set].moveToHead(blk);
+}
+
+void
+NMRU::invalidate(BlkType *blk)
+{
+    BaseSetAssoc::invalidate(blk);
+
+    // should be evicted before valid blocks
+    int set = blk->set;
+    sets[set].moveToTail(blk);
+}
+
+NMRU*
+NMRUParams::create()
+{
+    return new NMRU(this);
+}

src/mem/cache/tags/nmru.hh

@@ -0,0 +1,33 @@
+#ifndef __MEM_CACHE_TAGS_NMRU_HH__
+#define __MEM_CACHE_TAGS_NMRU_HH__
+
+#include "mem/cache/tags/base_set_assoc.hh"
+#include "params/NMRU.hh"
+
+class NMRU : public BaseSetAssoc
+{
+public:
+    /** Convenience typedef. */
+    typedef NMRUParams Params;
+
+    /**
+     * Construct and initialize this tag store.
+     */
+    NMRU(const Params *p);
+
+    /**
+     * Destructor
+     */
+    ~NMRU() {}
+
+    /**
+      * Required functions for this subclass to implement
+      */
+    BlkType* accessBlock(Addr addr, bool is_secure, Cycles &lat,
+                        int context_src);
+    BlkType* findVictim(Addr addr);
+    void insertBlock(PacketPtr pkt, BlkType *blk);
+    void invalidate(BlkType *blk);
+};
+
+#endif // __MEM_CACHE_TAGS_NMRU_HH__