https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63164
--- Comment #21 from Thomas de Bock <tdebock at DRWUK dot com> --- (In reply to Jakub Jelinek from comment #20) > (In reply to Thomas de Bock from comment #18) > > (In reply to Jason Merrill from comment #17) > > > As I commented at > > > > > > https://inbox.sourceware.org/gcc-patches/75ff8af8-af03-42fa-b68b- > > > e6c16a34c...@redhat.com/ > > > > > > we could optimize the dynamic_cast to type_info::operator== instead of > > > vtable comparison. > > > > I agree having dynamic_cast be optimized to type_info::operator== when it > > can (and when optimizations are enabled) would be good. Like I said though I > > also think having an optimal optimization that just checks vtables ptrs is > > important to have available behind some compiler flags as well. > > With <typeinfo> being included, that option would already exist, > -D__GXX_MERGED_TYPEINFO_NAMES=1 How so? The definition is: type_info::operator==(const type_info& __arg) const _GLIBCXX_NOEXCEPT { if (std::__is_constant_evaluated()) return this == &__arg; if (__name == __arg.__name) return true; ... so even with __GXX_MERGED_TYPEINFO_NAMES=1, the type_info name ptrs will be compared, requiring additional dereferences compared to just a vtable ptr comparison, no?
