sv: fix two struct access bugs

- preserve signedness of struct members
- fix initial width detection of struct members (e.g., in case expressions)

frontends/ast/ast.h

@@ -370,6 +370,9 @@ namespace AST
 
 	// Helper for setting the src attribute.
 	void set_src_attr(RTLIL::AttrObject *obj, const AstNode *ast);
+
+	// struct helper exposed from simplify for genrtlil
+	AstNode *make_struct_member_range(AstNode *node, AstNode *member_node);
 }
 
 namespace AST_INTERNAL

frontends/ast/genrtlil.cc

@@ -812,6 +812,10 @@ void AstNode::detectSignWidthWorker(int &width_hint, bool &sign_hint, bool *foun
 			this_width = id_ast->children[0]->range_left - id_ast->children[0]->range_right + 1;
 			if (children.size() > 1)
 				range = children[1];
+		} else if (id_ast->type == AST_STRUCT_ITEM) {
+			AstNode *tmp_range = make_struct_member_range(this, id_ast);
+			this_width = tmp_range->range_left - tmp_range->range_right + 1;
+			delete tmp_range;
 		} else
 			log_file_error(filename, location.first_line, "Failed to detect width for identifier %s!\n", str.c_str());
 		if (range) {

frontends/ast/simplify.cc

@@ -456,7 +456,7 @@ static AstNode *slice_range(AstNode *rnode, AstNode *snode)
 }
 
 
-static AstNode *make_struct_member_range(AstNode *node, AstNode *member_node)
+AstNode *AST::make_struct_member_range(AstNode *node, AstNode *member_node)
 {
 	// Work out the range in the packed array that corresponds to a struct member
 	// taking into account any range operations applicable to the current node
@@ -1693,6 +1693,8 @@ bool AstNode::simplify(bool const_fold, bool at_zero, bool in_lvalue, int stage,
 					newNode = new AstNode(AST_IDENTIFIER, range);
 					newNode->str = sname;
 					newNode->basic_prep = true;
+					if (item_node->is_signed)
+						newNode = new AstNode(AST_TO_SIGNED, newNode);
 					goto apply_newNode;
 				}
 			}