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Date: Mon, 19 Feb 2024 04:40:00 -0500
From: Kent Overstreet <kent.overstreet@linux.dev>
To: "Russell King (Oracle)" <linux@armlinux.org.uk>
Cc: Arnd Bergmann <arnd@arndb.de>, Calvin Owens <jcalvinowens@gmail.com>, 
	Dave Martin <Dave.Martin@arm.com>, linux-arm-kernel@lists.infradead.org, 
	linux-kernel@vger.kernel.org, 
	"linux-bcachefs@vger.kernel.org" <linux-bcachefs@vger.kernel.org>
Subject: Re: [PATCH] arm: Silence gcc warnings about arch ABI drift
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On Mon, Feb 19, 2024 at 09:26:51AM +0000, Russell King (Oracle) wrote:
> On Mon, Feb 19, 2024 at 07:21:11AM +0100, Arnd Bergmann wrote:
> > I think these should be addressed in bcachefs instead.
> > While it's not the fault of bcachefs that the calling
> > convention changed between gcc versions, have a look at
> > the actual structure layout:
> > 
> > struct bch_val {
> >         __u64           __nothing[0];
> > };
> > struct bpos {
> >         /*
> >          * Word order matches machine byte order - btree code treats a bpos as a
> >          * single large integer, for search/comparison purposes
> >          *
> >          * Note that wherever a bpos is embedded in another on disk data
> >          * structure, it has to be byte swabbed when reading in metadata that
> >          * wasn't written in native endian order:
> >          */
> > #if __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
> >         __u32           snapshot;
> >         __u64           offset;
> >         __u64           inode;
> > #elif __BYTE_ORDER__ == __ORDER_BIG_ENDIAN__
> >         __u64           inode;
> >         __u64           offset;         /* Points to end of extent - sectors */
> >         __u32           snapshot;
> > #else
> > #error edit for your odd byteorder.
> > #endif
> > } __packed
> > struct bch_backpointer {
> >         struct bch_val          v;
> >         __u8                    btree_id;
> >         __u8                    level;
> >         __u8                    data_type;
> >         __u64                   bucket_offset:40;
> >         __u32                   bucket_len;
> >         struct bpos             pos;
> > } __packed __aligned(8);
> > 
> > This is not something that should ever be passed by value
> > into a function.
> 
> +1 - bcachefs definitely needs fixing. Passing all that as an argument
> not only means that it has to be read into registers, but also when
> accessing members, it has to be extracted from those registers as well.
> 
> Passing that by argument is utterly insane.

If the compiler people can't figure out a vaguely efficient way to pass
a small struct by value, that's their problem - from the way you
describe it, I have to wonder at what insanity is going on there.