diff --git a/vlib/v/checker/mark_used.v b/vlib/v/checker/mark_used.v
index e53231b57c..841042f705 100644
--- a/vlib/v/checker/mark_used.v
+++ b/vlib/v/checker/mark_used.v
@@ -39,6 +39,7 @@ fn (mut c Checker) mark_used(ast_files []ast.File) {
 		'main.main',
 		'__new_array',
 		'__new_array_with_default',
+		'__new_array_with_array_default',
 		'new_array_from_c_array',
 		'memdup',
 		'vstrlen',
@@ -46,19 +47,41 @@ fn (mut c Checker) mark_used(ast_files []ast.File) {
 		'tos2',
 		'tos3',
 		'isnil',
+		'opt_ok2',
 		/* utf8_str_visible_length is used by c/str.v */
 		'utf8_str_visible_length',
+		'compare_ints',
+		'compare_u64s',
+		'compare_strings',
+		'compare_ints_reverse',
+		'compare_u64s_reverse',
+		'compare_strings_reverse',
 		'builtin_init',
 		/* byteptr and charptr */
 		'3.vstring',
 		'3.vstring_with_len',
 		'4.vstring',
 		'4.vstring_with_len',
+		'5.str', /* i8_str */
+		'6.str', /* i16_str */
+		'7.str', /* int_str */
+		'8.str', /* i64_str */
+		'9.str', /* byte_str */
+		'10.str', /* u16_str */
+		'11.str', /* u32_str */
+		'12.str', /* u64_str */
+		'13.str', /* f32_str */
+		'14.str', /* f64_str */
+		'15.str', /* char_str */
+		'16.str', /* bool_str */
+		/* byte. methods */
+		'9.str_escaped',
 		/* string. methods */
 		'18.add',
 		'18.trim_space',
 		'18.replace',
 		'18.clone',
+		'18.clone_static',
 		'18.trim',
 		'18.substr',
 		'18.at',
@@ -78,16 +101,31 @@ fn (mut c Checker) mark_used(ast_files []ast.File) {
 		'19.le',
 		'19.ge',
 		'19.add',
+		'19.str',
+		'20.str',
 		/* other array methods */
 		'21.get',
 		'21.set',
 		'21.get_unsafe',
 		'21.set_unsafe',
+		'21.clone_static',
+		'21.first',
+		'21.last',
+		'21.reverse',
+		'21.repeat',
 		'21.slice',
 		'21.slice2',
+		'26.str', /* float_literal_str */
+		'27.str', /* int_literal_str */
+		'29.str', /* array_string_str */
 		'59.get',
 		'59.set',
+		'65557.last',
+		'65557.pop',
 		'65557.push',
+		'65557.insert_many',
+		'65557.prepend_many',
+		'65557.reverse',
 		'65557.set',
 		'65557.set_unsafe',
 		/* TODO: process the _vinit const initializations automatically too */
@@ -108,9 +146,19 @@ fn (mut c Checker) mark_used(ast_files []ast.File) {
 		if k.ends_with('.free') {
 			all_fn_root_names << k
 		}
-		if c.pref.is_test && (k.starts_with('test_') || k.contains('.test_')) {
+		if k.ends_with('.lock') || k.ends_with('.unlock') || k.ends_with('.rlock')
+			|| k.ends_with('.runlock') {
 			all_fn_root_names << k
 		}
+		if c.pref.is_test {
+			if k.starts_with('test_') || k.contains('.test_') {
+				all_fn_root_names << k
+			}
+			if k.starts_with('testsuite_') || k.contains('.testsuite_') {
+				eprintln('>>> test suite: $k')
+				all_fn_root_names << k
+			}
+		}
 		if sb_mut_type != '' && k.starts_with(sb_mut_type) {
 			all_fn_root_names << k
 		}
@@ -144,8 +192,11 @@ fn (mut c Checker) mark_used(ast_files []ast.File) {
 	}
 	if walker.n_maps > 0 {
 		for k, mut mfn in all_fns {
-			if k == 'new_map_2' || k.starts_with('map_') || k.ends_with('set_1')
-				|| k.ends_with('exists_1') || k.ends_with('get_1') {
+			if k in ['new_map_2', 'new_map_init_2']
+				|| (k.starts_with('map_') || k.ends_with('clone') || k.ends_with('exists_1')
+				|| k.ends_with('keys') || k.ends_with('keys_1') || k.ends_with('get_1')
+				|| k.ends_with('set_1') || k.ends_with('key') || k.ends_with('value')
+				|| k.ends_with('has_index') || k.ends_with('expand') || k.ends_with('zeros_to_end')) {
 				walker.fn_decl(mut mfn)
 			}
 		}
diff --git a/vlib/v/checker/mark_used_walker/walker.v b/vlib/v/checker/mark_used_walker/walker.v
index 726f2b8902..af8a4a32a0 100644
--- a/vlib/v/checker/mark_used_walker/walker.v
+++ b/vlib/v/checker/mark_used_walker/walker.v
@@ -204,8 +204,17 @@ fn (mut w Walker) expr(node ast.Expr) {
 			}
 		}
 		ast.Ident {
-			if node.kind == .constant {
-				w.mark_const_as_used(node.name)
+			match node.kind {
+				.constant {
+					w.mark_const_as_used(node.name)
+				}
+				.function {
+					w.fn_by_name(node.name)
+				}
+				else {
+					// `.unresolved`, `.blank_ident`, `.variable`, `.global`, `.function`
+					// println('>>> else, ast.Ident kind: $node.kind')
+				}
 			}
 		}
 		ast.Likely {
@@ -347,15 +356,24 @@ pub fn (mut w Walker) call_expr(mut node ast.CallExpr) {
 	if w.used_fns[fn_name] {
 		return
 	}
+	w.mark_fn_as_used(fn_name)
 	stmt := w.all_fns[fn_name] or { return }
 	if stmt.name == node.name {
 		if !node.is_method || (node.receiver_type == stmt.receiver.typ) {
-			w.mark_fn_as_used(fn_name)
 			w.stmts(stmt.stmts)
 		}
 	}
 }
 
+pub fn (mut w Walker) fn_by_name(fn_name string) {
+	if w.used_fns[fn_name] {
+		return
+	}
+	stmt := w.all_fns[fn_name] or { return }
+	w.mark_fn_as_used(fn_name)
+	w.stmts(stmt.stmts)
+}
+
 pub fn (mut w Walker) struct_fields(sfields []ast.StructField) {
 	for sf in sfields {
 		if sf.has_default_expr {
diff --git a/vlib/v/gen/c/fn.v b/vlib/v/gen/c/fn.v
index b3b778687e..59964fce9a 100644
--- a/vlib/v/gen/c/fn.v
+++ b/vlib/v/gen/c/fn.v
@@ -17,9 +17,14 @@ fn (mut g Gen) is_used_by_main(node ast.FnDecl) bool {
 		}
 		if !is_used_by_main {
 			$if trace_skip_unused_fns_in_c_code ? {
-				g.writeln('// fn $node.name UNUSED')
+				g.writeln('// trace_skip_unused_fns_in_c_code, $node.name, fkey: $fkey')
 			}
 		}
+	} else {
+		$if trace_skip_unused_fns_in_c_code ? {
+			fkey := if node.is_method { '${int(node.receiver.typ)}.$node.name' } else { node.name }
+			g.writeln('// trace_skip_unused_fns_in_c_code, $node.name, fkey: $fkey')
+		}
 	}
 	return is_used_by_main
 }