Further fdivsqrt simplification after starting Sqrt at iteration 0

This commit is contained in:
David Harris 2024-03-04 16:40:49 -08:00
parent 9c04df8f69
commit 1a0097f6e7
4 changed files with 20 additions and 25 deletions

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@ -44,7 +44,7 @@ module fdivsqrtiter import cvw::*; #(parameter cvw_t P) (
logic [P.DIVb+3:0] WCNext[P.DIVCOPIES-1:0]; // Q4.DIVb
logic [P.DIVb+3:0] WS[P.DIVCOPIES:0]; // Q4.DIVb
logic [P.DIVb+3:0] WC[P.DIVCOPIES:0]; // Q4.DIVb
logic [P.DIVb:0] U[P.DIVCOPIES:0]; // U1.DIVb
logic [P.DIVb:0] U[P.DIVCOPIES:0]; // U1.DIVb // *** probably Q not U. See Table 16.26 notes
logic [P.DIVb:0] UM[P.DIVCOPIES:0]; // U1.DIVb
logic [P.DIVb:0] UNext[P.DIVCOPIES-1:0]; // U1.DIVb
logic [P.DIVb:0] UMNext[P.DIVCOPIES-1:0]; // U1.DIVb
@ -71,7 +71,7 @@ module fdivsqrtiter import cvw::*; #(parameter cvw_t P) (
flopen #(P.DIVb+4) wcreg(clk, FDivBusyE, WCN, WC[0]);
// UOTFC Result U and UM registers/initialization mux
// Initialize U to 1.0 and UM to 0 for square root; U to 0 and UM to -1 otherwise
// Initialize U to 0 = 0.0000... and UM to -1 = 1.00000... (in Q1.Divb)
assign initU ={(P.DIVb+1){1'b0}};
assign initUM = {{1'b1}, {(P.DIVb){1'b0}}};
mux2 #(P.DIVb+1) Umux(UNext[P.DIVCOPIES-1], initU, IFDivStartE, UMux);
@ -79,15 +79,10 @@ module fdivsqrtiter import cvw::*; #(parameter cvw_t P) (
flopen #(P.DIVb+1) UReg(clk, FDivBusyE, UMux, U[0]);
flopen #(P.DIVb+1) UMReg(clk, FDivBusyE, UMMux, UM[0]);
// C register/initialization mux
logic [1:0] initCUpper;
if(P.RADIX == 4) begin
assign initCUpper = 2'b00;
end else begin
assign initCUpper = 2'b10;
end
assign initC = {initCUpper, {P.DIVb{1'b0}}};
// C register/initialization mux: C = -R:
// C = -4 = 00.000000... (in Q2.DIVb) for radix 4, C = -2 = 10.000000... for radix2
if(P.RADIX == 4) assign initC = '0;
else assign initC = {2'b10, {{P.DIVb{1'b0}}}};
mux2 #(P.DIVb+2) cmux(C[P.DIVCOPIES], initC, IFDivStartE, NextC);
flopen #(P.DIVb+2) creg(clk, FDivBusyE, NextC, C[0]);

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@ -48,16 +48,16 @@ module fdivsqrtstage4 import cvw::*; #(parameter cvw_t P) (
logic [7:0] WCmsbs, WSmsbs; // U4.4
logic CarryIn;
logic [P.DIVb+3:0] WSA, WCA; // Q4.DIVb
logic j0,j1;
logic j0, j1; // step j = 0 or step j = 1
// Digit Selection logic
assign j0 = ~C[P.DIVb+1]; // first step of R digit selection: C = 00...0
assign j1 = C[P.DIVb] ^ C[P.DIVb-1]; // second step of R digit selection: C = 1100...0
assign j1 = C[P.DIVb] & ~C[P.DIVb-1]; // second step of R digit selection: C = 1100...0; *** could simplify to ~C[P.DIVb-1] because j=0 case takes priority
assign Smsbs = U[P.DIVb:P.DIVb-4]; // U1.4 most significant bits of square root
assign Dmsbs = D[P.DIVb-1:P.DIVb-3]; // U0.3 most significant fractional bits of divisor after leading 1
assign WCmsbs = WC[P.DIVb+3:P.DIVb-4]; // Q4.4 most significant bits of residual
assign WSmsbs = WS[P.DIVb+3:P.DIVb-4]; // Q4.4 most significant bits of residual
fdivsqrtuslc4cmp uslc4(.Dmsbs, .Smsbs, .WSmsbs, .WCmsbs, .SqrtE, .j1, .j0, .udigit);
fdivsqrtuslc4cmp uslc4(.Dmsbs, .Smsbs, .WSmsbs, .WCmsbs, .SqrtE, .j0, .j1, .udigit);
assign un = 1'b0; // unused for radix 4
// F generation logic

