Haven't tried it yet but it sounds good.

So, instead of traversing the macro_stack to check if a macro is ok to expand or not, there's a cheaper (and probably faster) algorithm that only use bit flag. And that bit flag is for cpp_macro, we can name it CPP_MACRO_EXPANDING.

The changes to the current codebase probably are:

• Add cpp_macro * to macro_stack.
• macro_stack_push() and macro_stack_pop() set and clear CPP_MACRO_EXPANDING bit flag, respectively.
• Delete is_active_macro() and replace its call in expand() to just checking the bit flag after looking up cpp_macro *.
• Move expand_arg() to expand() after calling collect_args().
• Add another cpp_token_array to cpp_macro_arg to store expanded tokens.