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| author | Mrinmoy Ghosh <mrinmoy.ghosh@arm.com> | 2012-02-13 12:26:24 -0600 |
|---|---|---|
| committer | Mrinmoy Ghosh <mrinmoy.ghosh@arm.com> | 2012-02-13 12:26:24 -0600 |
| commit | fd90c3676d94520b98a9af29af09c1f8a2858465 (patch) | |
| tree | ebda3f4fbd25b9135545bf2bfee4e316873e2130 /src/cpu/inorder | |
| parent | abc212461b865a47437a8dbf532b497ea4562137 (diff) | |
| download | gem5-fd90c3676d94520b98a9af29af09c1f8a2858465.tar.xz | |
BP: Fix several Branch Predictor issues.
1. Updates the Branch Predictor correctly to the state
just after a mispredicted branch, if a squash occurs.
2. If a BTB does not find an entry, the branch is predicted not taken.
The global history is modified to correctly reflect this prediction.
3. Local history is now updated at the fetch stage instead of
execute stage.
4. In the Update stage of the branch predictor the local predictors are
now correctly updated according to the state of local history during
fetch stage.
This patch also improves performance by as much as 17% on some benchmarks
Diffstat (limited to 'src/cpu/inorder')
| -rw-r--r-- | src/cpu/inorder/resources/bpred_unit.cc | 9 | ||||
| -rw-r--r-- | src/cpu/inorder/resources/bpred_unit.hh | 3 |
2 files changed, 7 insertions, 5 deletions
diff --git a/src/cpu/inorder/resources/bpred_unit.cc b/src/cpu/inorder/resources/bpred_unit.cc index 778366532..1a458e1d6 100644 --- a/src/cpu/inorder/resources/bpred_unit.cc +++ b/src/cpu/inorder/resources/bpred_unit.cc @@ -284,7 +284,8 @@ BPredUnit::update(const InstSeqNum &done_sn, ThreadID tid) // Update the branch predictor with the correct results. BPUpdate(predHist[tid].back().pc.instAddr(), predHist[tid].back().predTaken, - predHist[tid].back().bpHistory); + predHist[tid].back().bpHistory, + false); predHist[tid].pop_back(); } @@ -367,7 +368,7 @@ BPredUnit::squash(const InstSeqNum &squashed_sn, } BPUpdate((*hist_it).pc.instAddr(), actually_taken, - pred_hist.front().bpHistory); + pred_hist.front().bpHistory, true); // only update BTB on branch taken right??? if (actually_taken) @@ -425,12 +426,12 @@ BPredUnit::BPLookup(Addr inst_PC, void * &bp_history) void -BPredUnit::BPUpdate(Addr inst_PC, bool taken, void *bp_history) +BPredUnit::BPUpdate(Addr inst_PC, bool taken, void *bp_history, bool squashed) { if (predictor == Local) { localBP->update(inst_PC, taken, bp_history); } else if (predictor == Tournament) { - tournamentBP->update(inst_PC, taken, bp_history); + tournamentBP->update(inst_PC, taken, bp_history, squashed); } else { panic("Predictor type is unexpected value!"); } diff --git a/src/cpu/inorder/resources/bpred_unit.hh b/src/cpu/inorder/resources/bpred_unit.hh index 33ca4a0c6..b5d12d2db 100644 --- a/src/cpu/inorder/resources/bpred_unit.hh +++ b/src/cpu/inorder/resources/bpred_unit.hh @@ -160,9 +160,10 @@ class BPredUnit * @param taken Whether the branch was taken or not taken. * @param bp_history Pointer to the branch predictor state that is * associated with the branch lookup that is being updated. + * @param squashed if the branch in question was squashed or not * @todo Make this update flexible enough to handle a global predictor. */ - void BPUpdate(Addr instPC, bool taken, void *bp_history); + void BPUpdate(Addr instPC, bool taken, void *bp_history, bool squashed); /** * Updates the BTB with the target of a branch. |