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, 12 insertions, 2 deletions

diff --git a/src/mem/cache/blk.hh b/src/mem/cache/blk.hh
index 7dd0a92ae..a1e45028b 100644
--- a/src/mem/cache/blk.hh
+++ b/src/mem/cache/blk.hh
@@ -99,7 +99,7 @@ class CacheBlk
     /** The current status of this block. @sa CacheBlockStatusBits */
     State status;
 
-    /** Which curTick() will this block be accessable */
+    /** Which curTick() will this block be accessible */
     Tick whenReady;
 
     /**
@@ -108,7 +108,10 @@ class CacheBlk
      */
     int set, way;
 
-    /** whether this block has been touched */
+    /**
+      * Whether this block has been touched since simulation started.
+      * Used to calculate number of used tags.
+      */
     bool isTouched;
 
     /** Number of references to this block since it was brought in. */
@@ -117,8 +120,15 @@ class CacheBlk
     /** holds the source requestor ID for this block. */
     int srcMasterId;
 
+    /** Tick on which the block was inserted in the cache. */
     Tick tickInserted;
 
+    /**
+     * Replacement policy data. As of now it is only an update timestamp.
+     * Tick on which the block was last touched.
+     */
+    Tick lastTouchTick;
+
   protected:
     /**
      * Represents that the indicated thread context has a "lock" on