diff --git a/passes/opt/opt_expr.cc b/passes/opt/opt_expr.cc
index b62eae285..9d7248dc6 100644
--- a/passes/opt/opt_expr.cc
+++ b/passes/opt/opt_expr.cc
@@ -258,7 +258,29 @@ bool is_one_or_minus_one(const Const &value, bool is_signed, bool &is_negative)
 
 	return last_bit_one;
 }
-
+//if the signal has only one bit set, return the index of that bit.
+//otherwise return -1
+int get_onehot_bit_index(RTLIL::SigSpec signal){
+    if(!signal.is_fully_const())
+        return -1;
+    bool bit_set = false;
+    int bit_index = 0;
+    int i = 0;
+    for(auto bit: signal.bits()){
+        if(bit == RTLIL::State::S1){
+            if(bit_set)
+                return -1;
+            bit_index = i;
+            bit_set = true;
+        }
+        i++;
+    }
+    if(bit_set){
+        return bit_index;
+    }else{
+        return -1;
+    }
+}
 void replace_const_cells(RTLIL::Design *design, RTLIL::Module *module, bool consume_x, bool mux_undef, bool mux_bool, bool do_fine, bool keepdc, bool clkinv)
 {
 	if (!design->selected(module))
@@ -1166,6 +1188,86 @@ void replace_const_cells(RTLIL::Design *design, RTLIL::Module *module, bool cons
 					}
 			}
 		}
+        //replace a <0  or a >=0 with the top bit of a
+        if(do_fine && (cell->type == "$lt" || cell->type == "$ge" || cell->type == "$gt" || cell->type == "$le"))
+        {
+            bool is_lt = false; //used to decide whether the signal needs to be negated
+            RTLIL::SigSpec sigVar; //references the variable signal in the comparison
+            RTLIL::SigSpec sigConst; //references the constant signal in the comparison
+                 //note that this signal must be constant for the optimization 
+                 //to take place, but it is not checked beforehand.
+                 //If new passes are added, this signal must be checked for const-ness
+            int width; //width of the variable port
+            bool var_signed; 
+            if(cell->type == "$lt" || cell->type == "$ge"){
+                is_lt = cell->type == "$lt" ? 1 : 0;
+                sigVar = cell->getPort("\\A");
+                sigConst = cell->getPort("\\B");
+                width = cell->parameters["\\A_WIDTH"].as_int();
+                var_signed = cell->parameters["\\A_SIGNED"].as_bool();
+            }
+            if(cell->type == "$gt" || cell->type == "$le"){
+                is_lt = cell->type == "$gt" ? 1 : 0;  
+                sigVar = cell->getPort("\\B");
+                sigConst = cell->getPort("\\A");
+                width = cell->parameters["\\B_WIDTH"].as_int();
+                var_signed = cell->parameters["\\B_SIGNED"].as_bool();
+            }
+            //replace a(signed) < 0 with the high bit of a
+            if(sigConst.is_fully_const() && sigConst.is_fully_zero() && var_signed == true){
+                RTLIL::SigSpec a_prime(RTLIL::State::S0, cell->parameters["\\Y_WIDTH"].as_int());
+                a_prime[0] = sigVar[width-1];
+                if(is_lt){
+                    log("Optimizing a < 0 with a[%d]\n",width - 1);
+                    module->connect(cell->getPort("\\Y"), a_prime);
+                    module->remove(cell);
+                }
+                else{
+                    log("Optimizing a >= 0 with ~a[%d]\n",width - 1);
+                    module->addNot(NEW_ID, a_prime, cell->getPort("\\Y"));
+                    module->remove(cell);
+                } 
+                did_something = true;
+                goto next_cell;
+            }
+            else if(sigConst.is_fully_const() && sigConst.is_fully_def() && var_signed == false){           
+                int const_bit_set = get_onehot_bit_index(sigConst);
+                if(sigConst.is_fully_zero()){
+                    RTLIL::SigSpec a_prime(RTLIL::State::S0,1);
+                    if(is_lt){
+                        log("replacing a(unsigned) < 0 with constant false\n");
+                        a_prime[0] = RTLIL::State::S0;
+                    }
+                    else{
+                        log("replacing a(unsigned) >= 0 with constant true\n");
+                        a_prime[0] = RTLIL::State::S1;
+                    }
+                    module->connect(cell->getPort("\\Y"), a_prime);
+                    module->remove(cell);
+                    did_something = true;
+                    goto next_cell;
+                }
+               
+                else if(const_bit_set >= 0){ //if b has only 1 bit set
+                    int bit_set = const_bit_set;
+                    RTLIL::SigSpec a_prime(RTLIL::State::S0,width-bit_set);
+                    for(int i = bit_set; i < width; i++){
+                        a_prime[i-bit_set] = sigVar[i];
+                    }
+                    if(is_lt){
+                        log("replacing a < %d with !a[%d:%d]\n",sigConst.as_int(false),width-1,bit_set);
+                        module->addLogicNot(NEW_ID, a_prime,cell->getPort("\\Y"));
+                    }
+                    else{
+                        log("replacing a >= %d with |a[%d:%d]\n",sigConst.as_int(false),width-1,bit_set);
+                        module->addReduceOr(NEW_ID, a_prime,cell->getPort("\\Y")); 
+                    }
+                    module->remove(cell);
+                    did_something = true;
+                    goto next_cell;
+                }
+            }
+        }
 
 	next_cell:;
 #undef ACTION_DO