xmm registers in amd64 drivers

I realise this isn't quite the right list for this question.
But I suspect someone will know the answer!

I've just compiled one of our drivers (an ss7 protocol stack)
on a much newer system than the one we usually use
(ubunto 13.04 with gcc 4.7.3 rather than debian lenny).

I've bodged around some strict-aliasing and const-cast warnings
- the usage all look safe.

Looking at the object code I noticed that all the stdarg
functions now contain code to conditionally save the xmm
registers.

Is this now considered 'normal' or am I missing a compiler flag?

I know I've looked at the code before since I tried adding
enough fixed parameters to one of the functions to get all
the variable ones on-stack hoping that would optimise the
accesses - it didn't.

	David