Re: Help with an ABI peculiarity

[Eric Gallager <egall@gwmail.gwu.edu>]

On Mon, Jan 10, 2022 at 8:28 AM Iain Sandoe <iain@sandoe.co.uk> wrote:
>
> Hi Florian,
>
> > On 10 Jan 2022, at 08:38, Florian Weimer <fweimer@redhat.com> wrote:
> >
> > * Jeff Law via Gcc:
> >
> >> Most targets these days use registers for parameter passing and
> >> obviously we can run out of registers on all of them.  The key
> >> property is the size/alignment of the argument differs depending on if
> >> it's pass in a register (get promoted) or passed in memory (not
> >> promoted).  I'm not immediately aware of another ABI with that
> >> feature.  Though I haven't really gone looking.
> >
> > I think what AArch64 Darwin does is not compatible with a GCC extension
> > that allows calling functions defined with a prototype without it (for
> > pre-ISO-C compatibility).
>
> AFAIU the implementation:
>
> In the case that a call is built and no prototype is available, the assumption is
> that all parms are named.  The promotion is then done according to the C
> promotion rules.
>
> [for the number of args that can be passed in int regs] the callee will happen to
> observe the same rules in this case.
>
> It will, however, break once we overflow the number of int regs.. :/
>
> ====
>
> The case that is fundamentally broken from scratch is of a variadic function
> called without a prototype - since the aarch64-darwin ABI places unnamed
> parms differently.
>
> So that the absence of a prototype causes us to place all args as if they were
> named.
>
> ====
>
> Wmissing-prototype
> Wstrict-prototypes
>
> would wisely be promoted to errors for this platform,

Relevant bugs in this area:
82922, add -Wstrict-prototypes to -Wextra:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82922
91092, Error on implicit function declarations by default:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91092
(and the related bugs in the "See Also", "Depends On", "Duplicates",
etc. fields for each of them)

>
> (the ABI is obviously not up for change, since it’s already on millions of devices).
>
> >  Given that, anyone defining an ABI in
> > parallel with a GCC implementation probably has paused, reconsidered
> > what they were doing,
>
> My guess is that this step was omitted - i.e. the port was designed in the LLVM
> framework.  I can raise a query with the ABI owners, I guess.
>
> >  and adjusted the ABI for K&R compatibility.
>
> FWIW, we bootstrap sucessfully including the K&R code in intl/
> Given we have 8 int regs available, probably many calls will work ..
>
> ====
>
> As of now, I must assume that what is broken by the cases above will remain
> broken, and I just need to find a way to implement the cases that will work (i.e.
> when proper prototypes are available)
>
> thanks
> Iain
>