mem: Allowed tagged instruction prefetching in stride prefetcher

For systems with a tightly coupled L2, a stride-based prefetcher may observe
access requests from both instruction and data L1 caches.  However, the PC
address of an instruction miss gives no relevant training information to the
stride based prefetcher(there is no stride to train).  In theses cases, its
better if the L2 stride prefetcher simply reverted back to a simple N-block
ahead prefetcher.  This patch enables this option.

Committed by: Nilay Vaish <nilay@cs.wisc.edu>

1 files changed, 17 insertions, 0 deletions

diff --git a/src/mem/cache/prefetch/stride.cc b/src/mem/cache/prefetch/stride.cc
index a7abf4809..c4cf2023a 100644
--- a/src/mem/cache/prefetch/stride.cc
+++ b/src/mem/cache/prefetch/stride.cc
@@ -66,6 +66,23 @@ StridePrefetcher::calculatePrefetch(PacketPtr &pkt, std::list<Addr> &addresses,
     assert(master_id < Max_Contexts);
     std::list<StrideEntry*> &tab = table[master_id];
 
+    // Revert to simple N-block ahead prefetch for instruction fetches
+    if (instTagged && pkt->req->isInstFetch()) {
+        for (int d = 1; d <= degree; d++) {
+            Addr new_addr = data_addr + d * blkSize;
+            if (pageStop && !samePage(data_addr, new_addr)) {
+                // Spanned the page, so now stop
+                pfSpanPage += degree - d + 1;
+                return;
+            }
+            DPRINTF(HWPrefetch, "queuing prefetch to %x @ %d\n",
+                    new_addr, latency);
+            addresses.push_back(new_addr);
+            delays.push_back(latency);
+        }
+        return;
+    }
+
     /* Scan Table for instAddr Match */
     std::list<StrideEntry*>::iterator iter;
     for (iter = tab.begin(); iter != tab.end(); iter++) {