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, 2 insertions, 0 deletions

diff --git a/src/mem/cache/cache.cc b/src/mem/cache/cache.cc
index 9ee935961..cbc0ed90a 100644
--- a/src/mem/cache/cache.cc
+++ b/src/mem/cache/cache.cc
@@ -1816,6 +1816,7 @@ Cache::invalidateVisitor(CacheBlk &blk)
 CacheBlk*
 Cache::allocateBlock(Addr addr, bool is_secure, PacketList &writebacks)
 {
+    // Find replacement victim
     CacheBlk *blk = tags->findVictim(addr);
 
     // It is valid to return nullptr if there is no victim
@@ -2802,6 +2803,7 @@ Cache*
 CacheParams::create()
 {
     assert(tags);
+    assert(replacement_policy);
 
     return new Cache(this);
 }