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authorMitch Hayenga <mitch.hayenga@arm.com>2014-09-03 07:42:36 -0400
committerMitch Hayenga <mitch.hayenga@arm.com>2014-09-03 07:42:36 -0400
commitdaedc5a49127eb91036291af1619bbc98016aff0 (patch)
tree6bf2ac8d871cd9b4c49c3aa0dc48096f76dfe427
parentecd53009712da59a98ad3c13ed20aaa8e8cd7e29 (diff)
downloadgem5-daedc5a49127eb91036291af1619bbc98016aff0.tar.xz
cpu: Fix incorrect speculative branch predictor behavior
When a branch mispredicted gem5 would squash all history after and including the mispredicted branch. However, the mispredicted branch is still speculative and its history is required to rollback state if another, older, branch mispredicts. This leads to things like RAS corruption.
-rw-r--r--src/cpu/pred/2bit_local.hh5
-rw-r--r--src/cpu/pred/bpred_unit.hh14
-rw-r--r--src/cpu/pred/bpred_unit_impl.hh22
-rw-r--r--src/cpu/pred/tournament.cc15
-rw-r--r--src/cpu/pred/tournament.hh4
5 files changed, 41 insertions, 19 deletions
diff --git a/src/cpu/pred/2bit_local.hh b/src/cpu/pred/2bit_local.hh
index 23683cc67..e008c6232 100644
--- a/src/cpu/pred/2bit_local.hh
+++ b/src/cpu/pred/2bit_local.hh
@@ -1,5 +1,5 @@
/*
- * Copyright (c) 2011 ARM Limited
+ * Copyright (c) 2011, 2014 ARM Limited
* All rights reserved
*
* The license below extends only to copyright in the software and shall
@@ -92,6 +92,9 @@ class LocalBP : public BPredUnit
*/
void update(Addr branch_addr, bool taken, void *bp_history, bool squashed);
+ void retireSquashed(void *bp_history)
+ { assert(bp_history == NULL); }
+
void squash(void *bp_history)
{ assert(bp_history == NULL); }
diff --git a/src/cpu/pred/bpred_unit.hh b/src/cpu/pred/bpred_unit.hh
index 61b375f9b..f75ab79d5 100644
--- a/src/cpu/pred/bpred_unit.hh
+++ b/src/cpu/pred/bpred_unit.hh
@@ -178,6 +178,13 @@ class BPredUnit : public SimObject
*/
virtual void update(Addr instPC, bool taken, void *bp_history,
bool squashed) = 0;
+ /**
+ * Deletes the associated history with a branch, performs no predictor
+ * updates. Used for branches that mispredict and update tables but
+ * are still speculative and later retire.
+ * @param bp_history History to delete associated with this predictor
+ */
+ virtual void retireSquashed(void *bp_history) = 0;
/**
* Updates the BTB with the target of a branch.
@@ -200,7 +207,7 @@ class BPredUnit : public SimObject
ThreadID _tid)
: seqNum(seq_num), pc(instPC), bpHistory(bp_history), RASTarget(0),
RASIndex(0), tid(_tid), predTaken(pred_taken), usedRAS(0), pushedRAS(0),
- wasCall(0), wasReturn(0)
+ wasCall(0), wasReturn(0), wasSquashed(0)
{}
bool operator==(const PredictorHistory &entry) const {
@@ -234,7 +241,7 @@ class BPredUnit : public SimObject
/** Whether or not the RAS was used. */
bool usedRAS;
- /* Wether or not the RAS was pushed */
+ /* Whether or not the RAS was pushed */
bool pushedRAS;
/** Whether or not the instruction was a call. */
@@ -242,6 +249,9 @@ class BPredUnit : public SimObject
/** Whether or not the instruction was a return. */
bool wasReturn;
+
+ /** Whether this instruction has already mispredicted/updated bp */
+ bool wasSquashed;
};
typedef std::deque<PredictorHistory> History;
diff --git a/src/cpu/pred/bpred_unit_impl.hh b/src/cpu/pred/bpred_unit_impl.hh
index 18e221775..eaffb7ea1 100644
--- a/src/cpu/pred/bpred_unit_impl.hh
+++ b/src/cpu/pred/bpred_unit_impl.hh
@@ -372,8 +372,12 @@ BPredUnit::update(const InstSeqNum &done_sn, ThreadID tid)
while (!predHist[tid].empty() &&
predHist[tid].back().seqNum <= done_sn) {
// Update the branch predictor with the correct results.
