muxpack: reject slicing of predecessor Y ports in mux chain

passes/opt/muxpack.cc

@@ -150,6 +150,7 @@ struct MuxpackWorker
 			}
 		}
 
+		std::vector<Cell*> mux_cells;
 		for (auto cell : module->cells())
 		{
 			if (cell->type.in(ID($mux), ID($pmux)) && !cell->get_bool_attribute(ID::keep))
@@ -179,6 +180,7 @@ struct MuxpackWorker
 				}
 
 				sig_chain_prev[y_sig] = cell;
+				mux_cells.push_back(cell);
 				continue;
 			}
 
@@ -187,6 +189,30 @@ struct MuxpackWorker
 					for (auto bit : sigmap(conn.second))
 						sigbit_with_non_chain_users.insert(bit);
 		}
+
+		// A port can serve as a chain link only if its full sigspec matches
+		// some chain producer's Y. Bits used by any other input port
+		// have non-chain users.
+		for (auto cell : mux_cells)
+		{
+			SigSpec a_sig = sigmap(cell->getPort(ID::A));
+			SigSpec b_sig;
+			if (cell->type == ID($mux))
+				b_sig = sigmap(cell->getPort(ID::B));
+			SigSpec s_sig = sigmap(cell->getPort(ID::S));
+
+			bool a_is_chain_link = sig_chain_prev.count(a_sig);
+			bool b_is_chain_link = !b_sig.empty() && sig_chain_prev.count(b_sig);
+
+			if (!a_is_chain_link)
+				for (auto bit : a_sig)
+					sigbit_with_non_chain_users.insert(bit);
+			if (!b_is_chain_link)
+				for (auto bit : b_sig)
+					sigbit_with_non_chain_users.insert(bit);
+			for (auto bit : s_sig)
+				sigbit_with_non_chain_users.insert(bit);
+		}
 	}
 
 	bool is_start_cell(Cell* cell)

tests/various/muxpack.v

@@ -238,6 +238,16 @@ module case_overlap (
         end
 endmodule
 
+module chain_slice_self_input (
+        input [31:0] A,
+        input [1:0] S,
+        output [31:0] Y
+);
+        wire [31:0] tmp;
+        assign tmp = S == 1 ? {A[7:0], A[7:0], A[7:0], A[7:0]} : A;
+        assign Y = S == 2 ? {A[15:0], tmp[15:0]} : tmp;
+endmodule
+
 module case_overlap2 (
         input wire [2:0] x,
         input wire a, b, c, d, e,

tests/various/muxpack.ys

@@ -249,6 +249,21 @@ design -import gate -as gate
 miter -equiv -flatten -make_assert -make_outputs gold gate miter
 sat -verify -prove-asserts -show-ports miter
 
+design -load read
+hierarchy -top chain_slice_self_input
+prep
+design -save gold
+muxpack
+opt
+#stat
+select -assert-count 2 t:$mux
+select -assert-count 0 t:$pmux
+design -stash gate
+design -import gold -as gold
+design -import gate -as gate
+miter -equiv -flatten -make_assert -make_outputs gold gate miter
+sat -verify -prove-asserts -show-ports miter
+
 design -load read
 hierarchy -top case_overlap2
 #prep # Do not prep otherwise $pmux's overlapping entry will get removed