Vectorising non-vector C code is difficult (and likely relies on other expensive optimisations around flow control manipulation).
And it’s not a “sure fire” optimisation because the compiler can absolutely vectorise a loop whose 99p is an iteration or two making the vectorisation a complete waste of compilation and runtime.
So being an expensive and not perfectly reliable optimisation, it makes sense for it to be at O3.
That has nothing to do with ISA subsetting/compatibility.
Vectorising non-vector C code is difficult (and likely relies on other expensive optimisations around flow control manipulation).
And it’s not a “sure fire” optimisation because the compiler can absolutely vectorise a loop whose 99p is an iteration or two making the vectorisation a complete waste of compilation and runtime.
So being an expensive and not perfectly reliable optimisation, it makes sense for it to be at O3.
That has nothing to do with ISA subsetting/compatibility.