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	Sat, 22 Feb 2025 10:11:36 +0100
Date: Sat, 22 Feb 2025 10:11:36 +0100
From: Willy Tarreau <w@1wt.eu>
To: David Laight <david.laight.linux@gmail.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
        Jan Engelhardt <ej@inai.de>, "H. Peter Anvin" <hpa@zytor.com>,
        Greg KH <gregkh@linuxfoundation.org>,
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        Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>,
        Christoph Hellwig <hch@infradead.org>,
        rust-for-linux <rust-for-linux@vger.kernel.org>,
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Subject: Re: C aggregate passing (Rust kernel policy)
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User-Agent: Mutt/1.10.1 (2018-07-13)

On Sat, Feb 22, 2025 at 08:41:12AM +0000, David Laight wrote:
> On Sat, 22 Feb 2025 07:37:30 +0100
> Willy Tarreau <w@1wt.eu> wrote:
> 
> ...
> > Update: I found in my code a comment suggesting that it works when using
> > -freg-struct (which is in fact -freg-struct-return) which works both on
> > i386 and ARM.
> 
> The problem is that you need it to be an __attribute__(()) so it can
> be per-function without breaking ABI.

Yes I agree that it would be better.

> > I just didn't remember about this and couldn't find it when
> > looking at gcc docs.
> 
> I can never find anything in there either.
> And then I wish they say when it was introduced.

Same here. At least on gcc-2.95 it was already recognized and worked fine:

- without:
  fct:
        movl 4(%esp),%ecx
        movl $5,%eax
        movl $100,%edx
        movl %eax,(%ecx)
        movl %edx,4(%ecx)
        movl %ecx,%eax
        ret $4

- with:
  fct:
        movl $5,%eax
        movl $100,%edx
        ret

Willy