diff --git a/Makefile b/Makefile
index 5b4e047e3..abf68138d 100644
--- a/Makefile
+++ b/Makefile
@@ -738,6 +738,7 @@ OBJS += passes/cmds/plugin.o
 
 OBJS += passes/cmds/check.o
 OBJS += passes/cmds/clean_zerowidth.o
+OBJS += passes/cmds/selectconst.o
 OBJS += passes/cmds/setattr.o
 OBJS += passes/cmds/splitcells.o
 OBJS += passes/cmds/splitfanout.o
diff --git a/passes/cmds/Makefile.inc b/passes/cmds/Makefile.inc
index 2fde35346..4f0d14176 100644
--- a/passes/cmds/Makefile.inc
+++ b/passes/cmds/Makefile.inc
@@ -6,6 +6,7 @@ OBJS += passes/cmds/add.o
 OBJS += passes/cmds/delete.o
 OBJS += passes/cmds/design.o
 OBJS += passes/cmds/select.o
+OBJS += passes/cmds/selectconst.o
 OBJS += passes/cmds/show.o
 OBJS += passes/cmds/viz.o
 OBJS += passes/cmds/rename.o
diff --git a/passes/cmds/selectconst.cc b/passes/cmds/selectconst.cc
new file mode 100644
index 000000000..c6f0643ff
--- /dev/null
+++ b/passes/cmds/selectconst.cc
@@ -0,0 +1,85 @@
+/*
+ *  yosys -- Yosys Open SYnthesis Suite
+ *
+ *  Copyright (C) 2012  Claire Xenia Wolf <claire@yosyshq.com>
+ *
+ *  Permission to use, copy, modify, and/or distribute this software for any
+ *  purpose with or without fee is hereby granted, provided that the above
+ *  copyright notice and this permission notice appear in all copies.
+ *
+ *  THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+ *  WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+ *  MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+ *  ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+ *  WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
+ *  ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+ *  OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+ *
+ */
+
+#include "kernel/yosys.h"
+#include "kernel/celltypes.h"
+#include "kernel/sigtools.h"
+#include <string.h>
+#include <errno.h>
+
+USING_YOSYS_NAMESPACE
+PRIVATE_NAMESPACE_BEGIN
+
+struct SelectConstPass : public Pass {
+	SelectConstPass() : Pass("selectconst", "select objects with (mostly) constant input bits") { }
+	void help() override
+	{
+		//   |---v---|---v---|---v---|---v---|---v---|---v---|---v---|---v---|---v---|---v---|
+		log("\n");
+		log("    selectconst [ -frac <frac> ] {<selection>}\n");
+		log("\n");
+		log("Select cells with (mostly) constant input bits.\n");
+		log("\n");
+		log("    -frac\n");
+		log("        the fraction of constant input bits (default: 1.0).\n");
+		log("\n");
+	}
+	void execute(std::vector<std::string> args, RTLIL::Design *design) override
+	{
+		float frac = 1;
+
+		size_t argidx;
+		for (argidx = 1; argidx < args.size(); argidx++)
+		{
+			std::string arg = args[argidx];
+			if (arg == "-frac") {
+				frac = std::stof(args[++argidx]);
+				continue;
+			}
+			break;
+		}
+		extra_args(args, argidx, design);
+
+		for (auto module : design->selected_modules())
+		{
+			SigMap sigmap(module);
+
+			for (auto cell : module->selected_cells())
+			{
+				for (auto conn : cell->connections()) {
+					if (!cell->input(conn.first) || GetSize(conn.second) == 0)
+						continue;
+
+					SigPool sigpool;
+					for (int i = 0; i < GetSize(conn.second); i++) {
+						if (conn.second[i].wire == nullptr)
+							continue;
+						sigpool.add(sigmap(conn.second[i]));
+					}
+					if ((float) GetSize(sigpool) / GetSize(conn.second) >= frac) {
+						cell->set_bool_attribute("\\mostlyconst");
+						log("Marking %s as mostly constant (%d/%d bits constant).\n", log_id(cell), GetSize(sigpool), GetSize(conn.second));
+					}
+				}
+			}
+		}
+	}
+} SelectConstPass;
+
+PRIVATE_NAMESPACE_END
diff --git a/passes/techmap/bmuxmap.cc b/passes/techmap/bmuxmap.cc
index 1495e6247..39dbfd420 100644
--- a/passes/techmap/bmuxmap.cc
+++ b/passes/techmap/bmuxmap.cc
@@ -99,6 +99,7 @@ struct BmuxmapPass : public Pass {
 				while (module->count_id(new_id) > 0) new_id = IdString("$" + new_id.str());
 				RTLIL::Cell *pmux = module->addPmux(new_id, new_a, data, new_s, new_data);
 				pmux->add_strpool_attribute(ID::src, cell->get_strpool_attribute(ID::src));
+				pmux->set_bool_attribute(IdString("\\bmuxmap"));
 				data = new_data;
 			}
 			else
@@ -112,6 +113,7 @@ struct BmuxmapPass : public Pass {
 							sel[idx],
 							new_data.extract(i, width));
 						mux->add_strpool_attribute(ID::src, cell->get_strpool_attribute(ID::src));
+						mux->set_bool_attribute(IdString("\\bmuxmap"));
 					}
 					data = new_data;
 				}