Re: [Qemu-devel] [PATCH 3/3] qcow2: reorder fields in Qcow2CachedTable to reduce padding

[Max Reitz]

On 26.05.2015 18:12, Eric Blake wrote:
> On 05/26/2015 10:10 AM, Max Reitz wrote:
> > On 18.05.2015 18:48, Alberto Garcia wrote:
> > > Changing the current ordering saves 8 bytes per cache entry in x86_64.
> > Hm, not seven?
> >
> > > Signed-off-by: Alberto Garcia <address@hidden>
> > > ---
> > >    block/qcow2-cache.c | 2 +-
> > >    1 file changed, 1 insertion(+), 1 deletion(-)
> > >
> > > diff --git a/block/qcow2-cache.c b/block/qcow2-cache.c
> > > index a215f5b..43590ff 100644
> > > --- a/block/qcow2-cache.c
> > > +++ b/block/qcow2-cache.c
> > > @@ -31,9 +31,9 @@
> > >      typedef struct Qcow2CachedTable {
> > >        int64_t  offset;
> > > -    bool     dirty;
> > >        uint64_t lru_counter;
> > >        int      ref;
> > > +    bool     dirty;
> > >    } Qcow2CachedTable;
> > >      struct Qcow2Cache {
> > With "7" above, or an explanation why it actually is 8:
> Old layout:
>
> 0-7   offset
> 8     dirty
> 9-15  padding
> 16-23 lru_counter
> 24-27 dirty
> 28-31 padding
>
> New layout:
>
> 0-7   offset
> 8-15  lru_counter
> 16-19 ref
> 20    dirty
> 21-23 padding
>
> It indeed saves 8 bytes.

Oh, I forgot about the padding at the end. Thanks!

Max

> > Reviewed-by: Max Reitz <address@hidden>