r13 in the kernel

Hi,

while debugging lock_stubs.S I realized that r13 is always assumed to
contain the thread-id.

Is it really safe to do so? IIRC the V.4-ABI defines r13 as small data base
register, but when small data is not used it should be a normal
non-volatile register, like r14 - r31.
The kernel is compiled with -msdata=none.

What makes you sure that gcc's code generator wouldn't use r13 for storing
non-volatile data during a function?

-- 
Frank Wille