linux-kernel - Re: C aggregate passing (Rust kernel policy)

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Message-ID: <CAHk-=wjF0wjD4ko7MgrZ1wBZ9QOrQd_AnyhDDUJQ1L5+i-o22A@mail.gmail.com>
Date: Fri, 21 Feb 2025 11:12:27 -0800
From: Linus Torvalds <torvalds@...ux-foundation.org>
To: David Laight <david.laight.linux@...il.com>
Cc: Jan Engelhardt <ej@...i.de>, "H. Peter Anvin" <hpa@...or.com>, Greg KH <gregkh@...uxfoundation.org>, 
	Boqun Feng <boqun.feng@...il.com>, Miguel Ojeda <miguel.ojeda.sandonis@...il.com>, 
	Christoph Hellwig <hch@...radead.org>, rust-for-linux <rust-for-linux@...r.kernel.org>, 
	David Airlie <airlied@...il.com>, linux-kernel@...r.kernel.org, ksummit@...ts.linux.dev
Subject: Re: C aggregate passing (Rust kernel policy)

On Fri, 21 Feb 2025 at 10:34, David Laight <david.laight.linux@...il.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).

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