Consistent $mux undef handling

* Change simlib's $mux cell to use the ternary operator as $_MUX_
  already does
* Stop opt_expr -keepdc from changing S=x to S=0
* Change const eval of $mux and $pmux to match the updated simlib
  (fixes sim)
* The sat behavior of $mux already matches the updated simlib

The verilog frontend uses $mux for the ternary operators and this
changes all interpreations of the $mux cell (that I found) to match the
verilog simulation behavior for the ternary operator. For 'if' and
'case' expressions the frontend may also use $mux but uses $eqx if the
verilog simulation behavior is requested with the '-ifx' option.

For $pmux there is a remaining mismatch between the sat behavior and the
simlib behavior. Resolving this requires more discussion, as the $pmux
cell does not directly correspond to a specific verilog construct.

kernel/calc.cc

@@ -609,6 +609,36 @@ RTLIL::Const RTLIL::const_neg(const RTLIL::Const &arg1, const RTLIL::Const&, boo
 	return RTLIL::const_sub(zero, arg1_ext, true, signed1, result_len);
 }
 
+RTLIL::Const RTLIL::const_mux(const RTLIL::Const &arg1, const RTLIL::Const &arg2, const RTLIL::Const &arg3)
+{
+	log_assert(arg2.size() == arg1.size());
+	if (arg3[0] == State::S0)
+		return arg1;
+	else if (arg3[0] == State::S1)
+		return arg2;
+
+	RTLIL::Const ret = arg1;
+	for (int i = 0; i < ret.size(); i++)
+		if (ret[i] != arg2[i])
+			ret[i] = State::Sx;
+	return ret;
+}
+
+RTLIL::Const RTLIL::const_pmux(const RTLIL::Const &arg1, const RTLIL::Const &arg2, const RTLIL::Const &arg3)
+{
+	if (arg3.is_fully_zero())
+		return arg1;
+
+	if (!arg3.is_onehot())
+		return RTLIL::Const(State::Sx, arg1.size());
+
+	for (int i = 0; i < arg3.size(); i++)
+		if (arg3[i] == State::S1)
+			return RTLIL::Const(std::vector<RTLIL::State>(arg2.bits.begin() + i*arg1.bits.size(), arg2.bits.begin() + (i+1)*arg1.bits.size()));
+
+	log_abort(); // unreachable
+}
+
 RTLIL::Const RTLIL::const_bmux(const RTLIL::Const &arg1, const RTLIL::Const &arg2)
 {
 	std::vector<RTLIL::State> t = arg1.bits;

kernel/celltypes.h

@@ -488,16 +488,10 @@ struct CellTypes
 
 	static RTLIL::Const eval(RTLIL::Cell *cell, const RTLIL::Const &arg1, const RTLIL::Const &arg2, const RTLIL::Const &arg3, bool *errp = nullptr)
 	{
-		if (cell->type.in(ID($mux), ID($pmux), ID($_MUX_))) {
-			RTLIL::Const ret = arg1;
-			for (size_t i = 0; i < arg3.bits.size(); i++)
-				if (arg3.bits[i] == RTLIL::State::S1) {
-					std::vector<RTLIL::State> bits(arg2.bits.begin() + i*arg1.bits.size(), arg2.bits.begin() + (i+1)*arg1.bits.size());
-					ret = RTLIL::Const(bits);
-				}
-			return ret;
-		}
-
+		if (cell->type.in(ID($mux), ID($_MUX_)))
+			return const_mux(arg1, arg2, arg3);
+		if (cell->type == ID($pmux))
+			return const_pmux(arg1, arg2, arg3);
 		if (cell->type == ID($_AOI3_))
 			return eval_not(const_or(const_and(arg1, arg2, false, false, 1), arg3, false, false, 1));
 		if (cell->type == ID($_OAI3_))

kernel/rtlil.h

@@ -500,6 +500,8 @@ namespace RTLIL
 	RTLIL::Const const_pos         (const RTLIL::Const &arg1, const RTLIL::Const &arg2, bool signed1, bool signed2, int result_len);
 	RTLIL::Const const_neg         (const RTLIL::Const &arg1, const RTLIL::Const &arg2, bool signed1, bool signed2, int result_len);
 
+	RTLIL::Const const_mux         (const RTLIL::Const &arg1, const RTLIL::Const &arg2, const RTLIL::Const &arg3);
+	RTLIL::Const const_pmux        (const RTLIL::Const &arg1, const RTLIL::Const &arg2, const RTLIL::Const &arg3);
 	RTLIL::Const const_bmux        (const RTLIL::Const &arg1, const RTLIL::Const &arg2);
 	RTLIL::Const const_demux       (const RTLIL::Const &arg1, const RTLIL::Const &arg2);
 

passes/opt/opt_expr.cc

@@ -1494,7 +1494,7 @@ skip_identity:
 			RTLIL::SigSpec input = assign_map(cell->getPort(ID::S));
 			RTLIL::SigSpec inA = assign_map(cell->getPort(ID::A));
 			RTLIL::SigSpec inB = assign_map(cell->getPort(ID::B));
-			if (input.is_fully_const())
+			if (input.is_fully_const() && (!keepdc || input.is_fully_def()))
 				ACTION_DO(ID::Y, input.as_bool() ? cell->getPort(ID::B) : cell->getPort(ID::A));
 			else if (inA == inB)
 				ACTION_DO(ID::Y, cell->getPort(ID::A));

techlibs/common/simlib.v

@@ -1282,10 +1282,7 @@ input S;
 output reg [WIDTH-1:0] Y;
 
 always @* begin
-	if (S)
-		Y = B;
-	else
-		Y = A;
+	assign Y = S ? B : A;
 end
 
 endmodule