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Reduced number of cycles required for lower-precision sqrt

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This commit is contained in:
David Harris 2022-09-17 09:55:34 -07:00
parent ac78823f6c
commit b74a68ff0f
2 changed files with 44 additions and 3 deletions
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4
pipelined/src/fpu/fdivsqrt.sv
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@ -67,8 +67,8 @@ module fdivsqrt(
.clk, .DivStart(DivStartE), .Xm(XmE), .QeM, .Xe(XeE), .Fmt(FmtE), .Ye(YeE), .clk, .DivStart(DivStartE), .Xm(XmE), .QeM, .Xe(XeE), .Fmt(FmtE), .Ye(YeE),
.Sqrt(SqrtE), .Ym(YmE), .XZero(XZeroE), .X, .Dpreproc); .Sqrt(SqrtE), .Ym(YmE), .XZero(XZeroE), .X, .Dpreproc);
fdivsqrtfsm fdivsqrtfsm( fdivsqrtfsm fdivsqrtfsm(
.reset, .XsE, .SqrtE, .clk, .reset, .FmtE, .XsE, .SqrtE,
.DivBusy, .clk, .DivStart(DivStartE),.StallE, .StallM, .DivDone, .XZeroE, .YZeroE, .DivBusy, .DivStart(DivStartE),.StallE, .StallM, .DivDone, .XZeroE, .YZeroE,
.XNaNE, .YNaNE, .XNaNE, .YNaNE,
.XInfE, .YInfE, .EarlyTermShiftE(EarlyTermShiftM), .WZero); .XInfE, .YInfE, .EarlyTermShiftE(EarlyTermShiftM), .WZero);
fdivsqrtiter fdivsqrtiter( fdivsqrtiter fdivsqrtiter(

43
pipelined/src/fpu/fdivsqrtfsm.sv
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@ -33,6 +33,7 @@
module fdivsqrtfsm( module fdivsqrtfsm(
input logic clk, input logic clk,
input logic reset, input logic reset,
input logic [`FMTBITS-1:0] FmtE,
input logic XInfE, YInfE, input logic XInfE, YInfE,
input logic XZeroE, YZeroE, input logic XZeroE, YZeroE,
input logic XNaNE, YNaNE, input logic XNaNE, YNaNE,
@ -59,13 +60,53 @@ module fdivsqrtfsm(
// terminate immediately on special cases // terminate immediately on special cases
assign SpecialCase = XZeroE | (YZeroE&~SqrtE) | XInfE | YInfE | XNaNE | YNaNE | (XsE&SqrtE); assign SpecialCase = XZeroE | (YZeroE&~SqrtE) | XInfE | YInfE | XNaNE | YNaNE | (XsE&SqrtE);
assign cycles = (`DURLEN)'((`DIVN+2+(`LOGR*`DIVCOPIES)-1)/(`LOGR*`DIVCOPIES)+(`RADIX/4)); // DIVN = `NF+3
// NS = NF + 1
// N = NS or NS+2 for div/sqrt.
/* verilator lint_off WIDTH */
logic [`DURLEN+1:0] Nf, fbits; // number of fractional bits
if (`FPSIZES == 1)
assign Nf = `NF;
else if (`FPSIZES == 2)
always_comb
case (FmtE)
1'b0: Nf = `NF1;
1'b1: Nf = `NF;
endcase
else if (`FPSIZES == 3)
always_comb
case (FmtE)
`FMT: Nf = `NF;
`FMT1: Nf = `NF1;
`FMT2: Nf = `NF2;
endcase
else if (`FPSIZES == 4)
always_comb
case(FmtE)
`S_FMT: Nf = `S_NF;
`D_FMT: Nf = `D_NF;
`H_FMT: Nf = `H_NF;
`Q_FMT: Nf = `Q_NF;
endcase
always_comb begin
if (SqrtE) fbits = Nf + 2 + 2; // Nf + two fractional bits for round/guard + 2 for right shift by up to 2
else fbits = Nf + 2 + `LOGR; // Nf + two fractional bits for round/guard + integer bits - try this when placing results in msbs
if (SqrtE) cycles = (fbits + (`LOGR*`DIVCOPIES)-1)/(`LOGR*`DIVCOPIES); // ceiling(fbits / r*k)
else cycles = `FPDUR; // *** line above should work once otfc is used to put results in upper bits
end
/* verilator lint_on WIDTH */
always_ff @(posedge clk) begin always_ff @(posedge clk) begin
if (reset) begin if (reset) begin
state <= #1 IDLE; state <= #1 IDLE;
end else if (DivStart&~StallE) begin end else if (DivStart&~StallE) begin
step <= cycles; // *** this should be adjusted to depend on the precision; sqrt should use one fewer step becasue firststep=1 step <= cycles; // *** this should be adjusted to depend on the precision; sqrt should use one fewer step becasue firststep=1
// $display("Setting Nf = %d fbits %d cycles = %d FmtE %d FPSIZES = %d Q_NF = %d num = %d denom = %d\n", Nf, fbits, cycles, FmtE, `FPSIZES, `Q_NF,
// (fbits +(`LOGR*`DIVCOPIES)-1), (`LOGR*`DIVCOPIES));
if (SpecialCase) state <= #1 DONE; if (SpecialCase) state <= #1 DONE;
else state <= #1 BUSY; else state <= #1 BUSY;
end else if (DivDone) begin end else if (DivDone) begin