From 34e0b3bc61c81ccee377fb2ea2570ffcfa2cacd4 Mon Sep 17 00:00:00 2001
From: David Harris <david_harris@hmc.edu>
Date: Fri, 24 Mar 2023 08:27:30 -0700
Subject: [PATCH] Shifter sign simplification and capitalization

---
 src/ieu/alu.sv     |  5 +----
 src/ieu/shifter.sv | 19 +++++++++++--------
 2 files changed, 12 insertions(+), 12 deletions(-)

diff --git a/src/ieu/alu.sv b/src/ieu/alu.sv
index 11759b1d..47af5f4d 100644
--- a/src/ieu/alu.sv
+++ b/src/ieu/alu.sv
@@ -51,14 +51,11 @@ module alu #(parameter WIDTH=32) (
   logic             Carry, Neg;                                                   // Flags: carry out, negative
   logic             LT, LTU;                                                      // Less than, Less than unsigned
   logic             Asign, Bsign;                                                 // Sign bits of A, B
-  logic             ShiftSignA;
 
   // A, A sign bit muxes
   if (WIDTH == 64) begin
-    mux3 #(1) signmux(A[63], A[31], 1'b0, {~SubArith, W64}, ShiftSignA);
     mux3 #(64) extendmux({{32{1'b0}}, A[31:0]},{{32{A[31]}}, A[31:0]}, A, {~W64, SubArith}, CondExtA); // bottom 32 bits are always A[31:0], so effectively a 32-bit upper mux
   end else begin 
-    mux2 #(1) signmux(1'b0, A[31], SubArith, ShiftSignA);
     assign CondExtA = A;
   end
 
@@ -67,7 +64,7 @@ module alu #(parameter WIDTH=32) (
   assign {Carry, Sum} = CondShiftA + CondMaskInvB + {{(WIDTH-1){1'b0}}, SubArith};
   
   // Shifts (configurable for rotation)
-  shifter sh(.A(CondExtA), .Sign(ShiftSignA), .Amt(B[`LOG_XLEN-1:0]), .Right(Funct3[2]), .W64, .Y(Shift), .Rotate(BALUControl[2]));
+  shifter sh(.A(CondExtA), .Amt(B[`LOG_XLEN-1:0]), .Right(Funct3[2]), .W64, .SubArith, .Y(Shift), .Rotate(BALUControl[2]));
 
   // Condition code flags are based on subtraction output Sum = A-B.
   // Overflow occurs when the numbers being subtracted have the opposite sign 
diff --git a/src/ieu/shifter.sv b/src/ieu/shifter.sv
index d0a6471c..8dbdf88e 100644
--- a/src/ieu/shifter.sv
+++ b/src/ieu/shifter.sv
@@ -30,13 +30,16 @@
 `include "wally-config.vh"
 
 module shifter (
-  input  logic [`XLEN-1:0]     A,                           // shift Source
-  input  logic [`LOG_XLEN-1:0] Amt,                         // Shift amount
-  input  logic                 Right, Rotate, W64, Sign,    // Shift right, rotate signals
-  output logic [`XLEN-1:0]     Y);                          // Shifted result
+  input  logic [`XLEN-1:0]     A,                             // shift Source
+  input  logic [`LOG_XLEN-1:0] Amt,                           // Shift amount
+  input  logic                 Right, Rotate, W64, SubArith, // Shift right, rotate, W64-type operation, arithmetic shift
+  output logic [`XLEN-1:0]     Y);                            // Shifted result
 
-  logic [2*`XLEN-2:0]      z, zshift;                       // Input to funnel shifter, shifted amount before truncated to 32 or 64 bits
-  logic [`LOG_XLEN-1:0]    amttrunc, offset;                // Shift amount adjusted for RV64, right-shift amount
+  logic [2*`XLEN-2:0]      z, zshift;                         // Input to funnel shifter, shifted amount before truncated to 32 or 64 bits
+  logic [`LOG_XLEN-1:0]    amttrunc, Offset;                  // Shift amount adjusted for RV64, right-shift amount
+  logic                    Sign;                              // Sign bit for sign extension
+
+  assign Sign = A[`XLEN-1] & SubArith;  // sign bit for sign extension
 
   if (`ZBB_SUPPORTED) begin: rotfunnel
     if (`XLEN==32) begin // rv32 with rotates
@@ -76,10 +79,10 @@ module shifter (
   end
   
   // Opposite offset for right shifts
-  assign offset = Right ? amttrunc : ~amttrunc;
+  assign Offset = Right ? amttrunc : ~amttrunc;
   
   // Funnel operation
-  assign zshift = z >> offset;
+  assign zshift = z >> Offset;
   assign Y = zshift[`XLEN-1:0];    
 endmodule