From a80dbd3aec06deb87eab65534ff4fdb7b797d551 Mon Sep 17 00:00:00 2001 From: Ross Thompson Date: Mon, 13 Feb 2023 17:23:56 -0600 Subject: [PATCH] Further branch predictor improvements. --- src/ifu/bpred/bpred.sv | 59 +++--------------------------------------- 1 file changed, 4 insertions(+), 55 deletions(-) diff --git a/src/ifu/bpred/bpred.sv b/src/ifu/bpred/bpred.sv index a6cb0ccb..c86c0979 100644 --- a/src/ifu/bpred/bpred.sv +++ b/src/ifu/bpred/bpred.sv @@ -149,11 +149,9 @@ module bpred ( .InstrClassE); // the branch predictor needs a compact decoding of the instruction class. - ///// ********* THIS IS NOT THE ISSUE. -/* -----\/----- EXCLUDED -----\/----- if (`INSTR_CLASS_PRED == 0) begin : DirectClassDecode logic [4:0] CompressedOpcF; - logic [2:0] InstrClassF; + logic [3:0] InstrClassF; logic cjal, cj, cjr, cjalr, CJumpF, CBranchF; logic JumpF, BranchF; @@ -174,49 +172,12 @@ module bpred ( assign InstrClassF[2] = (JumpF & (PostSpillInstrRawF[19:15] & 5'h1B) == 5'h01) | // return must return to ra or r5 (`C_SUPPORTED & (cjalr | cjr) & ((PostSpillInstrRawF[11:7] & 5'h1B) == 5'h01)); - //assign InstrClassF[3] = (JumpF & (PostSpillInstrRawF[11:07] & 5'h1B) == 5'h01) | // jal(r) must link to ra or x5 - // (`C_SUPPORTED & (cjal | (cjalr & (PostSpillInstrRawF[11:7] & 5'h1b) == 5'h01))); - - - assign PredInstrClassF = InstrClassF; - assign SelBPPredF = (PredInstrClassF[0] & DirPredictionF[1]) | - PredInstrClassF[1]; - end else begin - assign PredInstrClassF = BTBPredInstrClassF; - assign SelBPPredF = (PredInstrClassF[0] & DirPredictionF[1] & PredValidF) | - PredInstrClassF[1] & PredValidF; - end - -----/\----- EXCLUDED -----/\----- */ - // the branch predictor needs a compact decoding of the instruction class. - if (`INSTR_CLASS_PRED == 0) begin : DirectClassDecode - logic [4:0] CompressedOpcF; - logic [3:0] InstrClassF; - logic cjal, cj, cjr, cjalr; - - assign CompressedOpcF = {PostSpillInstrRawF[1:0], PostSpillInstrRawF[15:13]}; - - assign cjal = CompressedOpcF == 5'h09 & `XLEN == 32; - assign cj = CompressedOpcF == 5'h0d; - assign cjr = CompressedOpcF == 5'h14 & ~PostSpillInstrRawF[12] & PostSpillInstrRawF[6:2] == 5'b0 & PostSpillInstrRawF[11:7] != 5'b0; - assign cjalr = CompressedOpcF == 5'h14 & PostSpillInstrRawF[12] & PostSpillInstrRawF[6:2] == 5'b0 & PostSpillInstrRawF[11:7] != 5'b0; - - assign InstrClassF[0] = PostSpillInstrRawF[6:0] == 7'h63 | - (`C_SUPPORTED & CompressedOpcF[4:1] == 4'h7); - - assign InstrClassF[1] = (PostSpillInstrRawF[6:0] == 7'h67 & (PostSpillInstrRawF[19:15] & 5'h1B) != 5'h01 & (PostSpillInstrRawF[11:7] & 5'h1B) != 5'h01) | // jump register, but not return - (PostSpillInstrRawF[6:0] == 7'h6F & (PostSpillInstrRawF[11:7] & 5'h1B) != 5'h01) | // jump, RD != x1 or x5 - (`C_SUPPORTED & (cj | (cjr & ((PostSpillInstrRawF[11:7] & 5'h1B) != 5'h01)) )); - - assign InstrClassF[2] = PostSpillInstrRawF[6:0] == 7'h67 & (PostSpillInstrRawF[19:15] & 5'h1B) == 5'h01 | // return must return to ra or r5 - (`C_SUPPORTED & (cjalr | cjr) & ((PostSpillInstrRawF[11:7] & 