r9 on ARM32?

[Jay K]

I'm wondering what is the role of r9 on ARM32, on Linux and Android.
  On Apple it is documented as long ago reserved, these days available for scratch.

I've looked around a bit but haven't gotten the full answer.

It is "the PIC register", I see.

What does that imply? Volatile? Von-volatile?

In particular I'm looking for a spare register, to pass an extra "special" parameter in, that can be considered volatile and never otherwise has a parameter.

Most ABIs have a few candidates, but arm32 comes up relatively short.

Intra procedural scratch (r12) probably cannot work for me.
I know gcc uses it for nested function context and that is laudable. I wish I could guarantee no code between me setting it and it being consumed.

And if it is volatile, I'd want the dynamic linker stubs to still preserve it incoming.

Thank you,
 - Jay