[Bug target/110899] RFE: Attributes preserve_most and preserve_all

["matz at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110899

--- Comment #9 from Michael Matz <matz at gcc dot gnu.org> ---
(In reply to Florian Weimer from comment #8)
> (In reply to Michael Matz from comment #7)
> > > > Does the clang implementation take into account the various problematic
> > > > cases that arise when calling a normal function from a (say) preserve_all
> > > > function
> > > > (hint: such call can't usually be done)?
> > > 
> > > How so? We need to version the __preserve_most__ attribute with the ISA
> > > level, of course.
> > 
> > I don't see how that helps.  Imagine a preserve_all function foo that calls
> > printf.  How do you propose that 'foo' saves all parts of the SSE registers,
> > even those that aren't invented yet, or those that can't be touched by the
> > current ISA?  (printf might clobber all of these)
> 
> Vector registers are out of scope for this.

Why do you say that?  From clang: "Furthermore it also preserves all
floating-point registers (XMMs/YMMs)."  (for preserve_all, but this bugreport
does
include that variant of the attribute).

> But lets look at APX. If printf is recompiled to use APX, then it will
> clobber the extended register file. If we define __preserve_most__ the way
> we do in my psABI proposal (i.e., *not* as everything but %r11), the
> extended APX registers are still caller-saved.

Right, for preserve_most _with your wording_ it works out.  preserve_all
or preserve_most with clang wording doesn't.

> (These details are the main reason why I want this in the psABI
> documentation. This stuff is out there and will be used, so let's make sure
> that it's somewhat reasonable.)

I agree with that.  I just want a little hashing-out-the-details before
putting anything in the psABI.