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, 5 insertions, 1 deletions

diff --git a/src/mem/cache/Cache.py b/src/mem/cache/Cache.py
index bac6c73e1..faee0925b 100644
--- a/src/mem/cache/Cache.py
+++ b/src/mem/cache/Cache.py
@@ -43,6 +43,7 @@ from m5.params import *
 from m5.proxy import *
 from MemObject import MemObject
 from Prefetcher import BasePrefetcher
+from ReplacementPolicies import *
 from Tags import *
 
 class BaseCache(MemObject):
@@ -74,7 +75,10 @@ class BaseCache(MemObject):
     prefetch_on_access = Param.Bool(False,
          "Notify the hardware prefetcher on every access (not just misses)")
 
-    tags = Param.BaseTags(LRU(), "Tag store (replacement policy)")
+    tags = Param.BaseTags(BaseSetAssoc(), "Tag store")
+    replacement_policy = Param.BaseReplacementPolicy(LRURP(),
+        "Replacement policy")
+
     sequential_access = Param.Bool(False,
         "Whether to access tags and data sequentially")