O3: Keep around the last committed instruction and use for squashing.

Without this change 0 is always used for the youngest sequence number if
a squash occured and the ROB was empty (E.g. an instruction is marked
serializeAfter or a fetch stall prevents other instructions from issuing).
Using 0 there is a race to rename where an instruction that committed the
same cycle as the squashing instruction can have it's renamed state undone
by the squash using sequence number 0.

1 files changed, 15 insertions, 0 deletions

diff --git a/src/cpu/o3/commit.hh b/src/cpu/o3/commit.hh
index 7183889c6..659b0ad5f 100644
--- a/src/cpu/o3/commit.hh
+++ b/src/cpu/o3/commit.hh
@@ -1,4 +1,16 @@
 /*
+ * Copyright (c) 2010 ARM Limited
+ * All rights reserved.
+ *
+ * The license below extends only to copyright in the software and shall
+ * not be construed as granting a license to any other intellectual
+ * property including but not limited to intellectual property relating
+ * to a hardware implementation of the functionality of the software
+ * licensed hereunder.  You may use the software subject to the license
+ * terms below provided that you ensure that this notice is replicated
+ * unmodified and in its entirety in all distributions of the software,
+ * modified or unmodified, in source code or in binary form.
+ *
  * Copyright (c) 2004-2006 The Regents of The University of Michigan
  * All rights reserved.
  *
@@ -409,6 +421,9 @@ class DefaultCommit
     /** The sequence number of the youngest valid instruction in the ROB. */
     InstSeqNum youngestSeqNum[Impl::MaxThreads];
 
+    /** The sequence number of the last commited instruction. */
+    InstSeqNum lastCommitedSeqNum[Impl::MaxThreads];
+
     /** Records if there is a trap currently in flight. */
     bool trapInFlight[Impl::MaxThreads];