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, 3 insertions, 10 deletions

diff --git a/src/mem/cache/tags/Tags.py b/src/mem/cache/tags/Tags.py
index b19010f52..ff4eff44e 100644
--- a/src/mem/cache/tags/Tags.py
+++ b/src/mem/cache/tags/Tags.py
@@ -66,19 +66,12 @@ class BaseTags(ClockedObject):
 
 class BaseSetAssoc(BaseTags):
     type = 'BaseSetAssoc'
-    abstract = True
     cxx_header = "mem/cache/tags/base_set_assoc.hh"
     assoc = Param.Int(Parent.assoc, "associativity")
 
-class LRU(BaseSetAssoc):
-    type = 'LRU'
-    cxx_class = 'LRU'
-    cxx_header = "mem/cache/tags/lru.hh"
-
-class RandomRepl(BaseSetAssoc):
-    type = 'RandomRepl'
-    cxx_class = 'RandomRepl'
-    cxx_header = "mem/cache/tags/random_repl.hh"
+    # Get replacement policy from the parent (cache)
+    replacement_policy = Param.BaseReplacementPolicy(
+        Parent.replacement_policy, "Replacement policy")
 
 class FALRU(BaseTags):
     type = 'FALRU'