Re: [PATCH] arm: Silence gcc warnings about arch ABI drift

Russell King (Oracle)
2024-02-19T09:52:16Z

On Mon, Feb 19, 2024 at 04:40:00AM -0500, Kent Overstreet wrote:
> 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.

It sounds like you have a personal cruisade here.

Passing structures on through function arguments is never efficient.
The entire thing _has_ to be loaded from memory at function call and
either placed onto the stack (creating an effective memcpy() on every
function call) or into registers. Fundamentally. It's not something
compiler people can mess around with.

Sorry but it's bcachefs that's the problem here, and well done to the
compiler people for pointing out stupid code.

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