I don't know exactly what, but I know there is more than just that, because calling printf breaks with a misaligned stack even when you're not passing it any floating-point arguments. And even if it doesn't break anything for you today, you're basically committing UB by violating the compiler's assumptions.

Aren't there GCC command-line flags to specify alignment assumptions?

Yes (see https://gcc.gnu.org/onlinedocs/gcc-14.2.0/gcc/x86-Options.ht...), but this article doesn't use them.

Code that uses vector registers