opt_expr: optimise 1-bit $xor or $_XOR_ with constant input

diff --git a/passes/opt/opt_expr.cc b/passes/opt/opt_expr.cc
index 4a2f170b83401acaa02b2a4944cc1ae8ca556235..e4daf6fe6da113dcdf9931f09b8971e92c29ed7a 100644 (file)
--- a/passes/opt/opt_expr.cc
+++ b/passes/opt/opt_expr.cc
@@ -496,6 +496,19 @@ void replace_const_cells(RTLIL::Design *design, RTLIL::Module *module, bool cons
                        }
                }
 
+               if (cell->type == ID($_XOR_) || (cell->type == ID($xor) && GetSize(cell->getPort(ID::A)) == 1 && GetSize(cell->getPort(ID::B)) == 1 && !cell->getParam(ID(A_SIGNED)).as_bool()))
+               {
+                       SigBit sig_a = assign_map(cell->getPort(ID::A));
+                       SigBit sig_b = assign_map(cell->getPort(ID::B));
+                       if (!sig_a.wire)
+                               std::swap(sig_a, sig_b);
+                       if (sig_b == State::S0 || sig_b == State::S1) {
+                               cover("opt.opt_expr.xor_buffer");
+                               replace_cell(assign_map, module, cell, "xor_buffer", ID::Y, sig_b == State::S1 ? module->NotGate(NEW_ID, sig_a) : sig_a);
+                               goto next_cell;
+                       }
+               }
+
                if (cell->type.in(ID($reduce_and), ID($reduce_or), ID($reduce_bool), ID($reduce_xor), ID($reduce_xnor), ID($neg)) &&
                                GetSize(cell->getPort(ID::A)) == 1 && GetSize(cell->getPort(ID::Y)) == 1)
                {
@@ -1590,7 +1603,7 @@ void replace_const_cells(RTLIL::Design *design, RTLIL::Module *module, bool cons
                                        }
 
                                        int const_bit_set = get_highest_hot_index(const_sig);
-                                       if(const_bit_set >= var_width)
+                                       if (const_bit_set >= var_width)
                                        {
                                                string cmp_name;
                                                if (cmp_type == ID($lt) || cmp_type == ID($le))