[guenthert]

> It's up to the compiler to decide how many registers it needs to preserve at a call.

But the compiler is bound by the ABI, isn't it? (at least for externally visible entrance points / calls to sub routines external to the current compilation unit)

[cvoss]

Someone decides how to define the ABI too, and that is a free choice. CPU register count doesn't constrain this particular system design question.