Re: [PATCH v2 1/3] compiler_types: Introduce the Clang __preserve_most function attribute

[Florian Weimer]

* Jakub Jelinek:

> On Mon, Aug 07, 2023 at 02:24:26PM +0200, Marco Elver wrote:
>> > | If the arguments are passed in callee-saved registers, then they will
>> > | be preserved by the callee across the call. This doesn’t apply for
>> > | values returned in callee-saved registers.
>> > |
>> > |  ·  On X86-64 the callee preserves all general purpose registers, except
>> > |     for R11. R11 can be used as a scratch register. Floating-point
>> > |     registers (XMMs/YMMs) are not preserved and need to be saved by the
>> > |     caller.
>> > |
>> > |  ·  On AArch64 the callee preserve all general purpose registers, except
>> > |     X0-X8 and X16-X18.
>> >
>> > Ideally, this would be documented in the respective psABI supplement.
>> > I filled in some gaps and filed:
>> >
>> >   Document the ABI for __preserve_most__ function calls
>> >   <https://gitlab.com/x86-psABIs/x86-64-ABI/-/merge_requests/45>
>> 
>> Good idea. I had already created
>> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110899, and we need
>> better spec to proceed for GCC anyway.
>
> "Registers used for passing arguments
> are preserved by the called function, but registers used for
> returning results are not."
>
> You mean just GPRs or also vector SSE or MMX registers?

I think this is pretty clear for x86-64:

| Floating-point registers (XMMs/YMMs) are not preserved and need to be
| saved by the caller.

The issue is more with future GPR extensions like APX.

Thanks,
Florian