Re: C aggregate passing (Rust kernel policy)

[David Laight <david.laight.linux@gmail.com>]

On Fri, 21 Feb 2025 11:12:27 -0800
Linus Torvalds <torvalds@linux-foundation.org> wrote:

> On Fri, 21 Feb 2025 at 10:34, David Laight <david.laight.linux@gmail.com> wrote:
> >
> > As Linus said, most modern ABI pass short structures in one or two registers
> > (or stack slots).
> > But aggregate returns are always done by passing a hidden pointer argument.
> >
> > It is annoying that double-sized integers (u64 on 32bit and u128 on 64bit)
> > are returned in a register pair - but similar sized structures have to be
> > returned by value.  
> 
> No, they really don't. At least not on x86 and arm64 with our ABI.
> Two-register structures get returned in registers too.
> 
> Try something like this:
> 
>   struct a {
>         unsigned long val1, val2;
>   } function(void)
>   { return (struct a) { 5, 100 }; }
> 
> and you'll see both gcc and clang generate
> 
>         movl $5, %eax
>         movl $100, %edx
>         retq
> 
> (and you'll similar code on other architectures).

Humbug, I'm sure it didn't do that the last time I tried it.

	David

> 
> But it really is just that the two-register case is special.
> Immediately when it grows past that size then yes, it ends up being
> returned through indirect memory.
> 
>                Linus