[Spice-devel] [PATCH spice-server 1/3] glz-encoder-dict: Remove useless __packed__ attribute

[Frediano Ziglio]

> 
> Hi,
> 
> On Mon, Jul 22, 2019 at 12:08:39PM +0100, Frediano Ziglio wrote:
> > The structure has no holes, adding this attribute could only
> > decrease efficiency.
> 
> Why does it decrease efficiency?
> 

If the giant array that contains these structure get unaligned as the
previous fields are not 4/8 bytes aligned the addresses won't be aligned.
And unaligned accesses are less efficient (even more for a hash table
as the read/write are not sequential).
Actually being a cache maybe would be worth forcing 8 bytes of alignment
instead of 4, today many architecture have 64 bit buses (with some
GPU having 128 or even 256 bits buses!).

> > Note that HashEntry structure is used for a large (8MB) array so
> > this won't affect much possible container size.
> > 
> > Signed-off-by: Frediano Ziglio <fziglio at redhat.com>
> > ---
> >  server/glz-encoder-priv.h | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/server/glz-encoder-priv.h b/server/glz-encoder-priv.h
> > index 08dc007ce..7757f232c 100644
> > --- a/server/glz-encoder-priv.h
> > +++ b/server/glz-encoder-priv.h
> > @@ -76,7 +76,7 @@ struct WindowImageSegment {
> >  };
> >  
> >  
> > -struct  __attribute__ ((__packed__)) HashEntry {
> > +struct HashEntry {
> >      uint32_t image_seg_idx;
> >      uint32_t ref_pix_idx;
> >  };
> > --
> > 2.20.1
> > 
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>