[gem5-dev] changeset in gem5: mem: Add additional tolerance to stride prefe...

[Mitch Hayenga]

changeset baaed1733069 in /z/repo/gem5
details: http://repo.gem5.org/gem5?cmd=changeset;node=baaed1733069
description:
        mem: Add additional tolerance to stride prefetcher
        Forces the prefetcher to mispredict twice in a row before resetting the
        confidence of prefetching.  This helps cases where a load PC strides by 
a
        constant factor, however it may operate on different arrays at times.
        Avoids the cost of retraining.  Primarily helps with small iteration 
loops.

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

diffstat:

 src/mem/cache/prefetch/stride.cc |  14 ++++++++++----
 src/mem/cache/prefetch/stride.hh |   1 +
 2 files changed, 11 insertions(+), 4 deletions(-)

diffs (52 lines):

diff -r b0b69dbafc08 -r baaed1733069 src/mem/cache/prefetch/stride.cc
--- a/src/mem/cache/prefetch/stride.cc  Wed Jan 29 23:21:26 2014 -0600
+++ b/src/mem/cache/prefetch/stride.cc  Wed Jan 29 23:21:26 2014 -0600
@@ -98,12 +98,17 @@
         bool stride_match = (new_stride == (*iter)->stride);
 
         if (stride_match && new_stride != 0) {
+            (*iter)->tolerance = true;
             if ((*iter)->confidence < Max_Conf)
                 (*iter)->confidence++;
         } else {
-            (*iter)->stride = new_stride;
-            if ((*iter)->confidence > Min_Conf)
-                (*iter)->confidence = 0;
+            if (!((*iter)->tolerance)) {
+                (*iter)->stride = new_stride;
+                if ((*iter)->confidence > Min_Conf)
+                    (*iter)->confidence = 0;
+            } else {
+                (*iter)->tolerance = false;
+            }
         }
 
         DPRINTF(HWPrefetch, "hit: PC %x data_addr %x (%s) stride %d (%s), "
@@ -118,7 +123,7 @@
             return;
 
         for (int d = 1; d <= degree; d++) {
-            Addr new_addr = data_addr + d * new_stride;
+            Addr new_addr = data_addr + d * (*iter)->stride;
             if (pageStop && !samePage(data_addr, new_addr)) {
                 // Spanned the page, so now stop
                 pfSpanPage += degree - d + 1;
@@ -160,6 +165,7 @@
         new_entry->isSecure = is_secure;
         new_entry->stride = 0;
         new_entry->confidence = 0;
+        new_entry->tolerance = false;
         tab.push_back(new_entry);
     }
 }
diff -r b0b69dbafc08 -r baaed1733069 src/mem/cache/prefetch/stride.hh
--- a/src/mem/cache/prefetch/stride.hh  Wed Jan 29 23:21:26 2014 -0600
+++ b/src/mem/cache/prefetch/stride.hh  Wed Jan 29 23:21:26 2014 -0600
@@ -72,6 +72,7 @@
         bool isSecure;
         int stride;
         int confidence;
+        bool tolerance;
     };
 
     std::list<StrideEntry*> table[Max_Contexts];
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