Keep strings as strings in const ternary and concat

frontends/ast/ast.cc

@@ -171,6 +171,7 @@ AstNode::AstNode(AstNodeType type, AstNode *child1, AstNode *child2)
 	is_output = false;
 	is_reg = false;
 	is_signed = false;
+	is_string = false;
 	range_valid = false;
 	port_id = 0;
 	range_left = -1;
@@ -591,6 +592,8 @@ bool AstNode::operator==(const AstNode &other) const
 		return false;
 	if (is_signed != other.is_signed)
 		return false;
+	if (is_string != other.is_string)
+		return false;
 	if (range_valid != other.range_valid)
 		return false;
 	if (port_id != other.port_id)
@@ -658,6 +661,14 @@ AstNode *AstNode::mkconst_bits(const std::vector<RTLIL::State> &v, bool is_signe
 	return node;
 }
 
+// create an AST node for a constant (using a string in bit vector form as value)
+AstNode *AstNode::mkconst_str(const std::vector<RTLIL::State> &v)
+{
+	AstNode *node = mkconst_str(RTLIL::Const(v).decode_string());
+	log_assert(node->bits == v);
+	return node;
+}
+
 // create an AST node for a constant (using a string as value)
 AstNode *AstNode::mkconst_str(const std::string &str)
 {
@@ -671,6 +682,7 @@ AstNode *AstNode::mkconst_str(const std::string &str)
 		}
 	}
 	AstNode *node = AstNode::mkconst_bits(data, false);
+	node->is_string = true;
 	node->str = str;
 	return node;
 }
@@ -702,7 +714,7 @@ RTLIL::Const AstNode::asAttrConst()
 	RTLIL::Const val;
 	val.bits = bits;
 
-	if (!str.empty()) {
+	if (is_string) {
 		val.flags |= RTLIL::CONST_FLAG_STRING;
 		log_assert(val.decode_string() == str);
 	}

frontends/ast/ast.h

@@ -143,7 +143,7 @@ namespace AST
 		// node content - most of it is unused in most node types
 		std::string str;
 		std::vector<RTLIL::State> bits;
-		bool is_input, is_output, is_reg, is_signed, range_valid;
+		bool is_input, is_output, is_reg, is_signed, is_string, range_valid;
 		int port_id, range_left, range_right;
 		uint32_t integer;
 
@@ -213,6 +213,7 @@ namespace AST
 		// helper functions for creating AST nodes for constants
 		static AstNode *mkconst_int(uint32_t v, bool is_signed, int width = 32);
 		static AstNode *mkconst_bits(const std::vector<RTLIL::State> &v, bool is_signed);
+		static AstNode *mkconst_str(const std::vector<RTLIL::State> &v);
 		static AstNode *mkconst_str(const std::string &str);
 
 		// helper function for creating sign-extended const objects

frontends/ast/simplify.cc

@@ -1176,6 +1176,7 @@ skip_dynamic_range_lvalue_expansion:;
 	// perform const folding when activated
 	if (const_fold && newNode == NULL)
 	{
+		bool string_op;
 		std::vector<RTLIL::State> tmp_bits;
 		RTLIL::Const (*const_func)(const RTLIL::Const&, const RTLIL::Const&, bool, bool, int);
 		RTLIL::Const dummy_arg;
@@ -1306,6 +1307,9 @@ skip_dynamic_range_lvalue_expansion:;
 					choice = children[2];
 				if (choice != NULL && choice->type == AST_CONSTANT) {
 					RTLIL::Const y = choice->bitsAsConst(width_hint, sign_hint);
+					if (choice->is_string && y.bits.size() % 8 == 0 && sign_hint == false)
+						newNode = mkconst_str(y.bits);
+					else
 						newNode = mkconst_bits(y.bits, sign_hint);
 				} else if (children[1]->type == AST_CONSTANT && children[2]->type == AST_CONSTANT) {
 					RTLIL::Const a = children[1]->bitsAsConst(width_hint, sign_hint);
@@ -1319,19 +1323,22 @@ skip_dynamic_range_lvalue_expansion:;
 			}
 			break;
 		case AST_CONCAT:
+			string_op = !children.empty();
 			for (auto it = children.begin(); it != children.end(); it++) {
 				if ((*it)->type != AST_CONSTANT)
 					goto not_const;
+				if (!(*it)->is_string)
+					string_op = false;
 				tmp_bits.insert(tmp_bits.end(), (*it)->bits.begin(), (*it)->bits.end());
 			}
-			newNode = mkconst_bits(tmp_bits, false);
+			newNode = string_op ? mkconst_str(tmp_bits) : mkconst_bits(tmp_bits, false);
 			break;
 		case AST_REPLICATE:
 			if (children.at(0)->type != AST_CONSTANT || children.at(1)->type != AST_CONSTANT)
 				goto not_const;
 			for (int i = 0; i < children[0]->bitsAsConst().as_int(); i++)
 				tmp_bits.insert(tmp_bits.end(), children.at(1)->bits.begin(), children.at(1)->bits.end());
-			newNode = mkconst_bits(tmp_bits, false);
+			newNode = children.at(1)->is_string ? mkconst_str(tmp_bits) : mkconst_bits(tmp_bits, false);
 			break;
 		default:
 		not_const: