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Review request for Default. Repository: gem5 Description ------- Changeset 11707:1d085f66c4ca --------------------------- cpu: disallow speculative update of the conditional 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). Diffs ----- src/cpu/minor/fetch2.cc 1d085f66c4ca src/cpu/pred/2bit_local.hh 1d085f66c4ca src/cpu/pred/2bit_local.cc 1d085f66c4ca src/cpu/pred/bi_mode.hh 1d085f66c4ca src/cpu/pred/bi_mode.cc 1d085f66c4ca src/cpu/pred/bpred_unit.hh 1d085f66c4ca src/cpu/pred/bpred_unit.cc 1d085f66c4ca src/cpu/pred/tournament.hh 1d085f66c4ca src/cpu/pred/tournament.cc 1d085f66c4ca Diff: http://reviews.gem5.org/r/3727/diff/ Testing ------- Fast regressions (SE) booting Linux (FS) Thanks, Arthur Perais _______________________________________________ gem5-dev mailing list gem5-dev@gem5.org http://m5sim.org/mailman/listinfo/gem5-dev
