diff --git a/README b/README
index cbd77bb17..97f2ba9b1 100644
--- a/README
+++ b/README
@@ -40,14 +40,25 @@ or the 2-clause BSD license).
 Getting Started
 ===============
 
-To build Yosys simply type 'make' in this directory. You need
-a C++ compiler with C++11 support (up-to-date CLANG or GCC is
-recommended) and some standard tools such as GNU Flex, GNU Bison,
-and GNU Make. It might be necessary to make some changes to
-the config section of the Makefile. The extensive tests require
-Icarus Verilog.
+You need a C++ compiler with C++11 support (up-to-date CLANG or GCC is
+recommended) and some standard tools such as GNU Flex, GNU Bison, and
+GNU Make. The extensive tests require Icarus Verilog.
+
+To configure the build system to use a specific set of compiler and
+build configuration, use one of
+
+	$ make config-clang-debug
+	$ make config-gcc-debug
+	$ make config-release
+
+For other compilers and build configurations it might be
+necessary to make some changes to the config section of the
+Makefile.
 
 	$ vi Makefile
+
+To build Yosys simply type 'make' in this directory.
+
 	$ make
 	$ make test
 	$ sudo make install
@@ -237,3 +248,5 @@ TODOs / Open Bugs
 
 - Better FSM state encoding
 
+- For pass' "fsm_detect" help: add notes what criteria lets it detect an FSM
+
diff --git a/kernel/register.cc b/kernel/register.cc
index ebb834c8f..a61548b64 100644
--- a/kernel/register.cc
+++ b/kernel/register.cc
@@ -132,7 +132,7 @@ void Pass::extra_args(std::vector<std::string> args, size_t argidx, RTLIL::Desig
 void Pass::call(RTLIL::Design *design, std::string command)
 {
 	std::vector<std::string> args;
-	char *s = strdup(command.c_str()), *saveptr;
+	char *s = strdup(command.c_str()), *sstart = s, *saveptr;
 	s += strspn(s, " \t\r\n");
 	if (*s == 0 || *s == '#')
 		return;
@@ -160,7 +160,7 @@ void Pass::call(RTLIL::Design *design, std::string command)
 		} else
 			args.push_back(str);
 	}
-	free(s);
+	free(sstart);
 	call(design, args);
 }
 
diff --git a/passes/opt/opt_const.cc b/passes/opt/opt_const.cc
index 4d00807ab..0effd964b 100644
--- a/passes/opt/opt_const.cc
+++ b/passes/opt/opt_const.cc
@@ -125,6 +125,11 @@ void replace_const_cells(RTLIL::Design *design, RTLIL::Module *module)
 			if (input.match("  1")) ACTION_DO("\\Y", input.extract(1, 1));
 #ifdef MUX_UNDEF_SEL_TO_UNDEF_RESULTS
 			if (input.match("01 ")) ACTION_DO("\\Y", input.extract(0, 1));
+			// TODO: "10 " -> replace with "!S" gate
+			// TODO: "0  " -> replace with "B AND S" gate
+			// TODO: " 1 " -> replace with "A OR S" gate
+			// TODO: "1  " -> replace with "B OR !S" gate (?)
+			// TODO: " 0 " -> replace with "A AND !S" gate (?)
 			if (input.match("  *")) ACTION_DO_Y(x);
 #endif
 		}
@@ -169,6 +174,31 @@ void replace_const_cells(RTLIL::Design *design, RTLIL::Module *module)
 			}
 		}
 
+		if ((cell->type == "$eq" || cell->type == "$ne") && cell->parameters["\\Y_WIDTH"].as_int() == 1 &&
+				cell->parameters["\\A_WIDTH"].as_int() == 1 && cell->parameters["\\B_WIDTH"].as_int() == 1)
+		{
+			RTLIL::SigSpec a = assign_map(cell->connections["\\A"]);
+			RTLIL::SigSpec b = assign_map(cell->connections["\\B"]);
+
+			if (a.is_fully_const()) {
+				RTLIL::SigSpec tmp = a;
+				a = b, b = tmp;
+			}
+
+			if (b.is_fully_const()) {
+				if (b.as_bool() == (cell->type == "$eq")) {
+					RTLIL::SigSpec input = b;
+					ACTION_DO("\\Y", cell->connections["\\A"]);
+				} else {
+					cell->type = "$not";
+					cell->parameters.erase("\\B_WIDTH");
+					cell->parameters.erase("\\B_SIGNED");
+					cell->connections.erase("\\B");
+				}
+				goto next_cell;
+			}
+		}
+
 #define FOLD_1ARG_CELL(_t) \
 		if (cell->type == "$" #_t) { \
 			RTLIL::SigSpec a = cell->connections["\\A"]; \
@@ -237,11 +267,15 @@ void replace_const_cells(RTLIL::Design *design, RTLIL::Module *module)
 		FOLD_1ARG_CELL(pos)
 		FOLD_1ARG_CELL(neg)
 
+		// be very conservative with optimizing $mux cells as we do not want to break mux trees
 		if (cell->type == "$mux") {
-			RTLIL::SigSpec input = cell->connections["\\S"];
-			assign_map.apply(input);
+			RTLIL::SigSpec input = assign_map(cell->connections["\\S"]);
+			RTLIL::SigSpec inA = assign_map(cell->connections["\\A"]);
+			RTLIL::SigSpec inB = assign_map(cell->connections["\\B"]);
 			if (input.is_fully_const())
 				ACTION_DO("\\Y", input.as_bool() ? cell->connections["\\B"] : cell->connections["\\A"]);
+			else if (inA == inB)
+				ACTION_DO("\\Y", cell->connections["\\A"]);
 		}
 
 	next_cell:;