linux-kernel - Re: [RFC-PATCH 1/5] unaligned: introduce common header

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Date:	Mon, 10 Nov 2008 10:51:24 -0800
From:	Harvey Harrison <harvey.harrison@...il.com>
To:	Will Newton <will.newton@...il.com>
Cc:	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [RFC-PATCH 1/5] unaligned: introduce common header

On Mon, 2008-11-10 at 18:35 +0000, Will Newton wrote:
> On Mon, Nov 10, 2008 at 4:51 PM, Harvey Harrison
> <harvey.harrison@...il.com> wrote:
> >
> > In this particular case, packed isn't right as you know big_data is
> > aligned (as long as you can guarantee the struct alignment), so you'd
> > probably want:
> >
> > struct foo {
> >        u64 big_data;
> >        u8 small_data;
> >        u32 medium_data __attribute__((__packed__));
> > }
> >
> > But that's not what we're talking about in the kernel's case.
> 
> Perhaps that would be a neater way of expressing what is required in
> my simple example, but it's fairly common to use packed on the whole
> struct which could be because a field that is "packed" by default on
> one architecture might not be on another. You could mark every field
> as packed but few people seem to do that and as far as I am aware
> there is no documented difference between packing all members and the
> whole struct. The gcc documentation for packed is pretty short:

Actually it's documented that putting attribute(packed) on the struct
is equivalent to putting attribute(packed) on _every_ member of the
struct.

>   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.
> 
> I'd love to know if the pointer alignment behaviour is widespread and
> then maybe write a patch for the gcc manual.

Well, it's kind of the whole point of __packed isn't it?  Otherwise the
struct members get naturally (or some arch-dependent value) aligned,
which the compiler can rely on unless you say __packed.

So in my example above, the compiler _knows_ how it has aligned
big_data and small_data and can use whatever access is most efficient,
but it can't make any assumptions about medium_data, so access through
a pointer _must_ be done unaligned.

struct foo *bar;
bar->medium_data; // compiler must do this unaligned

Harvey

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