From 571c7d3be4687d7cfdda585ce58508f1175c07ef Mon Sep 17 00:00:00 2001
From: David Harris <David_Harris@hmc.edu>
Date: Sun, 12 Nov 2023 19:41:12 -0800
Subject: [PATCH] Divider cleanup

---
 config/shared/config-shared.vh      |  4 ++--
 src/fpu/fdivsqrt/fdivsqrtcycles.sv  |  6 +++---
 src/fpu/fdivsqrt/fdivsqrtpreproc.sv | 13 ++++++++++---
 src/fpu/fdivsqrt/fdivsqrtuslc4.sv   |  2 +-
 4 files changed, 16 insertions(+), 9 deletions(-)

diff --git a/config/shared/config-shared.vh b/config/shared/config-shared.vh
index 9635d706b..55bca569f 100644
--- a/config/shared/config-shared.vh
+++ b/config/shared/config-shared.vh
@@ -98,8 +98,8 @@ localparam LOGR        = $clog2(RADIX);                             // r = log(R
 localparam RK          = LOGR*DIVCOPIES;                            // r*k bits per cycle generated
 
 // intermediate division parameters not directly used in fdivsqrt hardware
-localparam FPDIVMINb   = NF + 3; // minimum length of fractional part: Nf result bits + guard and round bits + 1 extra bit because square root could be shifted right *** explain better
-//localparam FPDIVMINb   = NF + 2 + (RADIX == 2); // minimum length of fractional part: Nf result bits + guard and round bits + 1 extra bit for preshifting radix2 square root right
+localparam FPDIVMINb   = NF + 3; // minimum length of fractional part: Nf result bits + guard and round bits + 1 extra bit to allow sqrt being shifted right
+//localparam FPDIVMINb   = NF + 2 + (RADIX == 2); // minimum length of fractional part: Nf result bits + guard and round bits + 1 extra bit for preshifting radix2 square root right, if radix4 doesn't use a right shift.  This version saves one cycle on double-precision with R=4,k=4.  However, it doesn't work yet because C is too short, so k is incorrectly calculated as a 1 in the lsb after the last step.
 localparam DIVMINb     = ((FPDIVMINb<XLEN) & IDIV_ON_FPU) ? XLEN : FPDIVMINb; // minimum fractional bits b = max(XLEN, FPDIVMINb)
 localparam RESBITS     = DIVMINb + LOGR; // number of bits in a result: r integer + b fractional
 
diff --git a/src/fpu/fdivsqrt/fdivsqrtcycles.sv b/src/fpu/fdivsqrt/fdivsqrtcycles.sv
index 6043ebb4a..e8a430a91 100644
--- a/src/fpu/fdivsqrt/fdivsqrtcycles.sv
+++ b/src/fpu/fdivsqrt/fdivsqrtcycles.sv
@@ -66,12 +66,12 @@ module fdivsqrtcycles import cvw::*;  #(parameter cvw_t P) (
   // P.DIVCOPIES = k. P.LOGR = log(R) = r.  P.RK = rk.  
   // Integer division needs p fractional + r integer result bits
   // FP Division needs at least Nf fractional bits + 2 guard/round bits and one integer digit (LOG R integer bits) = Nf + 2 + r bits
-  // FP Sqrt needs at least Nf fractional bits, 2 guard/round bits, and *** shift bits
+  // FP Sqrt needs at least Nf fractional bits and 2 guard/round bits.  The integer bit is always initialized to 1 and does not need a cycle.
   // The datapath produces rk bits per cycle, so Cycles = ceil (ResultBitsE / rk)
 
   always_comb begin 
-    if (SqrtE) FPResultBitsE = Nf + 2 + 0; // Nf + two fractional bits for round/guard; integer bit implicit
-    else       FPResultBitsE = Nf + 2 + P.LOGR; // Nf + two fractional bits for round/guard + integer bits - try this when placing results in msbs
+    if (SqrtE) FPResultBitsE = Nf + 2 + 0; // Nf + two fractional bits for round/guard; integer bit implicit because starting at n=1
+    else       FPResultBitsE = Nf + 2 + P.LOGR; // Nf + two fractional bits for round/guard + integer bits 
 
