r9 on ARM32?

[Richard Earnshaw (lists)]

On 24/04/18 03:22, Jay K wrote:
> I'm wondering what is the role of r9 on ARM32, on Linux and Android.

It's another callee-saved register and has no other special purpose.

This is defined in the
https://sourcery.mentor.com/GNUToolchain/kbattach142/arm_gnu_linux_abi.pdf
which is the linux-specific platform binding information for the ARM
EABI
(http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.subset.swdev.abi/index.html)

R.

> Â  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,
> Â - Jay
>