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Major cleanup of branch predictor.

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This commit is contained in:
Ross Thompson 2023-01-30 23:37:34 -06:00
parent 42828e6ec4
commit a15889e0aa
1 changed files with 24 additions and 37 deletions
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61
pipelined/src/ifu/bpred/speculativegshare.sv
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@ -48,17 +48,13 @@ module speculativegshare #(parameter int k = 10 ) (
logic [1:0] TableDirPredictionF, DirPredictionD, DirPredictionE;
logic [1:0] NewDirPredictionF, NewDirPredictionD, NewDirPredictionE;
logic [k-1:0] GHRF;
logic GHRExtraF;
logic [k-1:0] GHRD, GHRE, GHRM, GHRW;
logic [k-1:0] GHRNextF;
logic [k-1:0] GHRNextD;
logic [k-1:0] GHRNextE, GHRNextM, GHRNextW;
logic [k-1:0] IndexNextF, IndexF;
logic [k-1:0] IndexD, IndexE;
logic [k-1:0] GHRF, GHRD, GHRE;
logic GHRLastF;
logic [k-1:0] GHRNextF, GHRNextD, GHRNextE;
logic [k-1:0] IndexNextF, IndexF, IndexD, IndexE;
logic [1:0] ForwardNewDirPrediction, ForwardDirPredictionF;
logic FlushDOrDirWrong;
assign IndexNextF = GHRNextF ^ {PCNextF[k+1] ^ PCNextF[1], PCNextF[k:2]};
assign IndexF = GHRF ^ {PCF[k+1] ^ PCF[1], PCF[k:2]};
@ -102,41 +98,32 @@ module speculativegshare #(parameter int k = 10 ) (
satCounter2 BPDirUpdateE(.BrDir(PCSrcE), .OldState(DirPredictionE), .NewState(NewDirPredictionE));
// GHR pipeline
// this version fails the regression test do to pessimistic x propagation.
// assign GHRNextF = FlushD | DirPredictionWrongE ? GHRNextE[k-1:0] :
// BranchInstrF ? {DirPredictionF[1], GHRF[k-1:1]} :
// GHRF;
always_comb begin
if(FlushD | DirPredictionWrongE) begin
GHRNextF = GHRNextE[k-1:0];
end else if(BranchInstrF) GHRNextF = {DirPredictionF[1], GHRF[k-1:1]};
else GHRNextF = GHRF;
end
// If Fetch has a branch, speculatively insert prediction into the GHR
// 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.
assign FlushDOrDirWrong = FlushD | DirPredictionWrongE;
mux3 #(k) GHRFMux(GHRF, {DirPredictionF[1], GHRF[k-1:1]}, GHRNextE[k-1:0],
{FlushDOrDirWrong, BranchInstrF}, GHRNextF);
// Need 1 extra bit to store the shifted out GHRF if repair needs to back shift.
flopenr #(k) GHRFReg(clk, reset, (~StallF) | FlushD, GHRNextF, GHRF);
flopenr #(1) GHRFExtraReg(clk, reset, (~StallF) | FlushD, GHRF[0], GHRExtraF);
flopenr #(1) GHRFLastReg(clk, reset, (~StallF) | FlushD, GHRF[0], GHRLastF);
// use with out instruction class prediction
//assign GHRNextD = FlushD ? GHRNextE[k-1:0] : GHRF[k-1:0];
// with instruction class prediction
assign GHRNextD = (FlushD | DirPredictionWrongE) ? GHRNextE[k-1:0] :
WrongPredInstrClassD[0] & BranchInstrD ? {DirPredictionD[1], GHRF[k-1:1]} : // shift right
WrongPredInstrClassD[0] & ~BranchInstrD ? {GHRF[k-2:0], GHRExtraF}: // shift left
GHRF[k-1:0];
// 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}, BranchInstrD, 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);
flopenr #(k) GHRDReg(clk, reset, (~StallD) | FlushD, GHRNextD, GHRD);
assign GHRNextE = BranchInstrE & ~FlushM ? {PCSrcE, GHRD[k-2:0]} : // if the branch is not flushed
FlushE ? GHRE : // branch is flushed
GHRD;
mux3 #(k) GHREMux(GHRD, GHRE, {PCSrcE, GHRD[k-2:0]}, {BranchInstrE & ~FlushM, FlushE}, GHRNextE);
flopenr #(k) GHREReg(clk, reset, (BranchInstrE & ~StallE) | FlushE, GHRNextE, GHRE);
//assign GHRNextM = FlushM ? GHRM : GHRE;
//flopenr #(k) GHRMReg(clk, reset, (BranchInstrM & ~StallM) | FlushM, GHRNextM, GHRM);
//assign GHRNextW = FlushW ? GHRW : GHRM;
//flopenr #(k) GHRWReg(clk, reset, (BranchInstrW & ~StallW) | FlushW, GHRNextW, GHRW);
assign DirPredictionWrongE = PCSrcE != DirPredictionE[1] & BranchInstrE;