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@ -31,7 +31,7 @@ module fdivsqrtuslc4 (
input logic [2:0] Dmsbs, // U0.3 fractional bits after implicit leading 1
input logic [4:0] Smsbs, // U1.4 leading bits of square root approximation
input logic [7:0] WSmsbs, WCmsbs, // Q4.4 redundant residual most significant bits
input logic Sqrt, j1,
input logic Sqrt, j0, j1,
output logic [3:0] udigit // {2, 1, -1, -2} digit is 0 if none are hot
);
logic [7:0] PreWmsbs; // Q4.4 nonredundant residual msbs
@ -102,11 +102,12 @@ module fdivsqrtuslc4 (
// Select A
always_comb
if (Sqrt) begin
if (j1) A = 3'b101; // on first sqrt iteration A = .101
else if (Smsbs == 5'b10000) A = 3'b111; // if S = 1.0, use A = .111
else A = Smsbs[2:0]; // otherwise use A = 2S (in U0.3 format)
end else A = Dmsbs; // division Unless A = D (IN U0.3 format, dropping leading 1)
if (j1) A = 3'b101; // on first sqrt iteration A = .101
else if (Smsbs[4] == 1) A = 3'b111; // if S = 1.0000, use A = .111
else A = Smsbs[2:0]; // otherwise use A = 2S (in U0.3 format)
end else A = Dmsbs; // division A = D (IN U0.3 format, dropping leading 1)
// Select quotient digit from lookup table based on A and W
assign udigit = USel4[{A,Wmsbs}];
// On step j = 0 for square root, always select u_0 = 1
assign udigit = (Sqrt & j0) ? 4'b0100 : USel4[{A,Wmsbs}];
endmodule

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@ -32,7 +32,7 @@ module fdivsqrtuslc4cmp (
input logic [4:0] Smsbs, // U1.4 leading bits of square root approximation
input logic [7:0] WSmsbs, WCmsbs, // Q4.4 residual most significant bits
input logic SqrtE,
input logic j0,j1, // are we on first (j0) or second step (j1) of digit selection
input logic j0, j1, // are we on first (j0) or second step (j1) of digit selection
output logic [3:0] udigit // {2, 1, -1, -2} digit is 0 if none are hot
);
logic [6:0] Wmsbs;
@ -71,23 +71,22 @@ module fdivsqrtuslc4cmp (
// handles special case when j = 0 or j = 1 for sqrt
assign mkj2 = 20; // when j = 1 use mk2[101] when j = 0 use anything bigger than 7.
assign mkj1 = j1 ? 8 : 0; // when j = 1 use mk1[101] = 8 and when j = 0 use 0 so we choose u_0 = 1
assign mkj1 = j0 ? 0 : 8; // when j = 1 use mk1[101] = 8 and when j = 0 use 0 so we choose u_0 = 1
assign sqrtspecial = SqrtE & (j1 | j0);
// Choose A for current operation
always_comb
if (SqrtE) begin
if (Smsbs[4]) A = 3'b111; // *** can we get rid of SMSBs case?
if (Smsbs[4]) A = 3'b111; // for S = 1.0000 *** can we optimize away this case?
else A = Smsbs[2:0];
end else A = Dmsbs;
// Choose selection constants based on a
assign mk2 = sqrtspecial ? mkj2 : mks2[A];
assign mk1 = sqrtspecial ? mkj1 : mks1[A];
assign mk0 = -mk1;
assign mkm1 = (A == 3'b000) ? -13 : -mk2; // asymmetry in table *** can we hide?
assign mkm1 = (A == 3'b000) ? -13 : -mk2; // asymmetry in table *** can we hide from critical path
// Compare residual W to selection constants to choose digit
always_comb