https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90436
--- Comment #4 from Marc Glisse <glisse at gcc dot gnu.org> --- (side note not related to the redundant size checking) It is surprising how, in the code from comment 2, adding v.reserve(1000) does not help, it even slows the program down slightly here (yes, that's rather hard to believe). To reap the benefits, I also need to add in the loop: if(v.size()==v.capacity())__builtin_abort(); which enables the compiler to remove the reallocation code, and once that code is removed it can actually prove that size never reaches capacity and remove the call to abort! We don't even need __builtin_unreachable there. And once all that dead code is removed, it can finally vectorize.
