Added heuristics to further reduce LUT equations.

1 files changed, 12 insertions, 5 deletions

diff --git a/icebox/icebox_vlog.py b/icebox/icebox_vlog.py
index 4a79cd4..4d1a772 100755
--- a/icebox/icebox_vlog.py
+++ b/icebox/icebox_vlog.py
@@ -810,17 +810,24 @@ for lut in luts_queue:
     elif not "0" in lut_bits:
         const_assigns.append([net_out, "1'b1"])
     else:
+        def equals_1(expr):
+            return (expr == "1") or (expr == "1'b1")
+        def equals_0(expr):
+            return (expr == "0") or (expr == "1'b0")
         def make_lut_expr(bits, sigs):
             if not sigs:
                 return "1'b%s" % bits[0]
             l_expr = make_lut_expr(bits[0:len(bits)//2], sigs[1:])
             h_expr = make_lut_expr(bits[len(bits)//2:len(bits)], sigs[1:])
             if h_expr == l_expr: return h_expr
-            if sigs[0] == "0": return l_expr
-            if sigs[0] == "1": return h_expr
-            if h_expr == "1" and l_expr == "0": return sigs[0]
-            if h_expr == "0" and l_expr == "1": return "!" + sigs[0]
-            return "%s ? %s : %s" % (sigs[0], h_expr, l_expr)
+            if equals_0(sigs[0]): return l_expr
+            if equals_1(sigs[0]): return h_expr
+            if equals_1(h_expr) and equals_0(l_expr): return sigs[0]
+            if equals_0(h_expr) and equals_1(l_expr): return "!" + sigs[0]
+            if h_expr == "!" + l_expr: return "(%s ^ %s)" % (sigs[0], l_expr)
+            if equals_0(h_expr): return "(%s & !%s)" % (sigs[0], l_expr)
+            if equals_0(l_expr): return "(%s & %s)" % (sigs[0], h_expr)
+            return "(%s ? %s : %s)" % (sigs[0], h_expr, l_expr)
         lut_expr = make_lut_expr(lut_bits, [net_in3, net_in2, net_in1, net_in0])
         lut_assigns.append([net_out, "/* LUT   %2d %2d %2d */ %s" % (lut[0], lut[1], lut[2], lut_expr)])
     max_net_len = max(max_net_len, len(net_out))