[llvm-dev] Reserve ARM register for only section of the program

[Tim Northover via llvm-dev]

Hi Nisal,

(Adding llvm-dev back again; best to keep these things public in case
they help someone else in the future).

On 12 June 2017 at 19:43, Nisal Menuka <nisalmenuka23 at gmail.com> wrote:
> I am a first year graduate student at Rice University, I'm looking to
> reserve registers X9 and X10 for one of my research projects. I have
> some questions regarding this and greatly appreciate any help you can
> provide (I am a newbee when it comes to hacking compilers).

> 1. Is it possible to reserve X9 and X10 by the same methodology used
> to reserve X18 in LLVM?

It should be (x9 and x10 are very similar to x18, with no other
special purposes in the ABI), but you'll have to implement it
yourself. Searching for all uses of X18 in lib/Target/AArch64 and
adding your own code for the others would be a good place to get
started. For Clang the key phrase is "fixed_x18".

You're going to have more work to do for the interworking though. For
example if a function with -ffixed-x9-x10 calls one without, you'll
probably have to implement code to save those two registers at the
callsite and restore them afterwards because the callee will be
treating them as temporaries.

> 2. Else can you suggest a method to achieve it?

If you didn't need interworking I'd probably suggest just
unconditionally reserving them, which would be substantially simpler
(a 2-4 line change in LLVM).

But from what you've said so far I think following x18's
implementation is probably your best bet. It's most of the way there,
and already piped through to Clang in a usable way.

Cheers.

Tim.