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Optimized out second adder from IFU for PC+2

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This commit is contained in:
David Harris 2022-01-27 16:06:24 +00:00
parent 7f91170bab
commit bdd5796f3a
1 changed files with 45 additions and 41 deletions
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86
pipelined/src/ifu/ifu.sv
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@ -118,58 +118,60 @@ module ifu (
localparam integer SPILLTHRESHOLD = `MEM_ICACHE ? `ICACHE_LINELENINBITS/32 : 1; localparam integer SPILLTHRESHOLD = `MEM_ICACHE ? `ICACHE_LINELENINBITS/32 : 1;
if(`C_SUPPORTED) begin : SpillSupport if(`C_SUPPORTED) begin : SpillSupport
logic [`XLEN-1:0] PCFp2; logic [`XLEN-1:0] PCFp2;
logic Spill; logic Spill;
logic SelSpill, SpillSave; logic SelSpill, SpillSave;
logic [15:0] SpillDataLine0; logic [15:0] SpillDataLine0;
// this exists only if there are compressed instructions. // *** PLACE ALL THIS IN A MODULE
assign PCFp2 = PCF + `XLEN'b10; // this exists only if there are compressed instructions.
//assign PCFp2 = PCF + `XLEN'b10; **
assign PCNextFSpill = SelNextSpill ? PCFp2 : PCNextF; assign PCFp2 = PCF[1] ? {PCPlusUpperF, 2'b00} : {PCF[`XLEN-1:2], 2'b10}; // recode as mux
assign PCFSpill = SelSpill ? PCFp2 : PCF;
assign PCNextFSpill = SelNextSpill ? PCFp2 : PCNextF;
assign Spill = &PCF[$clog2(SPILLTHRESHOLD)+1:1]; assign PCFSpill = SelSpill ? PCFp2 : PCF;
assign Spill = &PCF[$clog2(SPILLTHRESHOLD)+1:1];
typedef enum {STATE_SPILL_READY, STATE_SPILL_SPILL} statetype; typedef enum {STATE_SPILL_READY, STATE_SPILL_SPILL} statetype;
(* mark_debug = "true" *) statetype CurrState, NextState; (* mark_debug = "true" *) statetype CurrState, NextState;
always_ff @(posedge clk) always_ff @(posedge clk)
if (reset) CurrState <= #1 STATE_SPILL_READY; if (reset) CurrState <= #1 STATE_SPILL_READY;
else CurrState <= #1 NextState; else CurrState <= #1 NextState;
always_comb begin always_comb begin
case(CurrState) case(CurrState)
STATE_SPILL_READY: if (Spill & ~(ICacheStallF | BusStall)) NextState = STATE_SPILL_SPILL; STATE_SPILL_READY: if (Spill & ~(ICacheStallF | BusStall)) NextState = STATE_SPILL_SPILL;
else NextState = STATE_SPILL_READY;
STATE_SPILL_SPILL: if(ICacheStallF | BusStall | StallF) NextState = STATE_SPILL_SPILL;
else NextState = STATE_SPILL_READY; else NextState = STATE_SPILL_READY;
STATE_SPILL_SPILL: if(ICacheStallF | BusStall | StallF) NextState = STATE_SPILL_SPILL; default: NextState = STATE_SPILL_READY;
else NextState = STATE_SPILL_READY; endcase
default: NextState = STATE_SPILL_READY; end
endcase
end
assign SelSpill = CurrState == STATE_SPILL_SPILL; assign SelSpill = CurrState == STATE_SPILL_SPILL;
assign SelNextSpill = (CurrState == STATE_SPILL_READY & (Spill & ~(ICacheStallF | BusStall))) | assign SelNextSpill = (CurrState == STATE_SPILL_READY & (Spill & ~(ICacheStallF | BusStall))) |
(CurrState == STATE_SPILL_SPILL & (ICacheStallF | BusStall)); (CurrState == STATE_SPILL_SPILL & (ICacheStallF | BusStall));
assign SpillSave = CurrState == STATE_SPILL_READY & (Spill & ~(ICacheStallF | BusStall)); assign SpillSave = CurrState == STATE_SPILL_READY & (Spill & ~(ICacheStallF | BusStall));
flopenr #(16) SpillInstrReg(.clk(clk), flopenr #(16) SpillInstrReg(.clk(clk),
.en(SpillSave), .en(SpillSave),
.reset(reset), .reset(reset),
.d(`MEM_ICACHE ? InstrRawF[15:0] : InstrRawF[31:16]), .d(`MEM_ICACHE ? InstrRawF[15:0] : InstrRawF[31:16]),
.q(SpillDataLine0)); .q(SpillDataLine0));
assign PostSpillInstrRawF = Spill ? {InstrRawF[15:0], SpillDataLine0} : InstrRawF; assign PostSpillInstrRawF = Spill ? {InstrRawF[15:0], SpillDataLine0} : InstrRawF;
assign CompressedF = PostSpillInstrRawF[1:0] != 2'b11; assign CompressedF = PostSpillInstrRawF[1:0] != 2'b11;
// end of spill support // end of spill support
end else begin : NoSpillSupport // line: SpillSupport end else begin : NoSpillSupport // line: SpillSupport
assign PCNextFSpill = PCNextF; assign PCNextFSpill = PCNextF;
assign PCFSpill = PCF; assign PCFSpill = PCF;
assign SelNextSpill = 0; assign SelNextSpill = 0;
assign PostSpillInstrRawF = InstrRawF; assign PostSpillInstrRawF = InstrRawF;
end end
@ -377,13 +379,15 @@ module ifu (
// pcadder // pcadder
// add 2 or 4 to the PC, based on whether the instruction is 16 bits or 32 // add 2 or 4 to the PC, based on whether the instruction is 16 bits or 32
assign PCPlusUpperF = PCF[`XLEN-1:2] + 1; // add 4 to PC assign PCPlusUpperF = PCF[`XLEN-1:2] + 1; // add 4 to PC
// choose PC+2 or PC+4 // choose PC+2 or PC+4 based on CompressedF, which arrives later.
// Speeds up critical path as compared to selecting adder input based on CompressedF
always_comb always_comb
if (CompressedF) // add 2 if (CompressedF) // add 2
if (PCF[1]) PCPlus2or4F = {PCPlusUpperF, 2'b00}; if (PCF[1]) PCPlus2or4F = {PCPlusUpperF, 2'b00};
else PCPlus2or4F = {PCF[`XLEN-1:2], 2'b10}; else PCPlus2or4F = {PCF[`XLEN-1:2], 2'b10};
else PCPlus2or4F = {PCPlusUpperF, PCF[1:0]}; // add 4 else PCPlus2or4F = {PCPlusUpperF, PCF[1:0]}; // add 4
// Decode stage pipeline register and logic // Decode stage pipeline register and logic
flopenrc #(`XLEN) PCDReg(clk, reset, FlushD, ~StallD, PCF, PCD); flopenrc #(`XLEN) PCDReg(clk, reset, FlushD, ~StallD, PCF, PCD);