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, 9 insertions, 2 deletions

diff --git a/src/mem/cache/tags/base.hh b/src/mem/cache/tags/base.hh
index 9e7242c1b..74fc7e0d0 100644
--- a/src/mem/cache/tags/base.hh
+++ b/src/mem/cache/tags/base.hh
@@ -54,6 +54,7 @@
 #include "base/callback.hh"
 #include "base/statistics.hh"
 #include "mem/cache/blk.hh"
+#include "mem/cache/replacement_policies/base.hh"
 #include "params/BaseTags.hh"
 #include "sim/clocked_object.hh"
 
@@ -249,6 +250,14 @@ class BaseTags : public ClockedObject
         occupancies[blk->srcMasterId]--;
     }
 
+    /**
+     * Find replacement victim based on address.
+     *
+     * @param addr Address to find a victim for.
+     * @return Cache block to be replaced.
+     */
+    virtual CacheBlk* findVictim(Addr addr) = 0;
+
     virtual CacheBlk* accessBlock(Addr addr, bool is_secure, Cycles &lat) = 0;
 
     virtual Addr extractTag(Addr addr) const = 0;
@@ -263,8 +272,6 @@ class BaseTags : public ClockedObject
      */
     virtual Addr regenerateBlkAddr(const CacheBlk* blk) const = 0;
 
-    virtual CacheBlk* findVictim(Addr addr) = 0;
-
     virtual int extractSet(Addr addr) const = 0;
 
     virtual void forEachBlk(CacheBlkVisitor &visitor) = 0;