[PATCH v2] libertas: Fix alignment issues in libertas core

[Erwin Authried]

Am Mittwoch, den 28.01.2009, 15:59 -0800 schrieb Colin McCabe:
> On Wed, Jan 28, 2009 at 3:47 PM, Erwin Authried <eauth at softsys.co.at> wrote:
> > Am Mittwoch, den 28.01.2009, 14:48 -0800 schrieb Colin McCabe:
> >> The argument for specifying struct layout explicitly is pretty clear.
> >> If we relied on gcc to get it right "by accident," we would find that
> >> gcc upgrades and minor code changes would break the driver in
> >> mysterious ways.
> > you are probably right, it's a bit fragile, and there's no warranty
> > that different/new architectures have the same rules for alignment of
> > structures.
> >
> >>
> >> I'm surprised that gcc generates poorer code when the layout is
> >> specified explicitly, since after all, it should be the same layout.
> >> Have you tried using __attribute__ ((aligned (8))) or similar?
> >>
> > it's not surprising because with the packed attribute gcc doesn't make
> > any assumptions about the the alignment of the start of the structure.
> > For example, if you compare the code for the following simple structure:
> >
> > struct a {
> >  int i;
> > };
> >
> > struct b {
> >  int i;
> > } __attribute__ ((packed));
> 
> That's very insightful.
> 
> I wonder if there is an __attribute__ or pragma that would force gcc
> to do the right thing here.
> Can you put __attribute__((aligned)) on a member of a packed struct? I
> might have to test that out later...
> 
> Colin

Colin,
great idea! I did a short test, the "aligned" attribute on the first
struct member seems to do the job!
This way, gcc knows that the struct is aligned and optimizes access to
all members of the struct. We only have to make sure that the buffer for
the struct is aligned.

struct a {
  int i  __attribute__ ((aligned));
  char x,y,z;
  int j;
} __attribute__ ((packed));

-Erwin