5'h1B) == 5'h01)); - - assign InstrClassF[3] = ((PostSpillInstrRawF[6:0] & 7'h77) == 7'h67 & (PostSpillInstrRawF[11:07] & 5'h1B) == 5'h01) | // jal(r) must link to ra or x5 + assign InstrClassF[3] = (JumpF & (PostSpillInstrRawF[11:07] & 5'h1B) == 5'h01) | // jal(r) must link to ra or x5 (`C_SUPPORTED & (cjal | (cjalr & (PostSpillInstrRawF[11:7] & 5'h1b) == 5'h01))); + assign PredInstrClassF = InstrClassF; assign SelBPPredF = (PredInstrClassF[0] & DirPredictionF[1]) | - PredInstrClassF[2] | PredInstrClassF[1] | - PredInstrClassF[3]; end else begin assign PredInstrClassF = BTBPredInstrClassF; assign SelBPPredF = (PredInstrClassF[0] & DirPredictionF[1] & PredValidF) | @@ -235,16 +196,6 @@ module bpred ( assign InstrClassD[2] = JumpD & (InstrD[19:15] & 5'h1B) == 5'h01; // return must return to ra or x5 assign InstrClassD[3] = JumpD & (InstrD[11:7] & 5'h1B) == 5'h01; // jal(r) must link to ra or x5 -/* -----\/----- EXCLUDED -----\/----- - assign InstrClassD[3] = (InstrD[6:0] & 7'h77) == 7'h67 & (InstrD[11:07] & 5'h1B) == 5'h01; // jal(r) must link to ra or x5 - assign InstrClassD[2] = InstrD[6:0] == 7'h67 & (InstrD[19:15] & 5'h1B) == 5'h01; // return must return to ra or r5 - assign InstrClassD[1] = (InstrD[6:0] == 7'h67 & (InstrD[19:15] & 5'h1B) != 5'h01 & (InstrD[11:7] & 5'h1B) != 5'h01) | // jump register, but not return - (InstrD[6:0] == 7'h6F & (InstrD[11:7] & 5'h1B) != 5'h01); // jump, RD != x1 or x5 - assign InstrClassD[0] = InstrD[6:0] == 7'h63; // branch - -----/\----- EXCLUDED -----/\----- */ - - - flopenrc #(4) InstrClassRegE(clk, reset, FlushE, ~StallE, InstrClassD, InstrClassE); flopenrc #(4) InstrClassRegM(clk, reset, FlushM, ~StallM, InstrClassE, InstrClassM); flopenrc #(1) BPPredWrongMReg(clk, reset, FlushM, ~StallM, BPPredWrongE, BPPredWrongM); @@ -256,9 +207,8 @@ module bpred ( // pipeline the class flopenrc #(4) PredInstrClassRegD(clk, reset, FlushD, ~StallD, PredInstrClassF, PredInstrClassD); - //flopenrc #(1) WrongInstrClassRegE(clk, reset, FlushE, ~StallE, AnyWrongPredInstrClassD, AnyWrongPredInstrClassE); + flopenrc #(1) WrongInstrClassRegE(clk, reset, FlushE, ~StallE, AnyWrongPredInstrClassD, AnyWrongPredInstrClassE); flopenrc #(4) PredInstrClassRegE(clk, reset, FlushE, ~StallE, PredInstrClassD, PredInstrClassE); - assign AnyWrongPredInstrClassE = InstrClassE != PredInstrClassE; // Check the prediction // if it is a CFI then check if the next instruction address (PCD) matches the branch's target or fallthrough address. @@ -299,7 +249,6 @@ module bpred ( // could be wrong or the fall through address selected for branch predict not taken. // By pipeline the BTB's PC and RAS address through the pipeline we can measure the accuracy of // both without the above inaccuracies. - //assign BTBPredPCWrongE = (PredPCE != IEUAdrE) & (InstrClassE[0] | InstrClassE[1] | InstrClassE[3]) & PCSrcE; assign BTBPredPCWrongE = (PredPCE != IEUAdrE) & (InstrClassE[0] | InstrClassE[1] & ~InstrClassE[2]) & PCSrcE; assign RASPredPCWrongE = (RASPCE != IEUAdrE) & InstrClassE[2] & PCSrcE;