     if (P.IDIV_ON_FPU) ResultBitsE = IntDivE ? IntResultBitsE : FPResultBitsE;
     else               ResultBitsE = FPResultBitsE;
diff --git a/src/fpu/fdivsqrt/fdivsqrtpreproc.sv b/src/fpu/fdivsqrt/fdivsqrtpreproc.sv
index ecdf10f8b..145bf9a68 100644
--- a/src/fpu/fdivsqrt/fdivsqrtpreproc.sv
+++ b/src/fpu/fdivsqrt/fdivsqrtpreproc.sv
@@ -168,14 +168,20 @@ module fdivsqrtpreproc import cvw::*;  #(parameter cvw_t P) (
   // This also means only one extra fractional bit is needed becaue we never shift right by more than 1.
   // Radix      Exponent odd          Exponent Even
   // 2          x-2 = 2(x/2 - 1)      x/2 - 2 = 2(x/4 - 1)
-  // 4          2x-4 = 4(x/2 - 1))    x-4 = 4(x/4 - 1)
+  // 4          2(x)-4 = 4(x/2 - 1))  2(x/2)-4 = 4(x/4 - 1)
   // Summary: PreSqrtX = r(x/2or4 - 1)
 
+  logic [P.DIVb:0] PreSqrtX;
   assign EvenExp = Xe[0] ^ ell[0]; // effective unbiased exponent after normalization is even
-/*  mux2 #(P.DIVb+1) sqrtxmux(Xnorm, {1'b0, Xnorm[P.DIVb:1]}, EvenExp, PreSqrtX); // X if exponent odd, X/2 if exponent even
+  mux2 #(P.DIVb+1) sqrtxmux(Xnorm, {1'b0, Xnorm[P.DIVb:1]}, EvenExp, PreSqrtX); // X if exponent odd, X/2 if exponent even
   if (P.RADIX == 2) assign SqrtX = {3'b111, PreSqrtX};                          // PreSqrtX - 2 = 2(PreSqrtX/2 - 1)
-  else              assign SqrtX = {2'b11, PreSqrtX, 1'b0};                     // 2PreSqrtX - 4 = 4(PreSqrtX/2 - 1) */
+  else              assign SqrtX = {2'b11, PreSqrtX, 1'b0};                     // 2PreSqrtX - 4 = 4(PreSqrtX/2 - 1) 
 
+/*  
+  // Attempt to optimize radix 4 to use a left shift by 1 or zero initially, followed by no more left shift
+  // This saves one bit in DIVb because there is no initial right shift.
+  // However, C needs to be extended further, lest it create a k with a 1 in the lsb when C is all 1s.
+  // That is an optimization for another day.
   if (P.RADIX == 2) begin
     logic [P.DIVb:0] PreSqrtX;    // U1.DIVb
     mux2 #(P.DIVb+1) sqrtxmux(Xnorm, {1'b0, Xnorm[P.DIVb:1]}, EvenExp, PreSqrtX); // X if exponent odd, X/2 if exponent even
@@ -185,6 +191,7 @@ module fdivsqrtpreproc import cvw::*;  #(parameter cvw_t P) (
     mux2 #(P.DIVb+2) sqrtxmux({Xnorm, 1'b0}, {1'b0, Xnorm}, EvenExp, PreSqrtX); // 2X if exponent odd, X if exponent even
     assign SqrtX = {2'b11, PreSqrtX};                     // PreSqrtX - 4 = 4(PreSqrtX/4 - 1)
   end
+*/
 
   // Initialize X for division or square root
   mux2 #(P.DIVb+4) prexmux(DivX, SqrtX, SqrtE, PreShiftX);                    
diff --git a/src/fpu/fdivsqrt/fdivsqrtuslc4.sv b/src/fpu/fdivsqrt/fdivsqrtuslc4.sv
index 268ca9ea2..b44b34a35 100644
--- a/src/fpu/fdivsqrt/fdivsqrtuslc4.sv
+++ b/src/fpu/fdivsqrt/fdivsqrtuslc4.sv
@@ -103,7 +103,7 @@ module fdivsqrtuslc4 (
     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 = S (in U0.3 format)
+      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)
 
   // Select quotient digit from lookup table based on A and W