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SubjectRe: [PATCH] Optimize generic get_unaligned / put_unaligned implementations.
On Thu, 15 Feb 2007 22:18:39 +0000
Ralf Baechle <> 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

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

> Qed?

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