diff --git a/frontends/ast/genrtlil.cc b/frontends/ast/genrtlil.cc
index 242514863..3c8f1fa16 100644
--- a/frontends/ast/genrtlil.cc
+++ b/frontends/ast/genrtlil.cc
@@ -400,10 +400,7 @@ struct AST_INTERNAL::ProcessGenerator
 		case AST_ASSIGN_EQ:
 		case AST_ASSIGN_LE:
 			{
-				std::map<RTLIL::SigBit, RTLIL::SigBit> new_subst_rvalue_map;
-				for (int i = 0; i < SIZE(subst_rvalue_to); i++)
-					new_subst_rvalue_map[subst_rvalue_from[i]] = subst_rvalue_to[i];
-
+				std::map<RTLIL::SigBit, RTLIL::SigBit> new_subst_rvalue_map = subst_rvalue_from.to_sigbit_map(subst_rvalue_to);
 				RTLIL::SigSpec unmapped_lvalue = ast->children[0]->genRTLIL(), lvalue = unmapped_lvalue;
 				RTLIL::SigSpec rvalue = ast->children[1]->genWidthRTLIL(lvalue.size(), &new_subst_rvalue_map);
 				lvalue.replace(subst_lvalue_from, subst_lvalue_to);
@@ -421,10 +418,7 @@ struct AST_INTERNAL::ProcessGenerator
 
 		case AST_CASE:
 			{
-				std::map<RTLIL::SigBit, RTLIL::SigBit> new_subst_rvalue_map;
-				for (int i = 0; i < SIZE(subst_rvalue_to); i++)
-					new_subst_rvalue_map[subst_rvalue_from[i]] = subst_rvalue_to[i];
-
+				std::map<RTLIL::SigBit, RTLIL::SigBit> new_subst_rvalue_map = subst_rvalue_from.to_sigbit_map(subst_rvalue_to);
 				RTLIL::SwitchRule *sw = new RTLIL::SwitchRule;
 				sw->signal = ast->children[0]->genWidthRTLIL(-1, &new_subst_rvalue_map);
 				current_case->switches.push_back(sw);
@@ -478,9 +472,7 @@ struct AST_INTERNAL::ProcessGenerator
 						else if (node->type == AST_BLOCK)
 							processAst(node);
 						else {
-							std::map<RTLIL::SigBit, RTLIL::SigBit> new_subst_rvalue_map;
-							for (int i = 0; i < SIZE(subst_rvalue_to); i++)
-								new_subst_rvalue_map[subst_rvalue_from[i]] = subst_rvalue_to[i];
+							std::map<RTLIL::SigBit, RTLIL::SigBit> new_subst_rvalue_map = subst_rvalue_from.to_sigbit_map(subst_rvalue_to);
 							current_case->compare.push_back(node->genWidthRTLIL(sw->signal.size(), &new_subst_rvalue_map));
 						}
 					}
diff --git a/kernel/rtlil.cc b/kernel/rtlil.cc
index 297537f00..f4f32f600 100644
--- a/kernel/rtlil.cc
+++ b/kernel/rtlil.cc
@@ -2687,6 +2687,22 @@ std::vector<RTLIL::SigBit> RTLIL::SigSpec::to_sigbit_vector() const
 	return bits_;
 }
 
+std::map<RTLIL::SigBit, RTLIL::SigBit> RTLIL::SigSpec::to_sigbit_map(const RTLIL::SigSpec &other) const
+{
+	cover("kernel.rtlil.sigspec.to_sigbit_map");
+
+	unpack();
+	other.unpack();
+
+	log_assert(width_ == other.width_);
+
+	std::map<RTLIL::SigBit, RTLIL::SigBit> new_map;
+	for (int i = 0; i < width_; i++)
+		new_map[bits_[i]] = other.bits_[i];
+
+	return new_map;
+}
+
 RTLIL::SigBit RTLIL::SigSpec::to_single_sigbit() const
 {
 	cover("kernel.rtlil.sigspec.to_single_sigbit");
diff --git a/kernel/rtlil.h b/kernel/rtlil.h
index 8f780c822..3a0f0ff8c 100644
--- a/kernel/rtlil.h
+++ b/kernel/rtlil.h
@@ -1020,6 +1020,7 @@ public:
 
 	std::set<RTLIL::SigBit> to_sigbit_set() const;
 	std::vector<RTLIL::SigBit> to_sigbit_vector() const;
+	std::map<RTLIL::SigBit, RTLIL::SigBit> to_sigbit_map(const RTLIL::SigSpec &other) const;
 	RTLIL::SigBit to_single_sigbit() const;
 
 	static bool parse(RTLIL::SigSpec &sig, RTLIL::Module *module, std::string str);