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

[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?  Because if some
of those are to be preserved by callee, there is an issue that they need
to be e.g. handled during unwinding, with all the consequences.  It is hard
to impossible to guess what size needs to be saved/restored, both normally
or during unwinding.  As caller could be say -mavx512f and expect
preservation of all 512 bits and callee -msse2 or -mavx{,2},
or caller -mavx{,2} and expect preservation of all 256 bits and callee -msse2 etc.
MSABI "solves" that by making just the low 128 bits preserved and upper bits
clobbered.

	Jakub