https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97989
--- Comment #19 from Jakub Jelinek <jakub at gcc dot gnu.org> --- Note, clang like gcc documents -P to only disable line markers, and it is also what that option does in clang. Just try clang -dD -E -P, it will show the #define lines all over too. It is just that clang doesn't support -g3 at all, as can be seen by clang not producing any .debug_macinfo nor .debug_macro sections.
