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, 4 insertions, 5 deletions

diff --git a/src/mem/cache/tags/fa_lru.hh b/src/mem/cache/tags/fa_lru.hh
index 559f56e28..7f3f99ae0 100644
--- a/src/mem/cache/tags/fa_lru.hh
+++ b/src/mem/cache/tags/fa_lru.hh
@@ -67,8 +67,6 @@ public:
     FALRUBlk *prev;
     /** The next block in LRU order. */
     FALRUBlk *next;
-    /** Has this block been touched? */
-    bool isTouched;
 
     /**
      * A bit mask of the sizes of cache that this block is resident in.
@@ -204,9 +202,10 @@ public:
     CacheBlk* findBlock(Addr addr, bool is_secure) const override;
 
     /**
-     * Find a replacement block for the address provided.
-     * @param pkt The request to a find a replacement candidate for.
-     * @return The block to place the replacement in.
+     * Find replacement victim based on address.
+     *
+     * @param addr Address to find a victim for.
+     * @return Cache block to be replaced.
      */
     CacheBlk* findVictim(Addr addr) override;