Manually set sret parameter

[Andrew Pinski]

On Sun, Jun 29, 2025, 4:36 PM Antoni Boucher <bouanto@zoho.com> wrote:

>
>
> Le 2025-06-29 à 10 h 46, Andrew Pinski a écrit :
> >
> >
> > On Sun, Jun 29, 2025, 7:43 AM Andrew Pinski <pinskia@gmail.com
> > <mailto:pinskia@gmail.com>> wrote:
> >
> >     On Sun, Jun 29, 2025, 7:36 AM Antoni Boucher via Gcc
> >     <gcc@gcc.gnu.org <mailto:gcc@gcc.gnu.org>> wrote:
> >
> >         Hi.
> >         Is there a way in GENERIC to specify that a parameter will be
> >         passed in
> >         "sret", or is this solely controlled by the hook
> struct_value_rtx?
> >
> >
> >     It is only controlled by the hook.
> >     What exactly are trying to do?
> >     You could set the return slot optimization bit on the call
> >     expression if you want the lhs of a call not to be copied and just
> >     passed as the address via sret.
>
> I'm trying to follow the Rust ABI for rustc_codegen_gcc: they manually
> specify whether a param is "sret".
>

Not all ABI/targets have a sret specific register. So this is even more
confusing.



> >
> >     That is if you have:
> >     StructVar = func(...);
> >
> >     You set the return slot optimization bit on the call expr in generic
> >     and which will copy that bit to the gimple GIMPLE_CALL and then
> >     during expand will again copy it back to the generic call_expr and
> >     expand will use the target for the address.
> >
> >
> > CALL_EXPR_RETURN_SLOT_OPT is the macro.
>
> Is this a guaranteed optimization? I'm asking because this is for ABI
> correctness and I need a solution that will always work.
> If not, would there be another way to do this?
> Thanks.
>

Yes it is guaranteed that if the return type is returned via memory
reference to the other function it will use that memory location.

Even for things like a->b = func(...)[RSO].  It is even used by the c++
frontend that way.



> >
> >
> >
> >     Is that what you are looking for?
> >
> >
> >     Thanks,
> >     Andrew
> >
> >         Thanks.
> >
>
>