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Fixed a branch predictor performance issue.

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This commit is contained in:
Ross Thompson 2023-02-17 15:37:03 -06:00
parent 5d5e4580d4
commit 0d271130b9
2 changed files with 15 additions and 3 deletions
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6
src/ifu/bpred/bpred.sv
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@ -222,8 +222,14 @@ module bpred (
assign AnyWrongPredInstrClassD = |WrongPredInstrClassD;
// branch is wrong only if the PC does not match and both the Decode and Fetch stages have valid instructions.
//assign BPPredWrongE = (PredictionPCWrongE & |InstrClassE | (AnyWrongPredInstrClassE & ~|InstrClassE));
assign BPPredWrongE = PredictionPCWrongE & InstrValidE & InstrValidD;
logic BPPredWrongEAlt;
logic NotMatch;
assign BPPredWrongEAlt = PredictionPCWrongE & InstrValidE & InstrValidD; // this does not work for cubic benchmark
assign NotMatch = BPPredWrongE != BPPredWrongEAlt;
// Output the predicted PC or corrected PC on miss-predict.
// Selects the BP or PC+2/4.
mux2 #(`XLEN) pcmux0(PCPlus2or4F, BPPredPCF, SelBPPredF, PCNext0F);

12
src/ifu/bpred/speculativegshare.sv
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@ -101,9 +101,16 @@ module speculativegshare #(parameter int k = 10 ) (
// If the front end is flushed or the direction prediction is wrong, reset to
// most recent valid GHR. For a BP wrong this is GHRD with the correct prediction shifted in.
// For FlushE this is GHRE. GHRNextE is both.
logic [k-1:0] GHRClassWrongAndPredF;
logic [k-1:0] GHRClassWrong;
mux2 #(k) GHRClassWrongAndPredFMux({DirPredictionF[1], DirPredictionD[1], GHRF[k-1:2]}, {DirPredictionF[1], GHRF[k-2:0]}, ~InstrClassD[0], GHRClassWrongAndPredF);
assign FlushDOrDirWrong = FlushD | DirPredictionWrongE;
mux3 #(k) GHRFMux(GHRF, {DirPredictionF[1], GHRF[k-1:1]}, GHRNextE[k-1:0],
{FlushDOrDirWrong, PredInstrClassF[0]}, GHRNextF);
// mux3 #(k) GHRFMux(GHRF, {DirPredictionF[1], GHRF[k-1:1]}, GHRNextE[k-1:0],
// {FlushDOrDirWrong, PredInstrClassF[0]}, GHRNextF);
mux5 #(k) GHRFMux(GHRF, {DirPredictionF[1], GHRF[k-1:1]}, GHRClassWrong, GHRClassWrongAndPredF, GHRNextE[k-1:0],
{FlushDOrDirWrong, WrongPredInstrClassD[0], PredInstrClassF[0]}, GHRNextF);
// Need 1 extra bit to store the shifted out GHRF if repair needs to back shift.
flopenr #(k) GHRFReg(clk, reset, ~StallF | FlushDOrDirWrong, GHRNextF, GHRF);
@ -112,7 +119,6 @@ module speculativegshare #(parameter int k = 10 ) (
// With instruction class prediction, the class could be wrong and is checked in Decode.
// If it is wrong and branch does exist then shift right and insert the prediction.
// If the branch does not exist then shift left and use GHRLastF to restore the LSB.
logic [k-1:0] GHRClassWrong;
mux2 #(k) GHRClassWrongMux({DirPredictionD[1], GHRF[k-1:1]}, {GHRF[k-2:0], GHRLastF}, ~InstrClassD[0], GHRClassWrong);
// As with GHRF FlushD and wrong direction prediction flushes the pipeline and restores to GHRNextE.
mux3 #(k) GHRDMux(GHRF, GHRClassWrong, GHRNextE, {FlushDOrDirWrong, WrongPredInstrClassD[0]}, GHRNextD);