mem: Add write coalescing and write-no-allocate to the caches

Enable the cache to detect contiguous writes and hold on to the MSHR
long enough to allow the entire line to be written. If the whole line
is written, the MSHR will be sent out as an invalidation requests, as
it is part of a whole-line write, i.e. no-fetch-on-write.

The cache is also able to switch to a write-no-allocate policy on the
actual completion of the writes, and instead use the tempBlock and
turn the write operation into a writeback.

These policies are all well-known, and described in works such as
Jouppi, Cache Write Policies and Performance, vol 21, no 2, ACM, 1993.

Change-Id: I19792f2970b3c6798c9b2b493acdd156897284ae
Reviewed-on: https://gem5-review.googlesource.com/c/12907
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Maintainer: Nikos Nikoleris <nikos.nikoleris@arm.com>

1 files changed, 3 insertions, 2 deletions

diff --git a/src/mem/cache/cache.cc b/src/mem/cache/cache.cc
index d38680eb8..fb0eb9d05 100644
--- a/src/mem/cache/cache.cc
+++ b/src/mem/cache/cache.cc
@@ -623,8 +623,9 @@ Cache::handleAtomicReqMiss(PacketPtr pkt, CacheBlk *&blk,
 
                 // write-line request to the cache that promoted
                 // the write to a whole line
-                blk = handleFill(bus_pkt, blk, writebacks,
-                                 allocOnFill(pkt->cmd));
+                const bool allocate = allocOnFill(pkt->cmd) &&
+                    (!writeAllocator || writeAllocator->allocate());
+                blk = handleFill(bus_pkt, blk, writebacks, allocate);
                 assert(blk != NULL);
                 is_invalidate = false;
                 satisfyRequest(pkt, blk);