Re: [PATCH] Optimize generic get_unaligned / put_unaligned implementations.

[Andrew Morton <akpm@linux-foundation.org>]

On Thu, 15 Feb 2007 22:18:39 +0000
Ralf Baechle <ralf@linux-mips.org> wrote:

> On Thu, Feb 15, 2007 at 01:53:58PM -0800, Andrew Morton wrote:
> 
> > > The whole union thing was only needed to get rid of a warning but Marcel's
> > > solution does the same thing by attaching the packed keyword to the entire
> > > structure instead, so this patch is now using his macros but using __packed
> > > instead.
> > 
> > How do we know this trick will work as-designed across all versions of gcc
> > and icc (at least) and for all architectures and for all sets of compiler
> > options?
> > 
> > Basically, it has to be guaranteed by a C standard.  Is it?
> 
> Gcc info page says:
> 
> [...]
> `packed'
>      The `packed' attribute specifies that a variable or structure field
>      should have the smallest possible alignment--one byte for a
>      variable, and one bit for a field, unless you specify a larger
>      value with the `aligned' attribute.
> [...]
> 

hm.  So if I have

	struct bar {
		unsigned long b;
	} __attribute__((packed));

	struct foo {
		unsigned long u;
		struct bar b;
	};

then the compiler can see that foo.b.b is well-aligned, regardless of the
packedness.

Plus some crazy people compile the kernel with icc (or at least they used
to).  What happens there?

> Qed?

worried.