From 5bb521635e2839fe14b054a161fcc00e4230fc00 Mon Sep 17 00:00:00 2001
From: David Harris <david_harris@hmc.edu>
Date: Sun, 17 Apr 2022 21:43:12 +0000
Subject: [PATCH] Experiments with prefix comparator; minor fixes in WFI and
 testbench warnings

---
 pipelined/src/ieu/comparator.sv        | 43 +++++++++++++++++++++++++-
 pipelined/src/privileged/privileged.sv |  5 +--
 pipelined/testbench/testbench.sv       |  4 +--
 3 files changed, 47 insertions(+), 5 deletions(-)

diff --git a/pipelined/src/ieu/comparator.sv b/pipelined/src/ieu/comparator.sv
index f1a66861..67fd476d 100644
--- a/pipelined/src/ieu/comparator.sv
+++ b/pipelined/src/ieu/comparator.sv
@@ -52,6 +52,47 @@ module comparator #(parameter WIDTH=32) (
   assign overflow = (a[WIDTH-1] ^ b[WIDTH-1]) & (a[WIDTH-1] ^ diff[WIDTH-1]);
   assign lt = neg ^ overflow;
   assign ltu = ~carry;
-  assign flags = {eq, lt, ltu};
+//  assign flags = {eq, lt, ltu};
+
+  /* verilator lint_off UNOPTFLAT */
+
+  // prefix implementation
+  localparam levels=$clog2(WIDTH);
+  genvar i;
+  genvar level;
+  logic [WIDTH-1:0] ee[levels:0];
+  logic [WIDTH-1:0] ll[levels:0];
+  logic eq2, lt2, ltu2;
+
+
+  // Bitwise logic
+  for (i=0; i<WIDTH; i++) begin
+    assign ee[0][i] = a[i] ~^ b[i]; // bitwise equality
+    assign ll[0][i] = ~a[i] & b[i]; // bitwise less than unsigned
+  end
+
+  // Recursion
+  for (level = 1; level<=levels; level++) begin
+    for (i=0; i<WIDTH/(2**level); i++) begin
+      assign ee[level][i] = ee[level-1][i*2+1] & ee[level-1][i*2];
+      assign ll[level][i] = ll[level-1][i*2+1] | ee[level-1][i*2+1] & ll[level-1][i*2];
+    end
+  end
+
+  // Output logic
+  assign eq2 = ee[levels][0];
+  assign ltu2 = ll[levels][0];
+  assign lt2 = ltu2 & ~ll[0][WIDTH-1] | a[WIDTH-1] & ~b[WIDTH-1];
+
+  always_comb begin 
+    assert (eq2 === eq) else $display("a %h b %h eq %b eq2 %b\n", a, b, eq, eq2);
+    assert (ltu2 === ltu) else $display("a %h b %h ltu %b ltu2 %b\n", a, b, ltu, ltu2);
+    assert (lt2 === lt) else $display("a %h b %h lt %b lt2 %b ltu2 %b L31 %b\n", a, b, lt, lt2, ltu2, ll[0][WIDTH-1]);
+  end
+
+  assign flags = {eq2, lt2, ltu2};
+
+  /* verilator lint_on UNOPTFLAT */
+
 endmodule
 
diff --git a/pipelined/src/privileged/privileged.sv b/pipelined/src/privileged/privileged.sv
index c60f338e..b429a14c 100644
--- a/pipelined/src/privileged/privileged.sv
+++ b/pipelined/src/privileged/privileged.sv
@@ -136,8 +136,9 @@ module privileged (
   // WFI timeout Privileged Spec 3.1.6.5
   ///////////////////////////////////////////
   if (`U_SUPPORTED) begin
-    logic [`WFI_TIMEOUT_BIT:0] WFICount;
-    floprc #(`WFI_TIMEOUT_BIT+1) wficountreg(clk, reset, ~wfiM, WFICount+1, WFICount);  // count while in WFI
+    logic [`WFI_TIMEOUT_BIT:0] WFICount, WFICountPlus1;
+    assign WFICountPlus1 = WFICount + 1;
+    floprc #(`WFI_TIMEOUT_BIT+1) wficountreg(clk, reset, ~wfiM, WFICountPlus1, WFICount);  // count while in WFI
     assign WFITimeoutM = STATUS_TW & PrivilegeModeW != `M_MODE & WFICount[`WFI_TIMEOUT_BIT]; 
   end else assign WFITimeoutM = 0;
 
diff --git a/pipelined/testbench/testbench.sv b/pipelined/testbench/testbench.sv
index 2b8e6df8..bf4903e5 100644
--- a/pipelined/testbench/testbench.sv
+++ b/pipelined/testbench/testbench.sv
@@ -361,8 +361,8 @@ module riscvassertions;
 //    assert (`MEM_DCACHE == 0 | `MEM_DTIM == 0) else $error("Can't simultaneously have a data cache and TIM");
     assert (`DMEM == `MEM_CACHE | `VIRTMEM_SUPPORTED ==0) else $error("Virtual memory needs dcache");
     assert (`IMEM == `MEM_CACHE | `VIRTMEM_SUPPORTED ==0) else $error("Virtual memory needs icache");
-    assert (`DMEM == `MEM_CACHE | `DBUS ==0) else $error("Dcache rquires DBUS.");
-    assert (`IMEM == `MEM_CACHE | `IBUS ==0) else $error("Icache rquires IBUS.");    
+    //assert (`DMEM == `MEM_CACHE | `DBUS ==0) else $error("Dcache rquires DBUS.");
+    //assert (`IMEM == `MEM_CACHE | `IBUS ==0) else $error("Icache rquires IBUS.");    
   end
 endmodule