From 497cc2d373d1559aaae0263635b88f670fd239cd Mon Sep 17 00:00:00 2001 From: Arthur Perais Date: Wed, 21 Dec 2016 15:07:16 -0600 Subject: cpu: disallow speculative update of branch predictor tables (o3) The Minor and o3 cpu models share the branch prediction code. Minor relies on the BPredUnit::squash() function to update the branch predictor tables on a branch mispre- diction. This is fine because Minor executes in-order, so the update is on the correct path. However, this causes the branch predictor to be updated on out-of-order branch mispredictions when using the o3 model, which should not be the case. This patch guards against speculative update of the branch prediction tables. On a branch misprediction, BPredUnit::squash() calls BpredUnit::update(..., squashed = true). The underlying branch predictor tests against the value of squashed. If it is true, it restores any speculatively updated internal state it might have (e.g., global/local branch history), then returns. If false, it updates its prediction tables. Previously, exist- ing predictors did not test against the "squashed" parameter. To accomodate for this change, the Minor model must now call BPredUnit::squash() then BPredUnit::update(..., squashed = false) on branch mispredictions. Before, calling BpredUnit::squash() performed the prediction tables update. The effect is a slight MPKI improvement when using the o3 model. A further patch should perform the same modifications for the indirect target predictor and BTB (less critical). Signed-off-by: Jason Lowe-Power --- src/cpu/pred/2bit_local.cc | 6 ++++++ 1 file changed, 6 insertions(+) (limited to 'src/cpu/pred/2bit_local.cc') diff --git a/src/cpu/pred/2bit_local.cc b/src/cpu/pred/2bit_local.cc index 9e1c781c5..6f821e94c 100644 --- a/src/cpu/pred/2bit_local.cc +++ b/src/cpu/pred/2bit_local.cc @@ -123,6 +123,12 @@ LocalBP::update(ThreadID tid, Addr branch_addr, bool taken, void *bp_history, assert(bp_history == NULL); unsigned local_predictor_idx; + // No state to restore, and we do not update on the wrong + // path. + if (squashed) { + return; + } + // Update the local predictor. local_predictor_idx = getLocalIndex(branch_addr); -- cgit v1.2.3