Re: [patch 34/54] Staging: hv: remove STRUCT_PACKED and STRUCT_ALIGNED defines

Stephen Hemminger
2009-07-24T21:52:00Z

On Fri, 24 Jul 2009 23:32:19 +0200
Jörn Engel <joern@logfs.org> wrote:

> On Tue, 21 July 2009 01:46:41 +0200, Arnd Bergmann wrote:
> > On Friday 17 July 2009, Greg Kroah-Hartman wrote:
> > > @@ -43,7 +43,7 @@ typedef struct _RING_BUFFER {
> > > // volatile u32 InterruptMask;
> > > // Ring data starts here + RingDataStartOffset !!! DO NOT place any fields below this !!!
> > > u8 Buffer[0];
> > > -} STRUCT_PACKED RING_BUFFER;
> > > +} __attribute__((packed)) RING_BUFFER;
> > >
> >
> > The data structure is actually packed already, the attribute does not make it better
> > and could be removed. We also have __packed as a shortcut for __attribute__((packed)).
>
> Honestly, I don't know how useful __packed really is. In a shared
> kernel/userspace header, it is only defined for the kernel.
>

As I remember, gcc generates worse code for packed structures on many architectures
since it may have to do byte fetchs/recombining to avoid unaligned
accesses.

--