From fcb46138cebd57587d35489cef3a3a48ebe40bcf Mon Sep 17 00:00:00 2001
From: Clifford Wolf <clifford@clifford.at>
Date: Mon, 8 Sep 2014 16:59:39 +0200
Subject: Simplified $fa undef model

---
 kernel/consteval.h       |  4 ++++
 kernel/satgen.h          | 15 +--------------
 techlibs/common/simlib.v |  2 +-
 3 files changed, 6 insertions(+), 15 deletions(-)

diff --git a/kernel/consteval.h b/kernel/consteval.h
index 6e507bd5..2d29d3f7 100644
--- a/kernel/consteval.h
+++ b/kernel/consteval.h
@@ -214,6 +214,10 @@ struct ConstEval
 			RTLIL::Const t3 = const_and(sig_c.as_const(), t1, false, false, width);
 			RTLIL::Const val_x = const_or(t2, t3, false, false, width);
 
+			for (int i = 0; i < SIZE(val_y); i++)
+				if (val_y.bits[i] == RTLIL::Sx)
+					val_x.bits[i] = RTLIL::Sx;
+
 			set(sig_y, val_y);
 			set(sig_x, val_x);
 		}
diff --git a/kernel/satgen.h b/kernel/satgen.h
index 91f8ab40..692c6e7f 100644
--- a/kernel/satgen.h
+++ b/kernel/satgen.h
@@ -991,20 +991,7 @@ struct SatGen
 				std::vector<int> undef_x = importUndefSigSpec(cell->getPort("\\X"), timestep);
 
 				ez->assume(ez->vec_eq(undef_y, ez->vec_or(ez->vec_or(undef_a, undef_b), undef_c)));
-
-				std::vector<int> undef_t1 = ez->vec_or(undef_a, undef_b);
-
-				std::vector<int> a0 = ez->vec_and(ez->vec_not(a), ez->vec_not(undef_a));
-				std::vector<int> b0 = ez->vec_and(ez->vec_not(b), ez->vec_not(undef_b));
-				std::vector<int> undef_t2 = ez->vec_and(ez->vec_or(undef_a, undef_b), ez->vec_not(ez->vec_or(a0, b0)));
-
-				std::vector<int> c0 = ez->vec_and(ez->vec_not(c), ez->vec_not(undef_c));
-				std::vector<int> t10 = ez->vec_and(ez->vec_not(t1), ez->vec_not(undef_t1));
-				std::vector<int> undef_t3 = ez->vec_and(ez->vec_or(undef_c, undef_t1), ez->vec_not(ez->vec_or(c0, t10)));
-
-				std::vector<int> t21 = ez->vec_and(t2, ez->vec_not(undef_t2));
-				std::vector<int> t31 = ez->vec_and(t3, ez->vec_not(undef_t3));
-				ez->assume(ez->vec_eq(undef_x, ez->vec_and(ez->vec_or(undef_t2, undef_t3), ez->vec_not(ez->vec_or(t21, t31)))));
+				ez->assume(ez->vec_eq(undef_x, undef_y));
 
 				undefGating(y, yy, undef_y);
 				undefGating(x, xx, undef_x);
diff --git a/techlibs/common/simlib.v b/techlibs/common/simlib.v
index da745b5e..dd12bd39 100644
--- a/techlibs/common/simlib.v
+++ b/techlibs/common/simlib.v
@@ -453,7 +453,7 @@ output [WIDTH-1:0] X, Y;
 wire [WIDTH-1:0] t1, t2, t3;
 
 assign t1 = A ^ B, t2 = A & B, t3 = C & t1;
-assign Y = t1 ^ C, X = t2 | t3;
+assign Y = t1 ^ C, X = (t2 | t3) ^ (Y ^ Y);
 
 endmodule
 
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