- update(predHist[tid].back().pc, predHist[tid].back().predTaken,
- predHist[tid].back().bpHistory, false);
+ if (!predHist[tid].back().wasSquashed) {
+ update(predHist[tid].back().pc, predHist[tid].back().predTaken,
+ predHist[tid].back().bpHistory, false);
+ } else {
+ retireSquashed(predHist[tid].back().bpHistory);
+ }
predHist[tid].pop_back();
}
@@ -465,12 +469,15 @@ BPredUnit::squash(const InstSeqNum &squashed_sn,
update((*hist_it).pc, actually_taken,
pred_hist.front().bpHistory, true);
+ hist_it->wasSquashed = true;
+
if (actually_taken) {
if (hist_it->wasReturn && !hist_it->usedRAS) {
DPRINTF(Branch, "[tid: %i] Incorrectly predicted"
" return [sn:%i] PC: %s\n", tid, hist_it->seqNum,
hist_it->pc);
RAS[tid].pop();
+ hist_it->usedRAS = true;
}
DPRINTF(Branch,"[tid: %i] BTB Update called for [sn:%i]"
@@ -488,23 +495,16 @@ BPredUnit::squash(const InstSeqNum &squashed_sn,
" to: %i, target: %s.\n", tid,
hist_it->RASIndex, hist_it->RASTarget);
RAS[tid].restore(hist_it->RASIndex, hist_it->RASTarget);
-
+ hist_it->usedRAS = false;
} else if (hist_it->wasCall && hist_it->pushedRAS) {
//Was a Call but predicated false. Pop RAS here
DPRINTF(Branch, "[tid: %i] Incorrectly predicted"
" Call [sn:%i] PC: %s Popping RAS\n", tid,
hist_it->seqNum, hist_it->pc);
RAS[tid].pop();
+ hist_it->pushedRAS = false;
}
}
- DPRINTF(Branch, "[tid:%i]: Removing history for [sn:%i]"
- " PC %s Actually Taken: %i\n", tid, hist_it->seqNum,
- hist_it->pc, actually_taken);
-
- pred_hist.erase(hist_it);
-
- DPRINTF(Branch, "[tid:%i]: predHist.size(): %i\n", tid,
- predHist[tid].size());
} else {
DPRINTF(Branch, "[tid:%i]: [sn:%i] pred_hist empty, can't "
"update.\n", tid, squashed_sn);
diff --git a/src/cpu/pred/tournament.cc b/src/cpu/pred/tournament.cc
index e471d08f5..c6514b6ad 100644
--- a/src/cpu/pred/tournament.cc
+++ b/src/cpu/pred/tournament.cc
@@ -1,5 +1,5 @@
/*
- * Copyright (c) 2011 ARM Limited
+ * Copyright (c) 2011, 2014 ARM Limited
* All rights reserved
*
* The license below extends only to copyright in the software and shall
@@ -346,10 +346,10 @@ TournamentBP::update(Addr branch_addr, bool taken, void *bp_history,
}
}
+ } else {
+ // We're done with this history, now delete it.
+ delete history;
}
- // We're done with this history, now delete it.
- delete history;
-
}
assert(local_history_idx < localHistoryTableSize);
@@ -358,6 +358,13 @@ TournamentBP::update(Addr branch_addr, bool taken, void *bp_history,
}
void
+TournamentBP::retireSquashed(void *bp_history)
+{
+ BPHistory *history = static_cast<BPHistory *>(bp_history);
+ delete history;
+}
+
+void
TournamentBP::squash(void *bp_history)
{
BPHistory *history = static_cast<BPHistory *>(bp_history);
diff --git a/src/cpu/pred/tournament.hh b/src/cpu/pred/tournament.hh
index 39fff5bfb..88334a8c0 100644
--- a/src/cpu/pred/tournament.hh
+++ b/src/cpu/pred/tournament.hh
@@ -1,5 +1,5 @@
/*
- * Copyright (c) 2011 ARM Limited
+ * Copyright (c) 2011, 2014 ARM Limited
* All rights reserved
*
* The license below extends only to copyright in the software and shall
@@ -104,6 +104,8 @@ class TournamentBP : public BPredUnit
*/
void update(Addr branch_addr, bool taken, void *bp_history, bool squashed);
+ void retireSquashed(void *bp_history);
+
/**
* Restores the global branch history on a squash.
* @param bp_history Pointer to the BPHistory object that has the