From 1ce2ab5daa7cd8d7b708894d6443bc00916c6905 Mon Sep 17 00:00:00 2001
From: Alec Vercruysse <vercruysse.alec@gmail.com>
Date: Tue, 11 Apr 2023 23:05:56 -0700
Subject: [PATCH] Coverage and readability improvements to LRUUpdate logic

The genvar stuff was switched to readable names to make it easier
to understand for the first time. In the LRUUpdate logic for loop,
a special case was added for simpler logic in the case of the root
node, to hit coverage.
---
 src/cache/cacheLRU.sv | 40 +++++++++++++++++++++++++---------------
 1 file changed, 25 insertions(+), 15 deletions(-)

diff --git a/src/cache/cacheLRU.sv b/src/cache/cacheLRU.sv
index d8de983f..5f1c199a 100644
--- a/src/cache/cacheLRU.sv
+++ b/src/cache/cacheLRU.sv
@@ -89,16 +89,26 @@ module cacheLRU
     assign WayExpanded[StartIndex : EndIndex] = {{DuplicationFactor}{WayEncoded[row]}};
   end
 
-  genvar               r, a, s;
+  genvar               node;
   assign LRUUpdate[NUMWAYS-2] = '1;
-  for(s = NUMWAYS-2; s >= NUMWAYS/2; s--) begin : enables
-    localparam p = NUMWAYS - s - 1;
-    localparam g = log2(p);
-    localparam t0 = s - p;
-    localparam t1 = t0 - 1;
-    localparam r = LOGNUMWAYS - g;
-    assign LRUUpdate[t0] = LRUUpdate[s] & ~WayEncoded[r];
-    assign LRUUpdate[t1] = LRUUpdate[s] & WayEncoded[r];
+  for(node = NUMWAYS-2; node >= NUMWAYS/2; node--) begin : enables
+    localparam ctr = NUMWAYS - node - 1;
+    localparam ctr_depth = log2(ctr);
+    localparam lchild = node - ctr;
+    localparam rchild = lchild - 1;
+    localparam r = LOGNUMWAYS - ctr_depth;
+
+    // the child node will be updated if its parent was updated and
+    // the WayEncoded bit was the correct value.
+    // The if statement is only there for coverage since LRUUpdate[root] is always 1.
+    if (node == NUMWAYS-2) begin
+      assign LRUUpdate[lchild] = ~WayEncoded[r];
+      assign LRUUpdate[rchild] = WayEncoded[r];
+    end
+    else begin
+      assign LRUUpdate[lchild] = LRUUpdate[node] & ~WayEncoded[r];
+      assign LRUUpdate[rchild] = LRUUpdate[node] & WayEncoded[r];
+    end
   end
 
   // The root node of the LRU tree will always be selected in LRUUpdate. No mux needed.
@@ -106,15 +116,15 @@ module cacheLRU
   mux2 #(1) LRUMuxes[NUMWAYS-3:0](CurrLRU[NUMWAYS-3:0], ~WayExpanded[NUMWAYS-3:0], LRUUpdate[NUMWAYS-3:0], NextLRU[NUMWAYS-3:0]);
 
   // Compute next victim way.
-  for(s = NUMWAYS-2; s >= NUMWAYS/2; s--) begin
-    localparam t0 = 2*s - NUMWAYS;
+  for(node = NUMWAYS-2; node >= NUMWAYS/2; node--) begin
+    localparam t0 = 2*node - NUMWAYS;
     localparam t1 = t0 + 1;
-    assign Intermediate[s] = CurrLRU[s] ? Intermediate[t0] : Intermediate[t1];
+    assign Intermediate[node] = CurrLRU[node] ? Intermediate[t0] : Intermediate[t1];
   end
-  for(s = NUMWAYS/2-1; s >= 0; s--) begin
-    localparam int0 = (NUMWAYS/2-1-s)*2;
+  for(node = NUMWAYS/2-1; node >= 0; node--) begin
+    localparam int0 = (NUMWAYS/2-1-node)*2;
     localparam int1 = int0 + 1;
-    assign Intermediate[s] = CurrLRU[s] ? int1[LOGNUMWAYS-1:0] : int0[LOGNUMWAYS-1:0];
+    assign Intermediate[node] = CurrLRU[node] ? int1[LOGNUMWAYS-1:0] : int0[LOGNUMWAYS-1:0];
   end
 
   logic [NUMWAYS-1:0] FirstZero;