Re: Help with an ABI peculiarity

[Iain Sandoe <iain@sandoe.co.uk>]

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,

(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