Is r25 saved across syscalls?

[Rich Felker <dalias@aerifal.cx>]

Hi all,
The kernel syscall entry/exit code seems to always save and restore
r25. Is this stable/documented behavior I can rely on? If there's a
reason it _needs_ to be preserved, knowing that would help convince me
it's safe to assume it will always be done. The intended usage is to
be able to make syscalls (where the syscall # is not a constant that
could be loaded with lwi) without a stack frame, as in "move $2,$25 ;
syscall".

Rich